Physicochemical characterization of MFm--based ammonium ionic liquids

Article abstract of DOI:10.1021/je300993n

A series of ammonium-based ionic liquids (ILs), which share a homologous series of cations (CH₃CH₂)₃N⁺(C _nH_2n+1) with n = 2, 4, 6, 8 and the anions with either BF₄^-, PF₆^-, or SbF₆ ^-, was synthesized. Their structures were confirmed by ¹H and ¹³C NMR, ESI-MS, and elemental analysis. Meanwhile, the content of impurity (e.g., water and bromide ions) was also determined using Karl Fischer titrator and ion chromatography. The thermal properties of the ILs were determined by TGA and DSC. Five of the investigated ILs have been shown to have a low melting point (< 100 C): N,N,N,N-tetraethylammonium tetrafluoroborate, [N₂₂₂₂]BF₄, N,N,N,N-tetraethylammonium hexafluorophosphate, [N₂₂₂₂]PF₆, N,N,N- triethylhexylammonium tetrafluoroborate, [N₂₂₂₆]BF₄, N,N,N-triethyloctylammonium hexafluorophosphate, [N₂₂₂₈]PF ₆ and N,N,N-triethyloctylammonium hexafluoroantimonate, [N ₂₂₂₈]SbF₆. Densities, refractive indices, and miscibility of these 12 ILs were well studied systematically. Moreover, from the analysis of the structure-property relationship, the role of the alkyl chain length of the cation on these physical properties of the ILs has been assessed, and the influence of the nature of the anions on these experimental data of the ILs has been discussed. The studies may provide valuable contributions for the design and study of ILs.

Full text of DOI:10.1021/je300993n

Article
pubs.acs.org/jced
−
Physicochemical Characterization of MF ‑Based Ammonium Ionic
m
Liquids
†
,‡
‡
,†
,‡
Haifang Li, Guoying Zhao, Fangfang Liu,* and Suojiang Zhang*
†
College of Chemical and Pharmacetical Engineering, Hebei University of Science and Technology, Shijiazhuang, 050018, China
Beijing Key Laboratory of Ionic Liquids Clean Process, State Key Laboratory of Multiphase Complex Systems, Institute of Process
‡
Engineering, Chinese Academy of Sciences, Beijing, 100190, China
*
S Supporting Information
ABSTRACT: A series of ammonium-based ionic liquids
(
(
ILs), which share a homologous series of cations
+
CH CH ) N (C H ) with n = 2, 4, 6, 8 and the anions
3
2 3
n
2n+1
−
−
−
with either BF , PF , or SbF , was synthesized. Their
structures were confirmed by H and C NMR, ESI-MS, and
4
6
6
1
13
elemental analysis. Meanwhile, the content of impurity (e.g.,
water and bromide ions) was also determined using Karl
Fischer titrator and ion chromatography. The thermal
properties of the ILs were determined by TGA and DSC.
Five of the investigated ILs have been shown to have a low melting point (< 100 °C): N,N,N,N-tetraethylammonium
tetrafluoroborate, [N2222]BF , N,N,N,N-tetraethylammonium hexafluorophosphate, [N ]PF , N,N,N-triethylhexylammonium
4
2222
6
tetrafluoroborate, [N2226]BF , N,N,N-triethyloctylammonium hexafluorophosphate, [N ]PF and N,N,N-triethyloctylammo-
4
2228
6
nium hexafluoroantimonate, [N2228]SbF . Densities, refractive indices, and miscibility of these 12 ILs were well studied
6
systematically. Moreover, from the analysis of the structure−property relationship, the role of the alkyl chain length of the cation
on these physical properties of the ILs has been assessed, and the influence of the nature of the anions on these experimental data
of the ILs has been discussed. The studies may provide valuable contributions for the design and study of ILs.
−
INTRODUCTION
BF , have been supported as adequate electrolytes for high-
4
■
energy devices owing to their sufficient electrochemical stability
compared with the imidazolium-based ILs. Recently, a significant
number of articles have been published on other topics about
ammonium ILs, such as CO capture, surfactants, lubricant,
acid catalysts, and solvent.
in which both the cation and the anion of ammonium ILs have
been systematically varied.
points of veiw, reliable and accurate data of ILs are the key for the
development of certain and economical process designs.
In this study, a variety of ammonium ILs which share a
In the past few years, a great effort has been devoted to the study
of ionic liquids (ILs) due to their peculiar chemical−physical
properties such as negligible vapor pressure, high ion
1
8
19
20
1
−3
2
conductivity, and high thermal stability. The most important
driving force of the booming research on ILs, typically including
those based on imidazolium, pyrrolidinium, ammonium,
phosphonium, guanidinium, and pyridinium, is their potential
in substituting for volatile organic solvents in chemical
2
1,22
However, there is little knowledge
2
3,24
From fundamental and industrial
25
4
,5
synthesis.
