Greener synthesis of new ammonium ionic liquids and their potential as extracting agents

Article abstract of DOI:10.1016/j.tetlet.2008.02.138

New hydrophobic ionic liquids were synthesized from tricaprylmethylammonium chloride (Aliquat 336) and selected Bronsted acids by a sustainable, simple and cost-saving deprotonation-metathesis route. Prepared ionic liquids were evaluated as pot

Full text of DOI:10.1016/j.tetlet.2008.02.138

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Tetrahedron Letters 49 (2008) 2782–2785
Greener synthesis of new ammonium ionic liquids and
their potential as extracting agents
Daniel Kogelnig *, Anja Stojanovic, Markus Galanski, Michael Groessl, Franz Jirsa,
Regina Krachler, Bernhard K. Keppler
University of Vienna, Institute of Inorganic Chemistry, Waehringer Strasse 42, 1090 Vienna, Austria
Received 24 January 2008; revised 19 February 2008; accepted 22 February 2008
Available online 29 February 2008
Abstract
New hydrophobic ionic liquids were synthesized from tricaprylmethylammonium chloride (Aliquat 336Ó) and selected Bronsted acids
by a sustainable, simple and cost-saving deprotonation-metathesis route. Prepared ionic liquids were evaluated as potential extracting
agents for cadmium from different aqueous solutions. High efficiency and selectivity were reached for the extraction of cadmium from
a natural river matrix with tricaprylmethylammonium thiosalicylate, [A336][TS], a thiol-containing task specific ionic liquid.
Ó 2008 Elsevier Ltd. All rights reserved.
The synthesis of ionic liquids (ILs), their characteriza-
tion, and possible applications have been developing pro-
gressively, as the properties of this class of organic salts
with melting points below the boiling point of water have
gained intensive attention in nearly all fields of chemistry.
Furthermore, through the incorporation of functional
groups, the synthesis of task specific ILs (TSILs) has been
a focus of research, leading to tailor-made substances for
desired applications.1
A widespread research field is the use of ILs as well as
TSILs incorporating disulfide-, thioether-, urea- or
hydroxybenzylamine groups as extracting agents for met-
als.2–7 Most of ILs or TSILs for extracting applications
described in the literature are based on imidazolium-, pipe-
ridinium-, pyrrolidinium- and pyridinium cations and fluo-
rine containing anions, whereas diverse functional groups
are generally appended to the cation.3–7 In contrast, the
newly synthesized ILs described here are based on a hydro-
phobic, long chain tetraalkylammonium cation with ali-
phatic and aromatic carboxylate anions. Furthermore,
tricaprylmethylammonium thiosalicylate, [A336][TS], pos-
sesses a thiol-containing anion.
New ammonium ILs were prepared by stirring equimo-
lar amounts of Aliquat 336Ó as a precursor and selected
Bronsted acids in a sodium hydroxide solution. Deproto-
nation of the acid followed by anion metathesis led to the
desired ionic liquids (Scheme 1). Aliquat 336Ó is actually
an ionic liquid itself and is regarded as a 2:1 mixture of
methyl trioctylammonium- and methyl tridecylammonium
chloride with
a proposed mean molar weight of
432 g/mol.8 We investigated the structure of the Aliquat
336Ó-cation and positive ion mode electrospray ioniza-
tion-mass spectrometry (ESI-MS) disclosed peaks that
could be assigned to methyl tricapryl cations containing
either octyl- or decylchains or mixtures of them. However,
one peak could clearly be assigned to an existing methyl
*
Corresponding author. Tel.: +43 1 4277 52626; fax: +43 1 4277 52620.
Scheme 1. Synthesis of tricaprylmethylammonium based ionic liquids.
E-mail address: daniel.kogelnig@univie.ac.at (D. Kogelnig).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.02.138

