Alkylguanidinium based ionic liquids in a screening study for the removal of anionic pollutants from aqueous solution

Article abstract of DOI:10.1039/c6ra03607d

Monoalkylguanidinium bis-trifluoromethane sulfonimides are water immiscible functional ionic liquids which appear as highly efficient phases for the sequestration of anionic pollutants from aqueous solutions. The new compounds show significantly enhanced extraction efficiency compared to conventional imidazolium based ionic liquids.

Full text of DOI:10.1039/c6ra03607d

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ARTICLE TYPE
Alkylguanidinium Based Ionic Liquids in a Screening Study for the
Removal of Anionic Pollutants from Aqueous Solution
a
a
a
Roza Bouchal, Bénédicte Prelot and Peter Hesemann*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
5
Monoalkylguanidinium bis-trifluoromethane sulfonimides
nanostructured ionosilica phases.
Finally, guanidinium
are water immiscible functional ionic liquids which appear as 50 sulfonimides have been immobilized on nanostructured silica
6
highly efficient phases for the sequestration of anionic
pollutants from aqueous solutions. The new compounds show
significantly enhanced extraction efficiency compared to
conventional imidazolium based ionic liquids.
materials via sol-gel approaches.
7
Ionic liquids are intensively studied in separation sciences. As
ILs are still rather expensive, special attention has been paid on
the limitation of IL quantity in separation processes. For this
reason, liquid-liquid microextraction processes play an
increasingly important role. On the other side, removal of
anionic dyes from aqueous solutions using imidazolium based
1
0
5
6
6
5
0
5
8
Ionic liquids are unique solvents which consist entirely of
1
ions. Due to their unique properties such as high chemical and
9
thermal stability, inflammability, low vapor pressure and high
ionic conductivity, ionic liquids found numerous applications as
solvents for organic synthesis and catalysis, in electrochemistry, as
electrolyte batteries, fuel cells, and as media for polymerization
processes. Due to their unusual self-aggregation properties, ionic
liquids are referred as ‘supramolecular solvents’ which also
opened new routes in materials science, in nanoparticle synthesis
or as reaction media in ionothermal syntheses.
ionic liquids has already been studied. It appears that the
extraction process is generally driven by hydrophilic/hydrophobic
interaction of the dyes with the ionic liquid phase. For this
reason, long chain substituted ionic liquids gave better results
compared to their short chain substituted counterparts. In this
work, we will show that the extraction processes involving
guanidinium based ionic liquids involve a different mechanism
and different types of interaction on the molecular level.
1
2
2
3
3
4
4
5
0
5
0
5
0
5
The concept of task-specific ionic liquids (TSILs) usually
consists of the chemical grafting of functional organic groups on
Imidazolium derivatives are by far the most studied class of
ionic liquids. In contrast, ionic liquids based on guanidinium
entities are much scarcer, and mostly peralkylated guanidinium
1
0
either cation or anion of the ionic liquid. We already used this
approach for accessing task-specific ionic liquids as recyclable
2
ions were reported. Here, we report monoalkyl guanidinium bis-
1
1
trifluoromethane sulfonimides as a new class of functional ionic 70 ligands in asymmetric catalysis and as extractant in liquid-liquid
1
2
separation. Here, we report guanidinium based ionic liquids as a
new class of task-specific ionic liquids. Due to the versatility and
the particular bonding properties of the guanidinium groups,
chemical functionality is conferred by the cationic substructure of
liquids. The particularity of these new ionic liquids is based on
the cationic guanidinium group which confers specific properties
to the ionic liquids. The guanidine/guanidinium substructure
exhibits very particular physico-chemical behaviour. It is a
relevant functional group widely present in Nature both in 75 the ionic liquid itself. For this reason, we can consider that
proteins via the essential amino acid arginine and in a variety of
natural products. Guanidine is a strong base (pKa=13.6) and
therefore exists in aqueous solution as protonated guanidinium
cation. The guanidine/guanidinium couple possesses unique
monoalkylguanidinium salts such as hexylguanidinium bis-
trifluoromethane sulfonimide (C Gua NTf scheme 1) are indeed
6
2,
task-specific ionic liquids. We investigated these new compounds
in liquid-liquid extraction of anionic pollutants from aqueous
electronic, physico-chemical and steric characteristics such as 80 solutions. We focused in particular on the sequestration of an
superbasicity and the ability to undergo -cation interactions. Its
ability to form strong H-bonding interactions together with the
planar arrangement with C2h symmetry are useful features in the
field of materials’ science, for example in view of the formation
organic dye (methyl orange, MO), an anionic drug (diclofenac,
DCF, scheme 1) and a metallic anion (chromate). Our work
highlights the particular properties of guanidinium based ionic
liquids compared to other water immiscible ionic liquids, i.e.