Recently, ILs integrated with functionalities for specific
+
homologous series of cations (CH CH ) N (C H ) with n =
2, 4, 6, 8 and the anions with either BF , PF , or SbF (Figure
) were synthesized and characterized. The physicochemical
properties including decomposition temperature (T ), phase
3
2 3
n
2n+1
purposes have been reviewed as promoters in various chemical
−
−
−
6
−14
4
6
6
reactions.
Notwithstanding having many successful efforts
1
about ILs as reaction media, catalysts, and promoters, they
currently have not been widely applied in any applications.
1
3,15
d
transition temperatures (T , and T ), density (ρ), and solubility
m
s‑s
To reach a better understanding of how to tune the properties of
ILs to meet industry requirements, knowledge about the origins
of these fundamental properties of ILs have begun to be reported
among researchers. But the investigated ILs were majorly focused
on the imidazolium cation. Among the imidazolium and
quaternary ammonium ILs, the basic difference in the properties
is that the former has a labile proton which limits its use in
organic reactions compounds. In 1997, MacFarlane began to
prepare a series of quaternary ammonium ILs (more than 16)
with NTf2− anion and then applied them in the field of
electrochemistry. Since then, the ILs based on the quaternary
in different solvents have been investigated, focusing on the
properties that depend on the change in the alkyl chain length of
cation and the anion nature, to know the relationships between
structure and property. Nowadays, many organic reactions are
performed in the presence of liquid acid coupled with ILs as
26
catalyst. So the refractive indexes (n ) of binary mixtures of the
presented ILs with trifluoromethanesulfonic acid at different
D
16
17
Received: September 12, 2012
Accepted: May 16, 2013
−
−
ammonium and robust anions, such as NTf , C(CN) , and
2
3
©
XXXX American Chemical Society
A
dx.doi.org/10.1021/je300993n | J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data
Article
+
Figure 1. General structure of ammonium ILs ([(CH CH ) N (C H )][MF ]); n is the number of carbon atoms in the side alky chain, m is the
3
2
3
n
2n+1
m
number of fluorine atoms of anion; M is the metal or nonmetal coordinated with fluorine.
Table 1. The Nomenclature, Acronyms, and Chemical Structure of the ILs Used in This Study
a
Representation of ionic liquids obtained from market.
mass fraction (w, w = mass of ionic liquid/(mass of ionic liquid +
mass of acid)) were measured and intended to obtain some
information about interaction behavior.
%), N-octyl bromide (≥ 99.0 %) and triethylamine (≥ 99.0 %)
were purchased from Sinopharm Chemical Reagent Co., Ltd.,
China. Ammonium fluoroborate (NH BF , ≥ 98.5 %),
4
4
potassium hexafluorophosphate (KPF , ≥ 98 %), and sodium
6
EXPERIMENTAL SECTION
hexafluoroantimonate (NaSbF , ≥ 98 %) were received from Alfa
■
6
Aesar. N,N,N,N-Tetraethylammonium bromide and N,N,N,N-
tetraethylammonium tetrafluoroborate with 99 % purity were
obtained from Shanghai Chengjie Chemical Co., Ltd., China and
Material and Apparatus. All the chemicals and solvents
mentioned in this study were used as received unless otherwise
noted. N-Butyl bromide (≥ 98.0 %), N-hexyl bromide (≥ 98.0
B
dx.doi.org/10.1021/je300993n | J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data
Article
dried under high vacuum for 24 h prior to use. Trifluorometha-
nesulfonic acid (TFSA, > 99.9 % in purity) was used as received
from 718th Research Institute of China Ship Building Industry
Corporation.
N,N,N,N-Tetraethylammonium Hexafluoroantimonate
1
[N2222]SbF . The yield was 70.2 %, white powder. H NMR (600
6
Hz, DMSO-d , TMS): δ (ppm) 3.21−3.18 (m, 8H), 1.16 (t, J =
6
2.10, 12H). 1 C NMR (600 Hz, DMSO-d , TMS): δ (ppm)
3
6
1
13
+
+
H and C NMR spectrum were measured using a JEOL ECA
00 spectrometer (25 °C). Electrospray ionization mass spectra
ESI-MS) was recorded on a micrOTOF-Q II spectrometer. C,
51.90, 7.55. ESI-MS m/z (%): 130.1564 [N2 ] (C H N
222 8 20
requires 130.1596, 100); 234.8980 [SbF6]− (SbF6 requires
−
6
(
234.8942, 100).
