2
A.S. Shalygin et al. / Journal of Molecular Liquids 315 (2020) 113694
pioneering work [25] in situ ATR-FTIR spectroscopy was used to an-
alyse the interaction of CO and ILs (С MIMPF and С MIMBF ) at
8 bar and 40 °C in the IL phase. These results indicated that there
is a weak Lewis acid−base interaction between CO and the anion,
where the anion serves as a Lewis base and CO acts as a Lewis
acid. This study was done almost immediately after the publication
of [26], where high solubility of CO in ILs was first reported. How-
ever, as the presence of water in IL has a significant effect of the sol-
ubility of CO , the advantage of ATR-FTIR spectroscopic technique
25] is the ability to detect and monitor the presence and removal
of water in situ. Furthermore, ATR-FTIR spectroscopy was applied
to demonstrate that under high pressures of CO , the melting tem-
research. FTIR spectroscopy was used to study the vibrational structure
and interionic interactions of individual homologous series of ILs: 1-
2
4
6
4
4
6
alkyl-3-methylimidazolium trifluoromethanesulfonate (C
[40], alkyl substituted imidazolium bis(trifluoromethanolsulfonyl)
imide (HC IMTf N (n = 0–12)) [41], and mixture of 1-alkyl-3-
methylimidazolium cations and water [42], C MIMTf N and methanol
[43], C MIMTf N and dimethyl sulfoxide [44], C MIMBF and
benzonitrile [45].
In this work, in situ ATR-FTIR spectroscopy was used to study the in-
teraction of high-pressure CO and the homological series of С MIMBF
(n = 2, 4, 6, 8, 10) ILs. In particular, С MIMBF ILs have been the most
n
MIMTfO)
2
2
n
2
n
2
2
n
2
n
4
2
[
2
n
4
n
4
2
comprehensively studied scientifically, commercially available and in
perature of ILs decreases [27]. In situ FTIR spectroscopy in both ATR
and transmission modes has been utilized to investigate the
demand [5]. A detailed analysis of the vibrational spectra acquired
from ILs and sorbed СО
the alkyl chain lengths and subjected to changing pressures of СО
temperature. For the first time, ATR-FTIR spectroscopy was used to de-
termine the thermodynamic parameters of CO sorption in ILs. A corre-
lation was found between the enthalpy of sorption of CO and the
wavenumber of the ν band.
2
was carried out on ionic liquids with different
partitioning of a model solute between С
pressure CO [28]. ATR-FTIR spectroscopy has been successfully ap-
plied to study the solubility of supercritical (SC) CO in an ionic liq-
uid, IL swelling under scCO , CO diffusion and molecular
interaction between IL and scCO [29], similar studies have been per-
formed for 1,3-dimethylimidazolium dimethylphosphate
4
MIMPF
6
and high-
2
and
2
2
2
2
2
2
2
3
(
(
(
C
C
C
1
2
2
MIMDMP), 1-ethyl-3-methylimidazolium dimethylphosphate
MIMDMP) and 1-ethyl-3-methylimidazolium diethylphosphate
MIMDEP) ILs [30]. Andanson et al. also used ATR-FTIR spectros-
2
. Experimental
2.1. Synthesis of ionic liquids
copy to study a triad system of IL – water – CO
It has been shown that, despite the limited solubility of water in su-
percritical CO , the IL can be effectively dried by extraction with
scCO even in the case of a hydrophilic ionic liquid. Andanson et al.
demonstrated the application of the ATR-FTIR spectroscopy to con-
trol the water and other impurity content in IL during the purifica-
2
[31] at high pressure.
n 4
The homologous series of С MIMBF (n = 2, 4, 6, 8, 10) ILs synthe-
2
sized using chloride-containing ionic liquids. Alkyl chlorides, 1-methyl
imidazole and C MIMCl were purchased from Acros Organics and used
without further purification. The ILs of C MIMCl, C MIMCl, C MIMCl
and C10MIMCl were synthesised using the methods reported in the lit-
erature method [46] from 1-methyl imidazole and corresponding alkyl
chloride.
A 100 ml round-bottomed flask fitted with magnetic stirrer bar and
reflux condenser was charged by 26.5 mmol of corresponding chloride
IL, 5.82 g (53 mmol) of NaBF
stirred for 3 h at 50 °C. The solid was filtered and washed with 10 ml
of acetone. The second portion of NaBF (2.20 g, 20 mmol) was added
to the filtrate and the resulting mixture was stirred for 3 h at 50 °C.