3
85
butyl-methyl-imidazolium bis-triflimide (C mim NTf ).
4 2
of molecular materials. Guanidinium salts therefore display big
differences compared to conventional imidazolium type ionic
liquids. In view of their self aggregation and coordination
properties, long-chain substituted guanidinium halides behave
differently compared to other cationic surfactants such as
4
ammonium or imidazolium based amphiphiles. Guanidinium
Scheme 1 Structures of hexylguanidinium bis-trifluoromethane
sulfonimide (C Gua NTf -left), the anionic dye methyl orange (MO-
middle) and the anionic drug diclofenac (DCF-right)
surfactants have also been used as structure directing agents in
template directed hydrolysis-polycondensation reactions of
6
2
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We firstly studied the synthesis of a variety of monoalkylated
and C mim NTf (right). At a first glance, the transfer of the
4 2
guanidinium based ionic liquids. Primary amines were efficiently 40 anionic dye from the water into the IL phase using the
converted
sulfonimides in two-step sequences. In
guanidinium chlorides were synthesized by reacting primary
into
alkylguanidinium
bis-trifluoromethane
guanidinium IL can directly be visualized by the decolouration of
a
first step, the
the water phase. In contrast, using the imidazolium IL C mim
4
5
NTf , the anionic dye is rather homogenously distributed between
2
1
3
amines with 1H-pyrazole-1-carboxamidine hydrochloride. The
the water and the IL phase (photo 1, right). This result gives a
resulting guanidinium chlorides were used for anion metathesis 45 first indication that the separation properties of guanidinium and
reactions with lithium bis-trifluoromethane sulfonimide anions
scheme 2). We synthesized different ionic liquids with variable
imidazolium ILs differ by a large extent.
(
1
0
alkyl chain length: hexyl, octyl and decylguanidinium bis-
trifluoromethane sulfonimides C Gua NTf (x = 6,8,10).
x
2
Scheme 2 Synthesis of the guanidinium type ionic liquids
The resulting ionic liquids were water immiscible and
moderately viscous compounds with viscosities from 420-480 cP
1
2
2
3
5
0
5
0
(
see ESI). These values, although being approx. six times higher
Photo 1 Biphasic water/IL solutions after contacting the MO containing
than those measured for imidazolium bis-trifluoromethane
6 2 4 2
water phase with C Gua NTf (left) and C mim NTf (right)
sulfonimides such as C mim NTf , are typical for a number of
4
2
1
4
ionic liquids. Similarly to imidazolium type ILs, increasing
alkyl chain length resulted in increasing viscosity of the
50
We than studied the sequestration of MO using the different
guanidinium based ILs more in detail. The distribution of the MO
between the water and the IL phase was quantified via UV-Vis-
1
5
guanidinium type ILs. The new compounds were characterized
1
13
by IR, H, C NMR, mass spectroscopy and differential scanning
spectroscopy (
= 466 nm). This technique allows the
calorimetry (DSC). All alkylguanidinium bis-trifluoromethane
sulfonimides are liquid at room temperature and do not display 55 general formula
any noticeable phase transition between -50 and 130°C (see ESI)
and therefore are room-temperature ionic liquids (RTILs). It has
to be mentioned that the guanidinium bis-trifluoromethane
sulfonimides are thermally highly stable. Thermogravimetric
determination of the distribution coefficient D according to the
D = ([MOinit] – [MO ]) / [MO ]
eq
eq
In a first series of experiments, we studied the extraction of
small quantities of MO, i.e. using a large excess of the IL (table
1). Under standard conditions, we contacted approx. 100 mg of
experiment with C Gua NTf indicate that degradation starts at 60 the ILs C Gua NTf , C Gua NTf and C Gua NTf with an
6
2
6
2
8
2
10
2
ca. 380°C. The thermal stability of this IL therefore is slightly
aqueous solution (3 mL) containing 1, 2, 4 and 10 mol-% of MO.