H, and N contents (wt %) were determined using a Vario EL
cube elemental analyzer (EA).
N,N,N-Triethylbutylammonium Tetrafluoroborate
1
[N2224]BF . The yield was 65.83 %, white powder. H NMR
4
Preparation of Triethylalkylammonium Bromides.
Triethylalkylammonium bromides were prepared by the
alkylation of triethylamine with the corresponding alkyl
bromides (N-butyl bromide, N-hexyl bromide, N-octyl bromide)
using previously published methods. The triethylalkylammo-
nium bromides were dried under high vacuum for 48 h prior to
further use. FT-IR data for the triethylalkylammonium bromides
are provided in the Supporting Information.
(600 Hz, CDCl , TMS): δ (ppm) 3.32−3.28 (m, 6H), 3.15−3.12
3
(m, 2H), 1.65−1.62 (m, 2H), 1.45−1.37 (m, 2H), 1.32 (t, J =
1.92 Hz, 9H), 0.99 (t, J = 7.56 Hz, 3H). 13C NMR (600 Hz,
CDCl , TMS): δ (ppm) 56.91, 52.93, 23.58, 19.69, 13.62, 7.45.
3
27
+
+
ESI-MS m/z (%): 158.1829 [N2224] (C H N requires
10 24
158.1909, 100).
N,N,N-Triethylbutylammonium Hexafluorophosphate
1
[N2224]PF . The yield was 65.7 %, white powder. H NMR (600
6
N,N,N-Triethylbutylammonium Bromide [N2224]Br. The
Hz, CDCl , TMS): δ (ppm) 3.28−3.24 (m, 6H), 3.10−3.08 (m,
3
1
yield was 94.9 %, white powder; mp, 173.26 °C. H NMR (600
2H), 1.62−1.54 (m, 2H), 1.43−1.40 (m, 2H), 1.31 (t, J = 6.84
13
Hz, CDCl , TMS): δ (ppm) 3.28−3.25 (m, 6H), 3.10−3.07 (m,
Hz, 9H), 0.99 (t, J = 7.56 Hz, 3H). C NMR (600 Hz, CDCl ,
3
3
2
H), 1.46−1.44 (m, 2H), 1.24−1.18 (m, 2H), 1.15 (t, J = 7.56
TMS): δ (ppm) 57.07, 53.32, 23.92, 19.94, 14.01, 7.96. ESI-MS
1
3
+
+
Hz, 9H), 0.76 (t, J = 7.56 Hz, 3H). C NMR (600 Hz, CDCl ,
m/z (%): 158.1866 [N ] (C H N requires 158.1909, 100);
3
2224
10 24
−
−
TMS): δ (ppm) 57.30, 53.48, 23.86, 19.68, 13.57, 8.04.
144.9682 [PF ] (PF requires 144.9642, 100).
6
6
Elemental analysis for CHN, analysis (% calculated): C 49.64
N,N,N-Triethylbutylammonium Hexafluoroantimo-
1
(
1
50.42), H 10.03 (10.16), N 5.95 (5.88). ESI-MS m/z (%):
nate [N2224]SbF . The yield was 81.3 %, white powder. H
6
+
+
58.1891 [N2224] (C H N requires 158.1909, 100).
NMR (600 Hz, CDCl , TMS): δ (ppm) 3.29−3.26 (m, 6H),
10
24
3
N,N,N-Triethylhexylammonium Bromide [N2226]Br. The
3.11−3.08 (m, 2H), 1.55−1.43 (m, 2H), 1.35−1.1.32 (m, 2H),
1
13
yield was 81.6 %, white solid; mp, 104.57 °C. H NMR (600 Hz,
1.28 (t, J = 4.86 Hz, 9H), 1.01 (t, J = 7.56 Hz, 3H). C NMR
CDCl , TMS): δ (ppm) 3.42−3.37 (m, 6H), 3.19−3.16 (m, 2H),
(600 Hz, CDCl , TMS): δ (ppm) 56.93, 52.88, 23.27, 19.50,
3
3
+
+
1
9
.59−1.55 (m, 2H), 1.33 (t, J = 7.56 Hz, 4H), 1.27 (t, J = 7.56 Hz,
13.30, 7.03. ESI-MS m/z (%): 158.1877 [N2 ] (C H N
224 10 24
1
3
requires 158.1909, 100); 234.8994 [SbF6]− (SbF6− requires
234.8942, 100).