The solid was filtered and washed with 10 ml of acetone. The filtrate
was evaporated under vacuum. The residue was dissolved in 30 ml of
dry dichloromethane (to remove NaCl and excess of starting salt) and
filtered. The filtrate was evaporated and product was dried under high
vacuum (10 bar) at 80 °C for 6 h. The water content in ionic liquids
was determined by coulometric titration by Fischer method using an
automatic titrator DL39KF, Mettler Toledo GmbH and amounted to
less than 0.2 mg/g of ionic liquid (mainly 0.01–0.18 mg/g for various
2
2
4
6
8
tion of IL by supercritical CO
the molecular interactions of dissolved CO
MIMPF 1-ethyl-3-methylimidazolium
trifluoromethanolsulfonyl)imide (C MIMTf N), and also for 1-n-
butylpyridinium tetrafluoroborate (C PYBF ) under scCO condi-
2
[32]. Baiker et al. [33] investigated
with ILs: C MIMBF
bis
2
4
4
,
C
(
4
6
,
2
2
4
4
2
4
and 80 ml of acetone. The mixture was
tions (50 °C, 12.0 MPa). It was found that the extent of
imidazolium-ring C\\H stretching band shifts and that of the anion
species B\\F, P\\F, C\\F, and S_O stretching band shifts after the
2 2
dissolution of CO . The formation of new anion species [X-CO ]
4
−
(
X = anion species of IL) which are more basic than X- has been pro-
posed. The combined use of Raman spectroscopy and DFT calcula-
tions to study CO sorption in 1-butyl-3-methyl-imidazolium-
trifluoro acetate IL showed that CO interacts with anions via acid-
base interaction, and another part of CO is adsorbed in the voids
existing among the ion pairs of IL [34]. It studies where CO asym-
2
2
−3
2
2
ν
3
metric stretching mode is established as a vibrational chromophore
for ultrafast two-dimensional infrared (2D-IR) spectroscopic studies
of local structure and dynamics in ILs, which are of interest for car-
bon capture applications have been reported in the literature [35].
At present, the use of in situ vibrational methods to study the inter-
IL). Yield of products are 75–85%. The reaction of sample of IL with
AgNO
IL media.
−
3
was negative, that demonstrates the absence of Cl ions in the
actions of CO
above.
2
and ILs is largely limited to key articles described
2.2. In situ ATR-FTIR spectroscopy at high-pressure
Vibrational spectroscopy (e.g. FTIR, Raman) has been demonstrated
as a powerful approach to determine the thermodynamic properties of
sorption processes in polymers and this has been discussed in a very
comprehensive recent review [36]. In addition to studying molecular in-
teractions, the in situ ATR-FTIR spectroscopic approach can be applied to
ATR-FTIR spectra were collected using a Bruker Vertex 70V FTIR
spectrometer with a mercury cadmium telluride single element detec-
tor. ATR-FTIR spectra were acquired using diamond ATR accessory
(Golden Gate, Specac) at a CO
2
pressure of 20, 30 and 40 atm and in
determine the thermodynamic parameters of the interaction of CO
with polymers [37,38]. The linear dependencies of CO absorbance of
the ν + 2ν + ν band as a function of temperature for triblock copol-
2
the temperature range from 25 to 65 °C accurate to (± 1 °C). A spectral
−1
2
resolution of 1 cm and 128 co-added scans were used for all measure-
ments of the ionic liquid samples throughout this investigation. The
high-pressure cell was positioned around the diamond ATR-crystal
upon which the ionic liquid was placed and sealed using a Teflon O-
ring. The cell was held in place by the clamp attached to the Golden
Gate accessory. The temperature of the system was varied using the
temperature controlled top-plate attachment to the ATR accessory.
1
2
3
ymer L121 were shown [39].
Despite the fact that ILs based on the 1-alkyl-3-methylimidazolium
cation have been discovered for a long time, their commercial availabil-
ity, easy modification of the cation structure and replacement of anions
makes these ILs a popular subject of various studies, including for poten-
tial CO
2
capture. To more fully understand the properties of ILs based on
High-pressure CO
2
was introduced and regulated using a syringe
their alkyl chain length, homologous series of ILs are often used for
pump (Teledyne ISCO D260) with pressure accuracy 0.5% full scale.