The biphasic solution was stirred during 15 h, and the quantity of
the MO remaining in the water phase was determined via
UV/Vis-spectroscopy. It clearly appears that all guanidinium
based ILs efficiently adsorb MO. Analysis of the supernatant
water phase shows that in all cases > 99% of MO is transferred
into the IL phase (D > 99, table 1, entries 1-12). The increasing D
values can be related to the fact that in all experiments, nearly
identical MO concentrations were found in the aqueous phase
after contacting with the guanidinium ILs. Interestingly, long
chain substituted ILs (C Gua NTf ) gave slightly higher
1
5
lower than observed for C mim NTf .
4
2
6
7
7
5
0
5
1
0
2
distribution coefficients compared to the short chain counterparts
C Gua NTf ). This trend can be related to the higher
(
6
2
hydrophobicity in the former ILs. Hydrophilic/hydrophobic
interaction has already shown to be the driving force of
Scheme 3 TGA plot of C
6
2
Gua NTf (heating rate: 10°C/min, under argon)
9
separation processes involving imidazolium based ILs. For seek
We then studied the use of the new ionic liquids in liquid-
liquid extraction of anionic pollutants. In a first time, we focused
on the sequestration of methyl orange (MO), an anionic dye
of comparison, we performed identical experiments using a
3
5
conventional IL, namely C
4
mim NTf . These experiments
2
showed that the use of C mim NTf
resulted in considerably
4
2
(
scheme 1). Photo 1 shows the biphasic water/IL solutions after 80 lower distribution coefficients indicating much less efficient MO
contacting the MO containing water phase with C Gua NTf (left)
transfer towards the IL phase (table 1, entries 13-16).
6
2
2
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Table 1 Liquid-liquid extraction of MO using C
NTf (exact quantities are given in the ESI)
x
Gua NTf
2
and C
4
mim
Table 2 Liquid-liquid extraction of MO using high C
35 ratios (exact quantities are given in the ESI)
6
Gua NTf
2
/ MO
D
2
Entry Quantity of Quantity of MO/IL
IL / MO/ ratio
mg (mmol) mg (mol)
C
init
/
C
eq
/
D
Entry Quantity of IL Quantity of MO/IL
MO/ ratio mmol/L mmol/L
mg (mol) mg (mol)
with C
1.58 (4.8)
3.24 (9.9)
C
init
/
eq
C /
mmol/L
mmol/L
/
a
a
with C
6
Gua NTf
2
6
Gua NTf
0.20
0.42
0.61
0.83
2
1
2
3
4
100.6 (0.237) 0.76 (2.33)
100.0 (0.236) 1.60 (4.90)
101.1 (0.238) 3.23 (9.88)
100.6 (0.237) 7.80 (23.8)
0.01
0.02
0.04
0.1
0.78
1.63
3.29
7.95
0.0047 165
0.0065 249
0.0061 542
0.0058 1375
1
2
3
4
5
10.0 (23.5)
9.9 (23.3)
10.3 (24.3) 4.88 (14.9)
10.1 (23.8) 6.49 (19.8)
10.4 (24.5) 7.82 (23.9)
with C
10.0 (22.1)
10.5 (23.2)
10.0 (22.1) 4.35 (13.3)
10.2 (22.6) 5.81 (17.8)
1.61
3.29
4.95
6.61
7.95
0.014
0.024
0.039
0.081