H), 1.23−1.28 (m, 2H), 0.77 (t, J = 6.90 Hz, 3H). C NMR
(600 Hz, CDCl , TMS): δ (ppm) 57.59, 53.58, 31.20, 26.10,
3
2
2.39, 22.05, 13.86, 8.14. Elemental analysis for CHN, analysis
N,N,N-Triethylhexylammonium Tetrafluoroborate
1
(% calculated): C 53.47 (54.13), H 10.36 (10.60), N 5.33 (5.26).
[N2226]BF . The yield was 74.2 %, white solid. H NMR (600
4
+
+
ESI-MS m/z (%): 186.2221 [N2226
]
(C H N requires
Hz, CDCl , TMS): δ (ppm) 3.32−3.29 (m, 6H), 3.14−3.11 (m,
12
28
3
186.2222, 100).
2H), 1.66−1.62 (m, 2H), 1.39−1.33 (m, 2H), 1.32 (t, J = 5.52
1
3
N,N,N-Triethyloctylammonium Bromide [N222 81 ]Br. The
Hz, 9H), 1.22 (t, J = 7.56 Hz, 4H), 0.89 (t, J = 6.84 Hz, 3H). C
yield was 65.4 %, white sheet solid; mp, 113.92 °C. H NMR
NMR (600 Hz, CDCl , TMS): δ (ppm) 57.14, 52.95, 31.20,
3
(
(
600 Hz, CDCl , TMS): δ (ppm) 3.73−3.69 (m, 6H), 3.53−3.50
26.01, 22.45, 21.68, 13.94, 7.47. ESI-MS m/z (%): 186.2118
3
+
+
m, 2H), 1.72−1.67 (m, 2H), 1.39 (t, J = 6.84 Hz, 8H), 1.31−
[N2226] (C H N requires 186.2222, 100).
12 28
1
.26 (m, 2H), 1.24 (t, J = 6.90 Hz, 9H), 0.88 (t, J = 6.90 Hz, 3H).
N,N,N-Triethylhexylammonium Hexafluorophosphate
1
3
1
C NMR (600 Hz, CDCl , TMS): δ (ppm) 57.56, 53.58, 31.66,
[N2226]PF . The yield was 60.5 %, white powder. H NMR (600
3
6
2
9.15, 29.08, 26.50, 22.60, 22.13, 14.10, 8.17. Elemental analysis
Hz, CDCl , TMS): δ (ppm) 3.28−3.24 (m, 6H), 3.09−3.07 (m,
3
for CHN, analysis (% calculated): C 56.05 (57.13), H 10.35
2H), 1.63−1.60 (m, 2H), 1.38−1.34 (m, 2H), 1.32 (t, J = 7.56
+
13
(
(
10.96), N 5.05 (4.76). ESI-MS m/z (%): 214.2521 [N2228
C H N requires 214.2535, 100).
]
Hz, 9H), 1.29 (t, J = 7.56 Hz, 4H), 0.89 (t, J = 6.90 Hz, 3H). C
+
NMR (600 Hz, CDCl , TMS): δ (ppm) 57.21, 52.98, 31.14,
14
32
3
−
Synthesis of MFm Based Ammonium Ionic Liquids.
25.95, 22.44, 21.60, 13.93, 7.35. ESI-MS m/z (%): 186.2114
−
+
+
−
The MF -based ammonium ILs were synthesized via rapid
anion metathesis reactions between the triethylalkylammonium
bromides and the corresponding nonmetallic salt (NH BF ) or
[N ] (C H N requires 186.2222, 100); 144.9667 [PF ]
m
2226
12 28
6
−
(PF6 requires 144.9658, 100).
N,N,N-Triethylhexylammonium Hexafluoroantimo-
4
4
1
metallic salts (KPF and NaSbF ). The nomenclature, acronyms,
nate [N2226]SbF . The yield was 81.2 %, white powder. H
6
6
6
and chemical structures of these ILs were listed in Table 1. All the
ILs samples were dried under high vacuum for 48 h and stored in
a desiccator prior to the measurements. The synthesis details and
FT-IR data of these ILs are supplied in the Supporting
Information.
NMR (600 Hz, CDCl , TMS): δ (ppm) 3.26−3.22(m, 6H),
3
3.15−3.98 (m, 2H), 1.63−1.56 (m, 2H), 1.34−1.32 (m, 2H),
1.28 (t, J = 7.56 Hz, 9H), 1.26 (t, J = 7.08 Hz, 4H), 0.88 (t, J =
13
6.90 Hz, 3H). C NMR (600 Hz, CDCl , TMS): δ (ppm) 57.22,
3
52.94, 30.97, 25.80, 22.33, 21.41, 13.79, 7.12. ESI-MS m/z (%):
+
+
N,N,N,N-Tetraethylammonium Hexafluorophosphate
186.2191 [N2 ] (C H N requires 186.2222, 100); 234.9009
226
12 28
1
−
−
[
N2222]PF . The yield was 75.8 %, white powder. H NMR
[SbF ] (SbF requires 234.8942, 100).