D. Kogelnig et al. / Tetrahedron Letters 49 (2008) 2782–2785
2783
1
dioctylhexylammonium cation. Interestingly, including a
methyl dioctylhexylammonium cation in molar weight cal-
culations, a mean value of 432 g/mol is also obtained for
Aliquat 336Ó and was therefore used throughout our
synthesis.
The deprotonation-metathesis route is simple, fast and
sustainable since water is the only solvent used and sodium
chloride the sole by-product. Tricaprylmethylammonium
thiosalicylate, [A336][TS], tricaprylmethylammonium benz-
oate, [A336][BA] and tricaprylmethylammonium hexano-
ate, [A336][Hex] were obtained in yields around 97% and
characterization data showed that ILs exhibit a good
quality.
In Figure 1a the H-proton spectrum of [A336][TS] is
shown. Chemical shifts between 6.68 and 7.87 ppm could
clearly be assigned to aromatic hydrogen atoms of the thio-
salicylate anion. FTIR spectra of precursors Aliquat 336Ó
and thiosalicylic acid as well as the resulting IL [A336][TS]
are shown in Figure 1b. Typical absorption bands of the car-
boxylic acid diminished after the reaction, indicating that
the deprotonation was successful. New absorption bands
at 743, 1466 and 1562 cmÀ1 could be assigned to stretching
and bending vibrations of aromatic C–H groups. Interest-
ingly, an expected absorption band of the thiol group near
2500 cmÀ1 could not be observed for the prepared
[A336][TS]. This may be attributable to the generally
Fig. 1a. 1H NMR spectrum of [A336][TS].
Fig. 1b. FTIR spectra of Aliquat 336Ó (A336), thiosalicylic acid (TS-acid) and ionic liquid [A336][TS].

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D. Kogelnig et al. / Tetrahedron Letters 49 (2008) 2782–2785
Table 1
Physicochemical properties of Aliquat 336Ó-based ionic liquids
Ionic liquid
Physical state (25 °C)
Decomp. T/°C
ClÀ cont. (wt %)
q/g cmÀ3 (20 °C)
Solubilitya
Miscible
Immiscible
[A336][TS]
[A336][BA]
[A336][Hex]
a
Liquid
Liquid
Liquid
202
161
148
<0.1
0.81
0.59
0.96
0.96
0.89
ea, ac, m, e, an
ea, ac, m, e, an
ea, ac, m, e, an
w
w
w
Observed solubility of ionic liquids: complete solubilization (miscible) or non-solubilization (immiscible), where w = water, ea = ethylacetate,
ac = acetone, m = methanol, e = ethanol, an = acetonitrile.
considered low intensity of this band. ESI-MS experiments
proved that the thiol group was not deprotonated: in case
of an additional deprotonation of the thiol group, a peak
at a lower mass to charge ratio should have been observable.
A single peak at m/z = 153.2 could clearly be assigned to the
thiosalicylate anion. Generally, negative ion mode ESI-MS
spectra of all prepared ILs showed signals corresponding
to the according aliphatic or aromatic anion.
Measured physicochemical properties of synthesized ILs
are given in Table 1. [A336][TS], [A336][BA] and
[A336][Hex] were liquid at room temperature and immisci-
ble with water but miscible with common organic solvents
such as ethyl acetate, acetone, ethanol, methanol and
acetonitrile. Thermogravimetric analysis showed decompo-
sition temperatures of prepared ILs between 148 °C and
202 °C. Measured chloride contents were between <0.10
and 0.81 wt %.9 These values are lower compared to most
of the reported values for ammonium ionic liquids pre-
pared from Aliquat 336Ó applying other synthesis routes.2
Obtained densities of ILs with aromatic anions [TS] and
[BA] were higher compared to the aliphatic IL
[A336][Hex]. These results are in accordance with the
assumption of Kulkarni et al.10 proposing an increase in
the bulkiness of the anion leading to an increase in density.
As prepared ILs contain a carboxylic functionality and
[A336][TS] additionally a thiol group, they were evaluated
as potential extracting agents for cadmium from different
aqueous solutions. Ultrapure water as well as a filtrated
(0.45 lm) natural fluvial water matrix was spiked with a cad-
mium standard solution obtaining a total concentration of
4 ppm/L. To evaluate the extraction potential of ILs under
neutral conditions and the influence of a natural buffered flu-
vial system, the pH-value of the water samples was kept at
the pH as measured after spiking with metal standard.
The extraction efficiency of ILs for cadmium from ultra-
pure water and natural river water (Fig. 2) follows the
order [A336][TS] ꢀ [A336][Hex] > [A336][BA].
Fig. 2. Extraction efficiency of evaluated ionic liquids for the extraction of
cadmium from ultrapure water and a natural river sample.
In ultrapure water, extraction efficiencies of >99.9%
were obtained for [A336][TS], 16% for [A336][Hex] and
13% for [A336][BA]. For the elimination of cadmium from
river water [A336][TS] showed an efficiency of >99.9%,
[A336][Hex] 14% and [A336][BA] 11%.
As the difference in structure between [A336][TS] and
[A336][BA] is a thiol group appended to the anion of
[A336][TS], it seems obvious that this group has an impor-
tant influence on the extraction process. There were no
remarkable differences in extraction efficiencies between
evaluated aliphatic and aromatic carboxylic ILs
[A336][Hex] and [A336][BA].
Interestingly, a natural river matrix did not lead to a
decrease of extraction efficiency for evaluated ILs. Calcu-
lated distribution coefficients for alkali- and earth alkali
metals with [A336][TS] are given in Table 2 and are below
a value of 2, whereas a distribution coefficient of >10,000
could be stated for cadmium. [A336][TS] is therefore very
efficient and selective for the extraction of cadmium from
a river matrix with natural occurring alkali- and earth
alkali cations. This may be interesting for extraction tasks
Table 2
0.5 ml of IL and 8 ml of cadmium containing water phase were shaken
for 10 min, followed by centrifugation at 3500 rpm. The water phase was
separated from the IL phase and acidified for measurements. Flame
atomic absorption spectroscopy (F-AAS) was used for quantization of
cadmium concentrations in the water phase before and after the extraction
process. Additionally, alkali and earth alkali elements of natural river
water were measured to evaluate the selectivity of ILs for the extraction of
cadmium compared to naturally occurring metals. Extraction efficiency of
>99.9% was assumed when measured values were below detection limit of
the corresponding method.
Distribution coefficients (D) for [A336][TS] for the extraction of metals
from a natural river matrix
Cation
D ([A336][TS])
Na+
K+
Ca2+
Mg2+
Cd2+
0.14
0.14
1.49
0.10
>10,000