0.964
114
136
127
81
with C
0.01
0.02
0.04
0.1
8
Gua NTf
2
0.97
8
5
6
7
8
100.5 (0.222) 0.73 (2.24)
100.6 (0.222) 1.47 (4.50)
100.1 (0.221) 2.93 (8.96)
101.0 (0.221) 7.27 (22.2)
1.11
2.23
4.44
11.06
0.0052 214
0.0060 369
0.0068 654
0.0057 1951
8
Gua NTf
0.20
2
6
7
8
9
1.46 (4.5)
2.92 (8.9)
2.23
4.45
6.64
8.85
11.06
0.0005 4930
0.38
0.60
0.79
0.97
0.002
0.003
0.010
0.612
2237
2276
871
17
with C10Gua NTf
2
9
100.3 (0.209) 0.69 (2.10)
0.01
0.02
0.04
0.1
1.05
2.08
4.17
0.0046 226
0.0058 355
0.0064 653
0.0042 2494
10 10.4 (23.0) 7.29 (22.3)
1
1
1
0 100.0 (0.208) 1.36 (4.15)
1 100.2 (0.209) 2.74 (8.38)
2 100.1 (0.208) 6.81 (20.8)
with C10Gua NTf
2
11 10.2 (21.2)
12 10.4 (21.7)
13 10.0 (20.8) 4.09 (12.5)
14 10.2 (21.2) 5.45 (16.6)
15 10.0 (20.8) 6.86 (21.0)
1.36 (4.2)
2.73 (8.3)
0.20
0.38
0.60
0.78
1.01
2.08 <0.0001 >10000
4.17 <0.0001 >10000
6.25
8.32
10.40
with C
0.01
0.018
0.04
4
mim NTf
2
0.005
0.004
0.545
1175
2007
18
1
1
1
1
3 104.5 (0.249) 0.80 (2.43)
4 110.6 (0.264) 1.55 (4.74)
5 102.3 (0.244) 3.15 (9.61)
6 105.5 (0.252) 7.75 (23.7)
1.21
2.37
4.79
11.87
0.940 0.29
1.808 0.31
3.879 0.23
8.834 0.34
10.40
a
dissolved in 3mL of water.
0.09
a
In order to generalize our approach, we investigated the
dissolved in 3 mL of water.
sequestration properties of the guanidinium type ILs for the
extraction of other anionic species, i.e. an anionic drug
(diclofenac, DCF) and a metallic anion (chromate). Both species
are of high environmental and sanitary concern: DCF is a widely
used non steroidal anti-inflammatory drug which is, together with
its metabolites, among the most frequently detected
After having ascertained the ability of the novel guanidinium
5
0
5
0
5
0
ILs to adsorb efficiently MO at low concentrations, we
investigated the anion exchange capacity of these novel
compounds. For this purpose, we performed extraction
experiments using higher molar ratios between the IL and MO
together with reduced IL quantity. In this second series of
experiments, we contacted approx. 10 mg of the C Gua NTf ILs
4
0
16
pharmaceutical residues in water bodies. Chromate, widely
1
1
2
2
3
x
2
45
present in the effluents of many industries including tanning,
electroplating paints, dyes etc., is a hazardous pollutant and
recognized as a human cancerogen. The contact time for all
experiences was 15 h.
with approx. 20, 40, 60, 80 mol-% and an equimolar amount of
MO dissolved in water (table 2). Once again, our results show
that the guanidinium ILs adsorb efficiently the anionic dye. For
all studied ILs, we observed decreasing D values with increasing
MO quantities. Furthermore, the highest distribution coefficients
were observed with the long chain substituted C Gua NTf .
