Synthesis of High-Purity Imidazolium Tetrafluoroborates and Bis(oxalato)borates

Article abstract of DOI:10.1002/chem.201604461

The synthesis and purification of imidazolium-based ionic liquids (ILs) with boron-containing anions is reported. The scope was the optimization of the meta thesis reaction and on the purification of the synthesized ILs. It was possible to reduce the reac

Full text of DOI:10.1002/chem.201604461

DOI: 10.1002/chem.201604461
Communication
&
Ionic Liquids
Synthesis of High-Purity Imidazolium Tetrafluoroborates and
Bis(oxalato)borates
Paulo Schmitz,[a, b] Rene Jakelski,[a, b] Kirsi Jalkanen,[a, b] Martin Winter,[a, b, c] and
Peter Bieker*[a, b]
On account of these characteristics, ILs are utilized in numer-
Abstract: The synthesis and purification of imidazolium-
ous research and application fields, such as in synthesis,[3c,5]
based ionic liquids (ILs) with boron-containing anions is
catalysis,[6] separation processes,[7] energy storage,[1a,8] or lubri-
reported. The scope was the optimization of the meta
cation[9] and metal deposition.[10] However, depending on the
thesis reaction and on the purification of the synthesized
application, there are various purity requirements on the ILs. It
ILs. It was possible to reduce the reaction and purification
is for instance inevitable to use strictly pure ILs for application
times, to avoid the use of acetonitrile as processing
in electrochemical energy storage systems,[11] for example, in
solvent, and to increase the yield compared to the known
lithium metal, lithium-ion, and dual-ion batteries.[12]
procedure for [BOB]À anion-containing ILs. Furthermore,
In general, ILs can be synthesized via two different routes:
to the best of our knowledge the flashpoints of the ILs
The first is a direct synthesis, while the alternative follows an
could be determined for the first time by the continuously
anion metathesis.[13] Halide-free ILs, for example, can be pro-
closed-cup flashpoint (CCCFP) method.
duced by a direct alkylation reaction of an amine with an alkyl
nitrate.[13a] As the variety of ILs that can be prepared by the
route of direct alkylation route is limited, most ILs are synthe-
Ionic liquids (ILs), and especially room-temperature ionic
liquids (RTILs), have been intensively investigated during the
last decade.[1] They often consist of organic cations and organic
or inorganic fluorine-containing anions, both with de-
localized charges.[2]
sized in two steps. First an organic halide salt is formed
through an alkylation of a base by a haloalkane, after which an
anion exchange by metathesis or an acid–base reaction is
performed.[14] Scheme 1 shows the two-step synthesis routes
for ILs based on 1-methylimidazole.
Depending on the detailed chemistry of the respective IL,
they show favorable physical properties, such as miscibility
with organic solvents, negligible vapor pressure also at elevat-
ed temperatures, chemical and thermal stability, recyclability,
wide electrochemical stability windows, and high conduct-
ivities.[3]
Herein we focused on the synthesis and purification
of 1-ethyl-3-methylimidazolium ([C2MIm]) and 1-butyl-3-
methylimidazolium ([C4MIm]) cations in combination with
bis(oxalato)borate (BOBÀ) and tetrafluoroborate (BF4À) anions.
The metathesis reaction was carried out both in the melt and
in suspension to obtain highly pure products, while optimizing
the reaction time, yield, and cost efficiency compared to the
reported procedure for BOBÀ anion-containing ILs.[3c] The
method investigated herein reduces the reaction and purifica-
tion times by two days, avoids the use of hazardous aceto-
nitrile, and increases the yield for more than 15%. Additionally,
the synthesized ILs were characterized with flashpoint and
refractive-index measurements, differential scanning calori
metry (DSC), and thermogravimetric analysis (TGA). To the best
of our knowledge this is the first time the flashpoints of the
respective ILs could be exactly determined by using the con-
tinuously closed-cup flashpoint (CCCFP) method.
