Thermally stable bis(trifluoromethylsulfonyl)imide salts and their mixtures

Article abstract of DOI:10.1039/c6nj00579a

We show that both tetraphenylphosphonium bis(trifluoromethylsulfonyl)imide ([PPh₄][NTf₂]) and Cs[NTf₂] are low melting salts of exceptionally high and also very similar thermal stability. This similarity indicates that the thermal stability is dominated by the anion. Moreover, eutectic mixtures of [PPh₄][NTf₂] and Cs[NTf₂] with melting points below 100 °C are presented. Surface analysis of the latter in the liquid state reveals a surprising depletion of [PPh₄]⁺ ions from the surface.

Full text of DOI:10.1039/c6nj00579a

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Thermally stable bis(trifluoromethylsulfonyl)imide salts and their
mixtures
Marlene Scheuermeyer,a Matthias Kusche,a Friederike Agel,a Patrick Schreiber,b Florian Maier,b
Hans-Peter Steinrück,b Jim H. Davis, Jr.,c Florian Heym,d Andreas Jess,d and Peter Wasserscheida*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
We show that both tetraphenylphosphonium bis(trifluoromethylsulfony)imide ([PPh4][NTf2]) and Cs[NTf2] are low melting
salts of exceptionally high and also very similar thermal stability. This similarity indicates that the thermal stability is
dominated by the anion. Moreover, eutectic mixtures of [PPh4][NTf2] and Cs[NTf2] with melting points below 100 °C are
presented. Surface analysis of the latter in the liquid state reveals a surprising depletion of [PPh4]+ ions from the surface.
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very stable and weakly nucleophilic anions, such as e.g. the
[NTf2]- ion, pyrolysis and cracking reactions of the organic
cation have been claimed to constitute the thermal stability
limit.5 Therefore, some research efforts have been directed
towards low melting alkali [NTf2]- salts to overcome this issue
by using non-decomposable alkali ions in combination with
one of the most stable IL anions.8
Introduction
Due to their unique structural properties, ionic liquids (ILs)
have established in the recent years as highly promising class
of solvents, materials and performance chemicals.1 However,
for many prospective applications in which the extremely low
vapor pressure of ILs would be of great advantage, e.g. as heat
transfer media,2 in high temperature catalysis3 or in separation
processes,4 their limited long-term operational stability at
Note that the determination of thermal decomposition
limits of ILs is not trivial. Apart from the very strong
dependence of all stability data on the IL quality9 and some
reported dependency on the sample pans used,5 there is a
strong influence on the heating ramp in thermogravimetric
analysis (TGA) experiments as both, decomposition and
evaporation, are kinetic processes that proceed to a different
degree with time. For example, MacFarlane and coworkers
have shown, that the temperature at which 1% thermal
degradation of [EMIM][NTf2] occurs is 307°C if the ionic liquid
is exposed to thermal stress for 1h while it is only 251°C if the
ionic liquid is exposed for 10h.10
temperatures above 250 °C has been found
limitation.5
a serious
The upper operational temperature limit of IL utilization is
defined by both IL thermal decomposition and IL evaporation
processes. Concerning IL decomposition, different mechanisms
are discussed in the literature depending on the type of IL
under investigation: For protic ionic liquids, thermal
decomposition occurs through the protonation equilibrium
between the salt form and the corresponding acid/base pair.