Table 3 Liquid-liquid extraction of DCF and chromate using C
6
Gua NTf
2
50
(exact quantities are given in the ESI)
1
0
2
More specifically, for extraction experiments involving C Gua
Entry Quantity of IL Quantity of Anion/IL
C
init
/
eq
C /
D
1
0
/
anionic
species/
mg (mol)
ratio mmol/L mmol/L
NTf with 20 and 38 mol-% of MO, the dye concentration in the
2
mg (mmol)
aqueous phase was below the detection limit (table 2, entries
11/12). In all cases, the distribution coefficients remain high up to
a dye/IL ratio of 0.8, but strongly decrease for equimolar dye/IL
amounts. These experiments clearly show that at least 80% of the
guanidinium cations of the ILs can be used for anion extraction.
It has to be mentioned that, with increasing IL/MO ratio, the IL
layer tends to solidify. This trend is due to the formation of
crystalline C Gua (NTf ) (MO)x systems. Indeed, the
a
DCF
0.01
1
2
3
4
5
6
99.8 (0.235) 0.9 (2.74)
101.7 (0.240) 1.8 (5.45)
99.9 (0.235) 3.6 (11.0)
102.3 (0.241) 5.4 (16.5)
101.8 (0.240) 7.0 (21.9)
101.3 (0.239) 8.6 (27.1)
0.91
1.82
3.65
5.44
7.24
9.02
0.033
0.022
0.016
0.031
0.032
0.046
27
82
228
172
225
196
0.02
0.05
0.07
0.09
0.11
Chromate
x
2 1-x
7
8
9
107.9 (0.254) 0.46 (2.4)
0.01
0.02
0.04
0.05
0.07
0.10
1.21
2.23
4.42
6.57
8.76
12.3
0.38
0.76
1.66
2.74
4.08
4.97
2.1
2.0
1.7
1.4
1.1
1.5
characterization of the formed C Gua (NTf ) (MO)0.1 phase via
6
2 0.9
1
103.2 (0.243) 0.86 (4.4)
102.1 (0.241) 1.7 (8.8)
102.4 (0.241) 2.5 (13.0)
H NMR spectroscopy nicely indicates a molar ratio between the
guanidinium cation and the MO anion of 1 : 0.12 (see ESI), what
is in very good agreement with the amount of added MO. This
result indicates that the extraction involves an anion exchange
mechanism and is driven by the high affinity of the guanidinium
groups towards the sulfonate groups of MO.
1
0
11 101.3 (0.239) 3.4 (17.3)
102.6 (0.242) 4.8 (24.6)
1
2
a
dissolved in 3 mL of water for experiments with DCF, in 2 mL of water
for experiments with chromate.
The results obtained with C Gua NTf and low anion/IL ratios
6
2
are given in table 3. Both DCF and chromate are extracted
towards the IL phase. However, results for DCF are slightly
lower compared to those of MO, and chromate is extracted in a
considerably lower extent. This behaviour is due to the
55
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hydrophobic character of the IL which favours the adsorption of
organic anions, whereas metallic or mineral anions display a
lower affinity. Similar results were obtained with C Gua NTf
6
2
and high anion/IL ratios (see ESI). It has to be mentioned that nor
DCF neither chromate are adsorbed in noticeable amounts by
imidazolium ILs (see ESI). The fact that guanidinium salts adsorb
these pollutants more efficiently is a clear sign of the contribution
of the guanidinium substructure towards the extraction
performances of these compounds. Finally, it should be
mentioned that the pH is hardly affected by the extraction
process. In all cases (MO, DCF, chromate), we observed neutral
pH or only very slight pH changes after the liquid-liquid
extraction. However, the extraction efficiency may depend of the
acidity/basicity of the solution. This aspect will be investigated in
detail in our future work.