As an additional major benefit, ILs are easily tunable to meet
special requirements, as cations and anions can be combined
and varied in different ways corresponding to the desired
application. Thus various properties, such as for example the
melting point, the viscosity, or the solubility can be custom-
tailored in the desired way.[4]
[a] P. Schmitz, R. Jakelski, Dr. K. Jalkanen, Prof. M. Winter, Dr. P. Bieker
Institute for Physical Chemistry
University Mꢀnster
Corrensstrasse 28/30, 48149 Mꢀnster (Germany)
E-mail: peter.bieker@uni-muenster.de
The ILs 1-ethyl-3-methylimidazolium bis(oxalato)borate
([C2MIm]BOB), 1-butyl-3-me-thylimidazolium bis(oxalato)borate
([C4MIm]BOB), 1-ethyl-3-methylimidazolium tetrafluoroborate
([C2MIm]BF4), and 1-butyl-3-methylimidazolium tetrafluorobo-
rate ([C4MIm]BF4) were synthesized both by a solvent-free
method and with dichloromethane as a solvent. The methods
were compared based on their yields. Furthermore, the purifi-
cation step was investigated to obtain highly pure ILs, which
fulfill the requirements for battery applications.[3b]
[b] P. Schmitz, R. Jakelski, Dr. K. Jalkanen, Prof. M. Winter, Dr. P. Bieker
MEET Battery Research Center
University Mꢀnster
Corrensstrasse 46, 48149 Mꢀnster (Germany)
[c] Prof. M. Winter
Helmholtz Institute Mꢀnster, IEK-12,
Forschungszentrum Jꢀlich GmbH
Corrensstrasse 46, 48149 Mꢀnster (Germany)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/chem.201604461.
Chem. Eur. J. 2017, 23, 1 – 5
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[C2MIm]-based ILs. Thus the solvent-free synthesis route in
combination with the used purification method can be stated
as suitable for [C4MIm]BOB and [C4MIm]BF4 (yield=95%). For
these ILs the solvent-based reaction route also delivered
acceptable yields (ca. 80%). For [C2MIm]BOB, yields up to 82%
were achieved by the solvent-free method. In contrast, for
[C2MIm]BF4 significantly lower yields were obtained (yieldmelt
=
50%, yieldsuspension =70%), because there was a much higher
product loss during the purification process compared to the
[C4MIm] ILs. This indicated that silica was not an optimal
column material in particular for [C2MIm]BF4, because the less
voluminous IL strongly adhered to the column material and
could also not be extracted from the column with an excess
amount of solvent. The smaller the ions and the lower the
asymmetry of the cation, the higher the loss inside the column
material. The yields of the synthesized ILs decreased in the
following sequence: [C4MIm]BOB ([C4MIm]BOB>[C4MIm]BF4
>[C2MIm]BOB>[C2MIm]BF4).
The refractive indices were measured for all the other ILs
except for the [C2MIm]BOB, as it exists in a solid form at the
measurement temperature of 208C. It can be seen that the
synthesis route, as expected, had no influence on the refractive
À
index. The indices for the BF4 containing ILs were almost
equal: A longer alkyl side chain in [C4MIm]BF4 led to a slight
increase of the refractive index in comparison to [C2MIm]BF4.
The BOBÀ anion showed a higher refraction compared to the
À
BF4 anion ([C4MIm]BOB 1.475, [C4MIm]BF4 1.417).
Scheme 1. Two-step synthesis routes for ILs based on 1-methylimidazol.
The thermal behavior of the ILs was investigated by DSC.
For the chosen settings only the [C2MIm]BF4 showed a cold
crystallization (Tcc =À448C) and a melting point (Tm =158C)
along with the glass transition (Tg). No differences between the
synthetic approaches were observed either in the DSC meas-
Table 1 summarizes the results with regard to the yields,
refractive indices, glass transition temperatures (Tg), and flash-
points of the synthesized ILs.
À
Considering the results from ion chromatography (IC) and
Karl Fischer titration, purities higher than 99.9% were est
imated for all of the synthesized ILs. The detected impurity
contents were below 20 ppm for chloride, 2 ppm for lithium,
and 10 ppm for water. In particular, low contents of chloride
and water are important for the quality of the ILs, since these
impurities have been found to influence their physical proper-
ties.[15] These results show that, except for [C2MIm]BF4, all ILs
could be synthesized in higher yields by the solvent-free
approach. This trend was more pronounced for the [C4MIm]-
based ILs, as their yields were higher compared to the
urements. The BF4 containing ILs showed much lower glass
transition temperatures than the BOBÀ anion including ones.