6
6
6
(
600 Hz, DMSO-d , TMS): δ (ppm) 3.21−3.18 (m, 8H), 1.16 (t,
N,N,N-Triethyloctylammonium Tetrafluoroborate
6
13
1
J = 2.10, 12H). C NMR (600 Hz, DMSO-d , TMS): δ (ppm)
5
[N2228]BF . The yield was 82.8 %, white powder. H NMR
6
4
+
+
1.87, 7.57. ESI-MS m/z (%): 130.1536 [N2222] (C H N
(600 Hz, CDCl , TMS): δ (ppm) 3.32−3.29 (m, 6H), 3.13−3.10
8
20
3
−
−
requires 130.1596, 100); 144.9670 [PF6] (PF6 requires
44.9642, 100).
(m, 2H), 1.63−1.61 (m, 2H), 1.33 (t, J = 6.84 Hz, 8H), 1.31 (t, J
= 7.56 Hz, 9H), 1.28−1.23 (m, 2H), 0.88 (t, J = 6.84 Hz, 3H).
1
C
dx.doi.org/10.1021/je300993n | J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data
Article
Scheme 1. Synthesis of MFm−-Based Ammonium Ionic Liquids
1
3
C NMR (600 Hz, CDCl , TMS): δ (ppm) 57.11, 52.93, 31.69,
analysis window was used to evaluate the data. The nickel and
calcium oxalate, supplied by TA, were used respectively for the
temperature and weight calibrations. The samples (typically
about 10 mg) were loaded into the crucible (platinum, 100 μL)
3
2
2
9.10, 26.36, 22.64, 21.74, 14.13, 7.47. ESI-MS m/z (%):
14.2406 [N2228] (C H N requires 214.2535, 100).
+
+
14
32
N,N,N-Triethyloctylammonium Hexafluorophosphate
1
−1
[
N2228]PF . The yield was 87.1 %, white solid. H NMR (600 Hz,
and heated at a rate of 10 °C·min for temperatures ranging
6
−1
CDCl , TMS): δ (ppm) 3.29−3.25 (m, 6H), 3.10−3.04 (m, 2H),
1
9
(
2
2
from 50 °C to 500 °C under a flow of nitrogen (60 mL·min ).
The mass and temperature precision of TGA is ± 0.1 μg and ±
0.1 °C, respectively.
3
.63−1.62 (m, 2H), 1.34 (t, J = 3.48 Hz, 8H), 1.29 (t, J = 6.90 Hz,
13
H), 1.26−1.22 (m, 2H), 0.88 (t, J = 6.90 Hz, 3H). C NMR
600 Hz, CDCl , TMS): δ (ppm) 57.16, 52.95, 31.67, 29.07,
Phase Transition Temperature Measurement. Measure-
ments of phase transition temperature were performed using a
Mettler-Toledo differential scanning calorimeter (DSC), model
3
9.04, 26.31, 22.63, 21.67, 14.12, 7.35. ESI-MS m/z (%):
14.2432 [N2 ] (C H N requires 214.2535, 100); 144.9667
PF ] (PF requires 144.9642, 100).
+
+
228
14 32
−
−
e
[
6 6
DSC1 (STAR system), and the data was evaluated using the
e
N,N,N-Triethyloctylammonium Hexafluoroantimonate
Mettler-Toledo STAR software version 10.00. The temperature
1
[
N2228]SbF . The yield was 77.2 %, solid with slight yellow. H
6
and heat flow were calibrated with zinc and indium reference
samples (purity, 99.999 %) provided by Mettler-Toledo. The
sample (typically around 4−8 mg) was placed in a 40 μL
aluminum pan with a pinhole at the top of the hermetically sealed
pan. A blank aluminum pan was used as the reference. The
sample inside the DSC furnace was heated from the beginning
temperature (about 0 or 25 °C) to the predetermined
temperature, close to the decomposition temperature of the
NMR (600 Hz, CDCl , TMS): δ (ppm) 3.26−3.23 (m, 6H),
3
3
8
6
5
.08−3.05 (m, 2H), 1.63−1.62 (m, 2H), 1.34 (t, J = 5.52 Hz,
H), 1.30 (t, J = 6.84 Hz, 9H), 1.29−1.24 (m, 2H), 0.88 (t, J =
13
.18 Hz, 3H). C NMR (600 Hz, CDCl , TMS): δ (ppm) 57.28,
3
3.04, 31.68, 29.07, 29.03, 26.31, 22.65, 21.66, 14.14, 7.35. ESI-
+
+
MS m/z (%): 214.2419 [N ] (C H N requires 214.2535,
1
2
228
14 32
−
−
00); 234.8976 [SbF ] (SbF requires 234.8942, 100).