5
0
5
0
5
0
5
0
5
0
1
1
2
2
3
3
4
4
5
55
6 2
Scheme 4 Extraction-regeneration cycle of C Gua NTf in the separation
of MO
In conclusion, we synthesized
a
series of new
For green and sustainable process engineering, the
regeneration of the extractants is of particular interest. Here, this
issue is of importance as, besides its notable toxicity, the bis-
trifluoromethane sulfonimide anion is not biodegradable and
monoalkylguanidinium bis-trifluoromethane sulfonimide ionic
liquids. These novel compounds appear as functional (‘task-
specific’) ionic liquids due to the presence of cationic
guanidinium groups, able to create strong interactions with
various anionic substrates via combined ionic interactions and
hydrogen bonding. Here, the high potential of guanidinium based
ILs in separation was monitored via the extraction of an anionic
dye (methyl orange, MO), an anionic drug (diclofenac) and a
metallic anion (chromate). Significant differences were found
regarding the extraction efficiency of these different anionic
species. The best results were found for MO, and DCF was also
efficiently sequestrated. In contrast, chromate was separated in
significantly lower extent. We attribute these results to the high
affinity of organic anions towards the hydrophobic IL phase. This
result is supported by the fact that decyl substituted guanidinium
IL C Gua NTf gave better results than related octyl or hexyl
6
6
7
7
8
0
5
0
5
0
1
7
tends to accumulate in the biosphere. We therefore studied the
recycling of the guanidinium ionic liquids in view of the
development of an extraction/regeneration cycle. Firstly, the
sequestration of anionic pollutants towards the IL phase involves
a transfer of the bis-trifluoromethane sulfonimide anion into the
water phase. We were able to form new guanidinium NTf ILs
2
via a simple addition of hexylguanidinium chloride. The newly
formed C Gua NTf show identical characteristics compared to
x
2
the initially synthesized material (see ESI). Another interesting
feature of the guanidinium ILs is their atypical miscibility with
some organic solvents. We observed that the guanidinium ILs are
completely miscible in diethyl ether in every IL/solvent ratio.
This behaviour opens new possibilities for a simple and
straightforward IL regeneration after liquid-liquid extraction. In
fact, treatment of the formed C Gua (NTf ) (MO) systems with
1
0
2
guanidinium compounds. In contrast, mineral or metallic anions
show a less pronounced affinity towards the guanidinium IL
phase. It has to be mentioned that in all cases, the guanidinium
based ILs gave considerably better results compared to their
imidazolium counterparts. Finally, we introduced a closed
separation-regeneration cycle involving the guanidinium based
ionic liquids which is of interest for sustainable process
engineering. Our ongoing work in this field focusing on the
separation of traces of pollutants of real environmental solutions
will be reported in due course.
x
2 1-x
x
+
-
an aqueous M NTf2 solution followed by addition of diethyl
ether gave a biphasic system containing the MO in the aqueous
phase and the newly formed C Gua NTf in the ether phase. We
x
2
also used the expelled bis-trifluoromethane sulfonimide anion
obtained in the water phase as described above for this purpose.
Evaporation of the organic solvent gave the recycled C Gua
x
NTf . In this way, our work demonstrates the possibility to re-use
2
both cation and anion of the guanidinium based ionic liquids and
to build up a closed extraction-regeneration cycle (scheme 4).
This extraction-regeneration cycle can also be performed with
other organic anions such as diclofenac, which behaves similarly
compared to MO in the liquid-liquid extraction process. Although
metallic anions such as chromate led to considerably lower
85
Notes and references
a
Institut Charles Gerhardt, UMR 5253 CNRS-UM-ENSCM;
Université de Montpellier, Place Eugène Bataillon, 34095 Montpellier
cedex 05, France ; Tel. +33-4.67.14.45.28; Fax +33-4.67.14.38.52;
email: peter.hesemann@umontpellier.fr.
distribution coefficients, the extraction-regeneration can be 90 ‡ The synthesis and spectroscopic details of the guanidinium based ILs
performed in a similar way as the cyclic process is driven by the
by the extraction of the IL into the ether phase. As a consequence,
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Graphical Contents
Alkylguanidinium Based Ionic Liquids in a Screening Study for
the Removal of Anionic Pollutants from Aqueous Solution
Roza Bouchal, Bénédicte Prelot and Peter Hesemann*
Monoalkylguanidinium bis-trifluoromethane sulfonimides are
highly efficient adsorbents for the sequestration of organic anions
from aqueous solutions and display strongly enhanced adsorption
properties compared to conventional imidazolium ILs.