D. Kogelnig et al. / Tetrahedron Letters 49 (2008) 2782–2785
2785
where hard water with high calcium and magnesium con-
centrations is present.
versus temperature charts of the three synthesized ionic liq-
uids and Aliquat336Ò as precursor are presented. Section
II: experimental part including applied analytical methods
(Flame-AAS and IC)) associated with this article can be
found, in the online version, at doi:10.1016/
j.tetlet.2008.02.138.
Compared to other cadmium extracting agents, working
well under neutral conditions, on which research was
recently published,11–15 [A336][TS] displays some consider-
able advantages: the selectivity for cadmium is well compa-
rable to calix[4]arene derivates bearing pyridinium or sulfur-
containing units in the presence of alkali metal ions.11–13
The extracting efficiency of [A336][TS] (>99.9%) lies even
above these compounds and its use may be regarded to as
more sustainable, as no volatile and flammable organic sol-
vents are needed. Other environmental friendly, low-cost
biomaterials, for example, biofilm covered granular acti-
vated carbon,15 or crab shells,14 showing adsorption of
organic residues or slow sorption kinetics, respectively, are
outnumbered by [A336][TS] displaying an undisturbed fast
metal uptake and a high affinity for cadmium under neutral
conditions, even in a complex natural matrix.
Current investigations deal with the affinity of
[A336][TS] for other metals. In addition, the elucidation
of the reaction mechanism as well as the design of a suit-
able recycling process for a reuse of the applied ILs by
metal stripping is of considerable interest.
In conclusion, we demonstrated the successful prepara-
tion of three new hydrophobic ammonium ionic liquids
from Aliquat 336Ó as a cation source and Bronsted acids
as anion sources. Moreover, preliminary investigations
have shown that the implementation of functional groups
(e.g., thiol) onto the anion is leading to task specificity in
ionic liquids usable as extracting agents.
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Due to our results, we are convinced that the presented
deprotonation-metathesis route may be an elegant way for
the preparation of further numerous hydrophobic Aliquat
336 Ó-based ILs with functionalities appended to the anion.
Acknowledgements
The authors are grateful to Florian Biba for performing
TGA measurements and the Austrian Federal Ministry of
Agriculture, Forestry, Environment and Water Manage-
ment (Project No. A600702) for financial support.
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