The higher the pKa difference between acid and base the
higher is the thermal stability of the protic salt.6 For non-protic
ionic liquids, elimination of alkyl groups by breaking a carbon-
hetero atom bond is the first major decomposition process
during heating-up. In this process, the IL stability is a function
of the counter-ion nucleophilicity with stronger nucleophilic
anions leading to lower decomposition temperatures.7 For
In addition to thermal decomposition, mass loss by IL
evaporation in form of neutral ion pairs also occurs at elevated
temperatures and reduced pressures.11 This process has been
found to be of high practical relevance, e.g. for the controlled
deposition of ultrathin IL films.12,13 It may, however, also limit
technical operation with ionic liquids in open systems at high
temperatures. Recently, it has been shown that special TG
experiments allow discriminating between thermal
decomposition and evaporation by applying a combination of
non-isothermal ambient pressure measurements with an
overflow of inert gas with isothermal experiments conducted
at high vacuum using a magnetic suspension balance.14
In this contribution we present a comparison of the long-
term thermal behavior of the two [NTf2]- salts, Cs[NTf2] and
[PPh4][NTf2]. Both salts are in principal known from the
literature: The solid state structure of Cs[NTf2] has been
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characterized in detail by Xue et al.,15 the melting point of thermal decomposition for
a component i during such non-
Cs[NTf2] as well as its binary8 and even ternary mixtures16 with isothermal TG-experiments is expressed by:
other alkali [NTf2]- salts have also been studied. As the lowest
1
ꢃꢀ
1
ꢃꢀ
1 ꢃꢀ
melting point of such ternary eutectics, a mixture of Cs[NTf2],
Li[NTf2], Na[NTf2] in the molar ratio of 0.9:0.05:0.05 has been
identified. The latter melts at 115 °C, as compared to 122 °C
for pure Cs[NTf2], reported in the same study.8 Hagiwara et al.
also give the thermal decomposition temperature of Cs[NTf2]
as 472 °C, which has been found to be the highest among the
alkali bis(trifluoromethylsulfonyl)imide salts.8
−
ꢂꢅ
=
=
−
ꢂꢅ
−
ꢂꢅ
ꢀꢁ,ꢂ ꢃꢄ
ꢀꢁ,ꢂ ꢃꢄ
ꢀꢁ,ꢂ ꢃꢄ
ꢌꢍꢎꢇꢏꢋꢇꢐꢂꢆꢂꢇꢑ
ꢆꢇꢆꢈꢉ
ꢊꢈꢋ
ꢗℎꢌꢘꢂ,ꢙ
ꢒꢂꢓꢔ
ꢀꢂ
ꢚꢊꢈꢋ,ꢂ + ꢛꢌꢍꢎꢇꢏꢋꢇꢐꢂꢆꢂꢇꢑ,ꢂ
.
ꢕꢖ ꢃꢔꢀꢁ,ꢂ
ꢀꢁ,ꢂ
(1)
M stands for the molar mass of the sample; AC for the crucible’s
surface; R the molar gas constant; Di,g for the binary diffusion
coefficient of the sample in the gas phase (N2 and He; in this work
the well-known correlation of Fuller, Schettler and Giddings was
used with a maximum average error of about 15% 19); dC for the
diameter of the crucible and T the sample temperature. The
Sherwood number Shd for a crucible (hC crucible’s height; hF
crucible’s actual filling height) in transversal gas flow is given by:
[PPh4][NTf2] has been reported by one of us in 2013 as
particularly thermally stable organic [NTf2]- salt together with a
number of other new phosphonium imide salts:17 The melting
point of [PPh4][NTf2] was reported to be 135 °C, and the
thermal stability was studied by an isothermal stability test at
350 °C for 96 h in new porcelain crucibles, with 2.3 % mass loss
found over this period. Undoubtedly, this result already gave a
strong hint to an extraordinary thermal robustness of
[PPh4][NTf2].
ꢣꢤ
ℎꢔ − ℎꢝ
ꢃꢔ
1
(2)
ꢗℎꢌ = ꢜ
+
ꢢ
ꢡ
√
For this work, we were interested in a direct comparison of
the thermal stability of Cs[NTf2] and [PPh4][NTf2] under strictly
comparable experimental conditions. With this comparative
study we aim to prove that some IL cations are thermally more
stable than the [NTf2]- anion, thus a shift in attention on the
anion is required in the development of thermally more stable
ionic liquids. Moreover, we present data on mutual solubility
and eutectic behaviour of mixtures of Cs[NTf2] and
[PPh4][NTf2] with the intention to qualify such mixtures as
suitable liquid materials for high temperature applications, e.g.
as heat transfer fluids with very low vapour pressures.