6 6
Water and Bromide Content Measurement. The water
−1
studied compound, at a rate of 10 °C·min under nitrogen flow.
contents of the investigated ILs were determined using a
coulometric Karl Fischer titrator (Mettler Toledo C20) in a
drying chamber. Prior to the measurement, desired milligrams of
the ILs were dissolved in redistilled acetonitrile (water content <
Density Measurement. Density (ρ) measurements are
based on the sweeping tube method with an automatic gas
displacement density analyzer (AccuPyc II 1340 Series
Pycnometers, Micromeritics). The densimeter was calibrated
with a standard stainless steel ball with a volume of 0.718517
1
5 ppm). Each water content was measured by introducing a few
milligrams of acetonitrile solution in the apparatus with the help
of an injector. All measurements for each IL were performed in
triplicate.
The bromide contents of the investigated ILs were determined
using ion chromatography (IC, ICS-900 Dionex, ThermoFisher)
with (4.0 × 250) mm analytical column, suppressed conductivity
detection, and ThermoFisher scientific ICS-900 software version
−3
cm provided by the supplier. The true volume of sample
−3
−3
(
typically 2/3 cm ) in a sample chamber (1 cm ) was
determined by nitrogen sweeping (0.1 MPa) and then a density
was obtained with the weight of the sample divided by its true
volume. Each density measurement was replicated five times and
the average values are reported for the densities and standard
deviations. The density of ILs was experimentally measured at T
2
011. The eluent used was a 4.5 mM Na CO + 1.4 mM
2 3
=
20 °C. The analyzer was validated by measuring the density of
NaHCO mixture. The bromide standard was obtained from the
3
the common ILs, including [BMim]Cl, [BMim]Br, [EMim]Cl,
National Institute of Metrology (China). Prior to the measure-
ment of bromide contents of ILs, a specification curve of the
bromide standards at different concentrations was constructed.
The IL samples for analysis were prepared by dissolving 0.1 g of
IL in 2 mL of HPLC grade acetonitrile and then being diluted to
and [BMMim]PF , which were in good agreement with the
6
reported values. Details were shown in the Supporting
Information.
Refractive Index Measurement. The refractive indexes nD
were measured with an Abbe refractometer (2W, Shanghai Tian
1
0 mL with ultrapure water (Watsons). About 0.01 mL of the IL
solution, treated with 0.2 μm syringe filter, was injected for each
measurement. The content of bromide ion was quantified using
the obtained specification curve.
Zhu Optical Instrument Co., Ltd.) which was calibrated using
ultrapure water with a known refractive index (1.333 at 20 °C).
The temperature of the samples was controlled using a
Decomposition Temperature Measurement. The de-
composition temperature was measured with a TA TGA/Q5000
thermal gravimetric analyzer (TGA) and the TA universal
laboratory thermostat with an accuracy of ± 0.05 °C. The n
of the binary mixtures of ILs and TFSA was experimentally
measured at T = 20 °C, P = 0.1 MPa.
D
D
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Table 2. Mass Fraction of Water (wH2O) and Mass Fraction of Bromide (w ) for MFm−-Based Ammonium Ionic Liquids
Br
[
N2222
PF6
]
[N2224
PF6
]
[N2226
PF6
]
[N2228
PF6
]
BF4
SbF6
BF4
SbF6
BF4
SbF6
BF4
SbF6
6
wH2O(10 w)
149
73
92
52
96
56
120
48
84
63
93
85
125
47
96
72
87
59
131
64
86
78
93
51
6
w (10 w)
Br
a
Table 3. Thermal Properties for Ionic Liquids
cmpd
Tm/°C
T /°C
s‑s
T /°C
d
cmpd
Tm/°C
T /°C
s‑s
T /°C
d
b
[
[
[
[
[
[
N2222]BF4
71.4
77.1
385.2
367.6
376.4
371.3
353.2
364.1
[N2226]BF4
[N2226]PF6
[N2226]SbF6
[N2228]BF4
[N2228]PF6
[N2228]SbF6
88.2
161.6
174.0
101.4
88.3
366.3
330.2
362.9
361.4
249.8
359.2
c
N2222]PF6
N2222]SbF6
N2224]BF4
N2224]PF6
N2224]SbF6
161.9
178.9
184.0
32.7
38.5
73.6
a
T , melting temperature (°C); T , solid−solid transition temperature (°C); T , decomposition temperature (°C). The uncertainty in determining
m
s‑s
d
the melting temperature and the solid−solid transition temperature is within the range ± 0.1 °C, and the uncertainty in determining the
decomposition temperature is within the range ± 0.2 °C. 72 °C T reported by ref 35. 70 °C T and 388 °C of T reported by ref 35.