Considering the difference in glass transition temperatures, it
can be concluded that an increasing alkyl chain length at the
À
imidazolium ring leads to an increase in the Tg of the BF4 ILs
and to a decrease in the Tg of the BOBÀ ILs. Considering the
glass transition temperatures of the synthesized ILs,
the observed temperatures (Table 1) fit well to the literature
values for [C2MIm]BF4 (À928C), [C4MIm]BF4 (À858C), and
[C4MIm]BOB (À298C).[3c,16] For [C2MIm]BOB no reference value
could be found.
Furthermore, the thermal stability was measured by TGA. No
influence of the synthesis route on the thermal stability was
observed. Figure 1 shows the TGA results for each IL.
These results display that [C2MIm]BF4 and [C4MIm]BF4 are
thermally stable up to more than 3508C, whereas [C2MIm]BOB
and [C4MIm]BOB decomposes below 3008C. Thus it can be
concluded that with the BOBÀ anion it is thermally less stable
than with the BF4À anion.
Table 1. Yields, refractive indices, glass transition temperatures, and flash-
points of the ILs synthesized by suspension or melt.
IL
Synthetic Yield
approach [%]
Refractive
index
Tg
[8C]
Flashpoint
[8C]
[C2MIm]BF4 suspension
[C2MIm]BF4 melt
[C2MIm]BOB suspension
[C2MIm]BOB melt
[C4MIm]BF4 suspension
[C4MIm]BF4 melt
[C4MIm]BOB suspension
[C4MIm]BOB melt
70
50
80
82
80
95
87
95
1.408
1.408
–
À93
À93
À23
À23
À85
À85
À30
À30
373
374
236
237
363
364
236
235
Finally, the flashpoints were determined according to CCCFP.
The flashpoints are in line with the TGA results: For the BOBÀ
containing ILs, flashpoints of about 2358C were observed,
–
1.417
1.417
1.475
1.475
À
whereas the BF4 ILs showed flashpoints higher than 3608C.
À
The BF4 ILs also showed a decreasing flashpoint with an
increasing alkyl chain length on the nitrogen substituent. This
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Communication
558C for two days. The water content of the final products was
below 10 ppm as determined by Karl Fischer titration. The chloride
content was below 20 ppm and the lithium content below 2 ppm,
as measured by ion chromatography (IC).
For more experimental details, see the Supporting Information.
Acknowledgements
For financial support of this work within the research project
“MEET Hi-EnD” (03X4634A) the authors wish to thank the Fed-
eral Ministry of Education and Research (BMBF).
Figure 1. TGA measurements of the synthesized ILs.
Keywords: borates
· flashpoints · ionic liquids · room-
temperature molten salts · synthetic methods
was to the best of our knowledge the first time the flashpoints
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refute the hypothesis that is often stated that ILs would be
nonflammable.[18] It can be noted that the synthesized ILs are
thermally stable and non-combustible at the operating
temperatures of room-temperature lithium batteries, but
elevated temperatures, which could be reached during a
battery thermal runaway, would lead to decomposition and
also
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Experimental Section
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Chem. Eur. J. 2017, 23, 1 – 5
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Manuscript received: September 21, 2016
Accepted Article published: January 9, 2017
Final Article published: && &&, 0000
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ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Communication
COMMUNICATION
IL? Better! The synthesis and purifica-
tion of imidazolium-based ionic liquids
(ILs) with boron-containing anions is
reported. It was possible to shorten the
reaction and purification times, to avoid
the use of acetonitrile as processing
solvent, and to increase the yield
compared to the known procedure for
bis(oxalato)borate-containing ILs.
Flashpoints of the ILs could be exactly
determined by the continuously
&
Ionic Liquids
P. Schmitz, R. Jakelski, K. Jalkanen,
M. Winter, P. Bieker*
&& – &&
Synthesis of High-Purity Imidazolium
Tetrafluoroborates and
Bis(oxalato)borates
closed-cup flashpoint method.
Chem. Eur. J. 2017, 23, 1 – 5
www.chemeurj.org
5
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