2.9 + 0.58 ꢕꢟ ꢗꢠ
ꢞ
ꢌ
with the Reynolds number
ꢥꢃꢔ
ꢕꢟꢌ
=
(3)
ꢦꢙ
using the superficial velocity u and the kinematic viscosity νg of the
carrier gas g. The Schmidt number Sc is defined as the ratio of the
kinematic viscosity νg to the binary diffusion coefficient Di,g. The
vapor pressure pvap was calculated by a simplified Antoine equation
assuming a constant (temperature independent) entropy term Cvap
and a constant enthalpy of vaporization ΔvapH:
Results and discussion
To start with, we determined viscosity and density data of the
two pure salts at 200 °C (Table 1).
ꢚꢊꢈꢋ
ꢚꢧꢍꢨ
Δꢊꢈꢋ
ꢕꢖ
ꢬ
= ꢩꢊꢈꢋ ∙ ꢟꢪꢚ ꢫ−
ꢭ with ꢚꢧꢍꢨ = 1 ꢮꢯ.
(4)
Table 1: Relevant physico-chemical properties of Cs[NTf2] and
[PPh4][NTf2] salts.
The rate of the thermal decomposition was calculated assuming a
first order reaction with the rate constant kdecomposition and the
formal kinetic parameters k0 and EA:
Property
[PPh4][NTf2]
Cs [NTf2]
125 (122)2
2.2
melting point (DSC) / °C
134 (135)1
Eꢰ
(5)
ꢛꢌꢍꢎꢇꢏꢋꢇꢐꢂꢆꢂꢇꢑ = ꢛꢁ ∙ ꢟꢪꢚ ꢫ− ꢭ .
ꢕꢖ
density (200 °C) / g cm-3
1.2
6
It should be emphasized that the formal kinetic estimates represent
kinetic parameters for a global model in order to describe only the
thermal decomposition at low conversion and not the complex
chemical reactions in the course of decomposition. Eq. 1 was solved
numerically using the software Berkeley Madonna (Robert Macey;
University of California). To get a first impression on the thermal
stability of both salts in direct comparison, Figure 1 shows TG
experiments of Cs[NTf2] and [PPh4][NTf2], along with the well-
established ionic liquid [EMIM][NTf2]. All three salts were first kept
in the TG set-up at 120 °C for one hour to allow traces of humidity
to be released. Then, the samples were heated with a defined
heating ramp (2 K min-1) under a constant nitrogen flow.
viscosity (200 °C) / mPa s
1 value in brackets is from reference17
14
;
2 value from reference8.
Furthermore, non-isothermal TG-experiments at different heating
rates (0.5, 2, 10 K min-1) were conducted. In order to discriminate
between thermal decomposition and evaporation during these
experiments, the latter were carried out in helium and nitrogen
flow (100 ml min-1 NTP). This method and the subsequent data
evaluation was already published by some of us recently.14,18 The
normalized rate of mass loss (derivation of mass signal m; DTG-
signal; normalized to initial mass m0) due to evaporation and/or
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1,0
0,9
0,8
0,7
0,6
0,5
0,4
0,3
135
130
125
120
115
110
105
liquid
liq.-[PPh4][NTf2]
liq.-Cs[NTf2]
[PPh4][NTf2]
0,2
0,1
0,0
100
95
Cs[NTf2]
[EMIM][NTf2]
Cs[NTf2] + [PPh4][NTf2]
90
150
200
250
300
350
400
450
500
550
600
Temperature / °C
0,0
Cs[NTf2]
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1,0
[PPh4][NTf2]
Χ ([PPh ][NTf ])
4
2
Figure 1: Comparative TG experiments of [PPh4][NTf2], Cs[NTf2] and
[EMIM][NTf2] in N2 flow (100 mL min-1), drying at 120 °C for 1 hour,
heating rate 2 K min-1.
Figure 2: Phase diagram of [PPh4][NTf2] and Cs[NTf2].