b
c
m
m
d
RESULTS AND DISCUSSION
■
Synthesis of Ionic Liquids. An efficient mean of generating
ILs, allowing ILs to be produced by simple acid−base reactions
and completely avoiding the use of halide ions, has been
2
8
reported. However, the relative limitation of this mentioned
29
methodology is more time consumption. Traditionally, two
steps, namely quaterization and anion metathesis reactions, are
still widely used during the synthesis of ILs. In this study, the
−
MFm based ammonium ILs were synthesized by a metathesis
reaction between desired anion compounds and the ammonium
bromide salts which were synthesized by alkylation of the
corresponding tertiary amines with bromoalkanes (Scheme 1).
1
13
All investigated ILs, which have been verified by H NMR, C
NMR, ESI-MS and FTIR as having been successful synthesized,
are solid state at ambient temperature.
Figure 2. TGA traces of BF anion-based ionic liquids: ●, [N2222]BF4;
4
Water and Bromide Content. The purity of ILs is a very
important issue on physical properties and the presence of
impurities can be extremely detrimental to the performance of
ILs. PF6− anion-based ILs started to decompose to release the
▲
, [N2224]BF ; Δ, [N ]BF ; ■, [N2228]BF4.
4
2226
4
3
0
toxic product in the presence of the traces of water at 60 °C.
3
1
Seddon et al. have reported that the increasing content of
chlorine ion to imidazolium ILs produced a nonlinear decrease of
the density. Huddleston et al. also have discussed the influence of
the common contaminants on physicochemical properties of
3
2
ILs.
The water and bromide contents were determined to supply
some information on the investigated ILs. The average contents
given in mass fraction of impurities in the IL (wH2O and w ) are
Br
presented in Table 2. According to these measurements, the mass
fraction purities of all of the ILs are determined to be more than
99.9 %. The residual water content of ILs is primarily related to
the nature of the anion. For every cation studied here, ILs
containing BF hydrophilic anion exhibited the highest water
contents (< 150 ppm).
4
●
Figure 3. TGA traces of PF anion-based ionic liquids: , [N2222]PF6;
6
▲
, [N2224]PF ; Δ, [N ]PF ; ■, [N2228]PF6.
6
2226
6
Decomposition Temperature. Thermal decomposition
behaviors of the samples were measured by TGA and the 10 %
weight loss temperature was defined as the decomposition
temperature range from 249 °C to 385 °C, depending on the
3
3
temperature (T ). Their T and TGA traces are shown in Table
structure of the cations and anions (Table 3).
d
d
3
and Figures 2 to 4, respectively. The TGA curves of all studied
Figure 5 depicts the T variation of the studied ILs with the
d
ILs were similar except for [N2228]PF . For [N ]PF , the
weight of the sample decreased at the start temperature but
cationic alkyl chain length. With the same anions, the Td
observed of the ILs decreased with increasing number of carbon
atoms in the alkyl chain from [N2222] to [N2228], where no
6
2228
6
decreased significantly about 250 °C. The T results show that
d
−
the thermal stability of the ammonium-based ILs is in the
noticeable change in the T except for PF -based ILs could be
d
6
E
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Journal of Chemical & Engineering Data
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PF anion. This was consistent with the finding in the literature
6
which showed that a comparative higher thermal stability of
−
−
35
BF -based ILs occurred than that of PF -based ILs.
4
6
Phase Transition Temperature. The phase-transition
temperatures were investigated as the inflection point of change
in the DSC trace (Figures 7 to 9) and the values were tabulated in
Figure 4. TGA traces of SbF anion-based ionic liquids: ●, [N2222]SbF6;
6
▲
, [N2224]SbF ; Δ, [N ]SbF ; ■, [N2228]SbF6.
6
2226
6
Figure 7. DSC scan for BF anion-based ionic liquid: (a) [N2222]BF4;
4
(
b) [N2224]BF ; (c) [N ]BF ; (d) [N2228]BF4.
4 2226 4
Figure 5. Decomposition temperature variation with the cation alkyl
−
−
−
chain length of ammonium ionic liquid: □, BF ; ○, PF ; Δ, SbF .