The behaviour of this eutectic mixture under thermal stress
has been determined by the same TGA-method already used
for the pure salts. Figure 3 shows the measured rate of mass
loss (normalized to initial mass; see refs.14,18
Both, Cs[NTf2] and [PPh4][NTf2], are thermally significantly more
stable than [EMIM][NTf2]. Moreover, Cs[NTf2] and [PPh4][NTf2]
show the same on-set of strong mass loss around 420 °C indicating
that the decomposition mechanism must be very similar for both
salts. We draw from this result the important conclusion that it is
not the phosphonium cation of [PPh4][NTf2] that limits thermal
stability but the imide anion. The results of our more detailed
kinetic evaluation of Cs[NTf2] and [PPh4][NTf2] decomposition are
given in Table 2. The agreement of the results of [PPh4][NTf2]
(Tmax,2% in 96h = 348 °C) with the isothermal measurements from the
literature17 is excellent. The vapor pressure of these two salts is by a
factor of 4 and 18, respectively, lower than the imidazolium based
IL [EMIM][NTf2] (pvap(350 °C) ≈ 30 Pa). Nevertheless, during TG-
experiments at low heating rates the evaporation is the major mass
loss.
) and the
calculated contributions of evaporation and thermal
decomposition. Interestingly, the obtained overall rate of mass
loss can be calculated over a wide range (until 60% mass loss)
applying Raoult’s law to an ideal mixture composed of
independent ILs; i.e. the normalized mass loss (eq.(1)) and the
effective available surface area AC in eq. (1) was calculated
iteratively for each IL after each time step separately. Figure 3
shows the results of the simulations. It can be seen that the
thermal stability of the mixture is determined by the thermal
stability of [PPh4][NTf2], and the overall mass loss at the
beginning is dominated by the evaporation of Cs[NTf2].
Table 2: Parameters of the vapor pressure (eqn. (1)), the kinetics of
thermal decomposition (eqn. (2)) and the corresponding temperature
range for [PPh4][NTf2] and Cs[NTf2].
[PPh4][NTf2]
1.2·1020
325
CsNTf2]
8.6·1019
325
[EMIM][NTf2]
1.2·1020
317
k0 / s-1
EA / kJ mol-1
Cvap / -
3.2·1011
135
2.2·1011
125
3.5·1011
120
ΔvapH / kJ mol-1
pvap (T=350°C) / Pa
Tmax, 1%/a / °C 1
1.6
7.2
30.5
301
304
286
1 1% mass loss due to thermal decomposition per year
Next, we studied the melting points of Cs[NTf2]-[PPh4][NTf2]
mixtures with the intention to combine extremely high thermal
stability with lower melting points. Figure 2 shows the obtained
phase diagram. From these experiments we conclude that Cs[NTf2]
and [PPh4][NTf2] form homogeneous liquid mixtures above the
melting point of each compound. Moreover, the mixture is
characterized by an eutectic composition [PPh4][NTf2] : Cs[NTf2] =
1:2.13 (32 mol-% [PPh4][NTf2]), with a melting point of 98.6 °C. The
viscosity of this mixture at 200 °C has been determined to be
9 mPa s.
Figure 3: TG-experiments (N2; 6 l h-1; heating ramp = 2 K min-1) with the
eutectic mixture of the ionic liquids Cs[NTf2] and [PPh4][NTf2] under
ambient pressure - solid line; dashed and dotted lines are the
corresponding contributions of evaporation and thermal
decomposition of the individual components.
In the context of applications where a thin salt film coats a
high surface-area support (e.g. SILP catalysis), the outermost
surface is the dominating interface for substrates entering the
reaction medium and for products leaving the reaction system.
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In order to elucidate the surface composition of our salt brings the melting point below 100 °C. Interestingly, the
mixtures, we performed angle-resolved X-ray photoelectron surface of the eutectic mixture in the liquid state favour the
spectroscopy (ARXPS) under ultra-high vacuum conditions, presence of Cs+ ions on expense of the bulky [PPh4]+ ions as
similar to earlier measurements for acetate salt mixtures.20 shown by ARXPS.
Figure 4 shows measurements with 25 mol% [PPh4][NTf2] bulk
content (according to XPS) at 130 °C, that is, in the liquid
Experimental
phase. The quantitative analysis of the bulk sensitive 0° spectra
shows no contaminations within the XPS probing depth. The
comparison of the spectra in the Cs 3d and C 1s regions at 0°
and 80° allows for investigation of selective enrichment/
depletion effects.
Characterisation. The purity and identity of the compounds were
determined by NMR, elemental analysis and DART-MS (Direct
Analysis in Real Time Mass Spectrometry). The 1H and 13C NMR
spectra were recorded on a Jeol ECX-400. The chemical shifts are in
ppm relative to aceton-d6 (1H: 2.05 ppm; 13C: 29.84 ppm, 206.26
ppm). The determination of the carbon, hydrogen, nitrogen and
sulfur content was performed on a Carlo Erba EA 1106, 1107 and
1108 instrument at the Analytical Laboratories of the Chair of
Inorganic and General Chemistry of the Friedrich-Alexander
Universität Erlangen-Nürnberg (Erlangen, Germany). The DART-MS
analysis was performed on a JMS-T100LP AccuTOF LC-plus 4G from
Jeol with a He plasma ionization from ionSense, operating in both
positive and negative-ion mode. The m/z values refer to the most
abundant isotope.
10
3
2
1
C 1s
Cs 3d5/2
0° (bulk sensitive)
80° (surface sensitive)
[PPh4]+ (-C6H5)
[Cs]+
5
[Ntf2]- (-CF3)
0
0
Synthesis. All starting materials were used as received, without
further treatment. The [PPh4][NTf2] salt was synthesized by an ion
exchange of 10.17 g (35.41 mmol) Li[NTf2] (Iolitec) and 13.27 g
(35.41 mmol) [PPh4]Cl (Sigma Aldrich) in water. The precipitated
product was filtrated and washed thoroughly with water to remove
chloride impurities, the filtrate was tested with AgNO3. The salt was
dried in vacuum. 1H NMR (δ, 293 K, aceton-d6): 7.78 ppm (16 H, m),
7.95 ppm (4H, t) 13C NMR (δ, 293 K, aceton-d6): 117.81 ppm (1 C),
118.70 ppm (1 C), 130.50 ppm (8 C, d), 134.91 ppm (8 C, d), 135.54
ppm (4 C, d); Elemental Analysis: calculated: %C: 50.41; %H: 3.25;
%N: 2.26; %S: 10.35; found: %C: 50.67; %H: 3.00; %N: 2.23; %S:
10,26; DART-MS (ESI+): m/z = 339.09 for [PPh4]+; (ESI-): m/z = 278.92
for [NTf2]-.
730
725
720
295
290
285
280
Binding energy / eV
Figure 4: ARXP Cs 3d and C 1s spectra of a 25 : 75 mol% mixture of
[PPh4][NTf2] : Cs[NTf2] at 130 °C.
A pronounced surface depletion of the phosphonium cation is
deduced from the 80° spectra, where more than ~80% of the
detected signal originates solely from the topmost layer:
Whereas the Cs cation signal and the anion’s CF3 signal at
293 eV do not exhibit any changes with emission angle, the
phosphonium C 1s signal at 285 eV decreases at 80°,
corresponding to a [PPh4][NTf2] surface content of only
20 mol% at 130 °C. This surface depletion of the bulky organic
cation is surprising: in alkali-acetate mixtures, the larger
cations were found to be surface enriched.20
For the synthesis of Cs[NTf2] the acid H[NTf2] was obtained by
sublimation from a reaction mixture of 10.23 g (35.63 mmol)
Li[NTf2] and 10 equivalents of 98 % H2SO4 (Sigma Aldrich) at 80 °C
and reduced pressure (0.5 mbar). Subsequently, H[NTf2] was
reacted with 5.79 g (17.82 mmol) Cs2CO3 (Sigma Aldrich) in 10 mL
methanol (Sigma Aldrich) at 50 °C. The solvent was removed at a
rotary evaporator and the raw material was recrystallized from
acetonitrile (Sigma Aldrich) and dried in vacuum. Elemental
Analysis: calculated: %C: 5.82; %H: 0.00; %N: 3.39; %S: 15.52;
found: %C: 5.77; %H: 0.00; %N: 3.50; %S: 15.63. DART-MS (ESI+):
m/z = 132.89 for Cs+, m/z = 545.67 for Cs2[NTf2]+; (ESI-): m/z =
148.01 for [NTf]-, m/z = 279.93 for [NTf2]-; m/z = 692.70 for
Conclusions
In conclusion, we could demonstrate that the thermal stability
of [PPh4][NTf2] is not limited by its cation but by the imide
anion. This finding confirms the exceptional thermal stability of
the [PPh4]+ ion and may guide the way to the synthesis of even
thermally more stable organic salts. In addition, it has been
found that the relatively high melting points of both Cs[NTf2]
and [PPh4][NTf2] can – at least partly – be circumvented by
applying binary mixtures of these two compounds. The fact
that inorganic and bulky organic imide salts show excellent
miscibility and eutectic behaviour is surprising and may open
new routes to salt mixtures of new properties. In the special
case of binary Cs[NTf2]-[PPh4][NTf2] mixtures this approach
lowers the melting point of [PPh4][NTf2] by around 40 °C and
-
Cs[NTf2]2 .
The binary mixtures of the salts were obtained by mixing the right
amounts and melting.
Viscosity measurements. The viscosity measurements were carried
out with a rotary viscosimeter DV-II+ Pro Extra (Brookfield) with a
home-made heating device using standard spindles in
temperature range from 120 to 250 °C.
a
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4
5
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a dynamic differential
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Chem. Phys., 2010, 12, 12089.
19 E.N. Fuller, P.D. Schettler and J.C. Giddings, Ind. Eng. Chem.,
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20 A. Deyko, S. Bajus, F. Rietzler, A. Bösmann, P. Wasserscheid,
TG-experiments. The thermo-gravimetric analysis at different
heating rates (0.5, 2, 10 K min-1) were conducted in a
horizontal thermobalance EXSTAR 6300 TG/DTA from Hitachi
High-Tech using quartz glass crucibles. The TG-experiments
were conducted with N2 (99.999 % purity) or He (99.996 %
purity) as carrier gas with a gas flow rate of 6 l h-1 NTP. The
temperature uncertainty in the TG-experiments was ± 0.1 K.
The standard uncertainty of the kinetic parameters was about
± 15 %.
ARXPS investigations. The surface studies were performed
using our new and unique laboratory electron spectrometer
DASSA (“Dual Analyser System for Surface Analysis”),
dedicated for investigations of liquid samples.21 It comprises
an ultrahigh vacuum chamber equipped with two electron
analysers (ARGUS, Scienta-Omicron), for simultaneous
measurements at 0° and 80°, relative to the surface normal of
a horizontally mounted liquid sample; measurements at 0° are
considered as bulk sensitive (information depth 7-9 nm), and
measurements at 80° as surface sensitive (information depth
1-1.5 nm). By employing monochromatic Al Kα radiation
(1486.6 eV), a combined energy resolution of 0.4 eV is
H.-P. Steinrück and F. Maier, J. Phys. Chem. C, 2013, 117
22939.
,
achieved. The sample temperature is controlled within ± 1°C,
which allows for following changes in bulk and surface
composition as a function of temperature.
21 I. Niedermaier, C. Kolbeck, H.-P. Steinrück and F. Maier, Rev.
Sci. Instr., 2016, accepted.
Acknowledgements
MS, MK, FA and PW thank the European Research Council
(ERC) for financial support (Advanced Investigator Grant No.
267376). PS, FM, HS thank the German Science Foundation
DFG for support within its Excellence Cluster for “Engineering
of Advanced Materials”. JHD thanks the National Science
Foundation (grant CHE-1464740), and the Petroleum Research
Fund, administered by the American Chemical Society, for
support of his work.
Notes and references
1
2
R. Hayes, G. G. Warr, and R. Atkin, Chem. Rev., 2015, 115,
6357.
E. B. Fox, A. E. Visser, N. J. Bridges and J. W. Amoroso, Energy
Fuels, 2013, 3385.
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DOI: 10.1039/C6NJ00579A
For the Table of Contents
Thermally stable bis(trifluoromethylsulfonyl)imide salts and their mixtures
Marlene Scheuermeyer, Matthias Kusche, Friederike Agel, Patrick Schreiber, Florian Maier, Hans-
Peter Steinrück, Jim H. Davis, Jr., Florian Heym, Andreas Jess and Peter Wasserscheid
The investigated salts form an eutectic mixture which melts below 100 °C and surface analysis show a
depletion of phosphonium cations.