4
6
6
34
seen. Joshi and Anderson also observed that the thermal
stability of ILs was mainly determined by the anion nature of the
ILs and little affected by the alkyl chain length of the cation.
Figure 6 shows the effect of the anion types of the ILs with fixed
cations on their T . For the fixed cations, the T observed of the
d
d
ILs with the SbF anion was in the middle, lower than that of the
6
Figure 8. DSC scan for PF anion-based ionic liquids: (a) [N2222]PF6;
6
ILs with the BF anion and higher than that of the ILs with the
(b) [N2224]PF ; (c) [N ]PF ; (d) [N2228]PF6.
4
6
2226
6
Figure 6. Decomposition temperature variation with the anion of
+
+
+
ammonium ionic liquid: □, [N2222] ; ■, [N ] ; Δ, [N ] ; ○,
Figure 9. DSC scan for SbF anion-based ionic liquids: (a) [N2222]SbF6;
2224
2226
6
+
[
N2228] .
(b) [N2224]SbF ; (c) [N ]SbF ; (d) [N2228]SbF6.
6
2226
6
F
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Article
a
Table 4. Experimental Density of Ionic Liquids at Temperature T = 20 °C and Pressure p = 0.1 MPa
ionic liquid
ρ/(g·cm−3)
σ
ionic liquid
ρ/(g·cm−3
)
σ
[
N2222]BF4
N2222]PF6
N2222]SbF6
N2224]BF4
N2224]PF6
N2224]SbF6
1.2204
1.4395
1.7152
1.1397
1.3662
1.6402
0.0007
0.0005
0.0008
0.0003
0.0005
0.0011
[N2226]BF4
[N2226]PF6
[N2226]SbF6
[N2228]BF4
[N2228]PF6
[N2228]SbF6
1.0935
1.3513
1.4882
1.0653
1.1902
1.4389
0.0003
0.0005
0.0012
0.0017
0.0005
0.0011
[
[
[
[
[
a
ρ is the density of ionic liquid, and σ is the standard deviation. Standard uncertainties u are u(T) = 0.1 °C, u(p) = 0.01 MPa, and the combined
−3
expanded uncertainty U is U (ρ) = 0.008 g·cm (0.95 level of confidence). The phase for each investigated IL was solid at the experimental
c
c
conditions, and the details were given in the section describing the synthesis of ILs.
Table 3. In this table, the melting temperature (T ) and the
m
solid−solid transition temperature (T ) were taken as the onset
s‑s
of the transition of an endothermic peak on heating curve. The
glass-transition temperature (T ), defined as the midpoint of an
g
endothermic peak on heating from the amorphous glass state to a
liquid state, was not found in the DSC traces of all investigated
ILs.
It is well-known that thermal properties of ILs depend on the
36
structure of the cation and anion. Illustrative DSC thermo-
grams in Figures 7 to 9 were all obtained on heating. Two
distinctive endothermic peaks on heating, which indicated that a
single Ts‑s transition was shown before the melting point, were
observed in [N2224]BF and [N ]PF DSC curves of Figure 7
4
2228
6
and Figure 8, respectively. In addition, no heat capacity change
was shown for the [N2222]SbF compound. Other ILs except the
6
aforementioned three compounds only exhibited a big broad
Figure 10. Density variation with the cation alkyl chain length of
−
−
−
endothermic peak corresponding to the T , where the
ammonium-based ionic liquids: □, BF ; ○, PF ; Δ, SbF .
m
4
6
6
coincidence of T with T was observed.
s‑s
m
The melting points of ILs depend largely on the specific
37
38
anions. Chiappe et al. and Ohno et al. made systematic
studies of the influence of the anion nature on the melting point.
In this study, when the nature of anion was changed with a fixed
cationic structure ranging from [N2222] to [N2226], the variation
order in temperature dependence of Tm was [cation]BF <
4
[
cation]PF < [cation]SbF . On the contrary, when the cationic
6 6
alkyl chain length was eight carbon atoms, the Tm began to
decrease ([N2228]BF4 > [N2228]PF6 > [N2228]SbF ). In
6
comparison to anions, cations also have a remarkable impact
on the melting temperature of the investigated ILs. The present
observations, which followed the order [N2224]PF > [N2226]PF6
6
>
[N2228]PF and [N ]SbF > [N ]SbF > [N2228]SbF6,
6 2224 6 2226 6
were in agreement with the finding that the presence of the
electron donating alkyl substituent on the ammonium cation led
3
9
to ILs with lower melting temperatures. So we assumed that the
studied phase transition temperatures reflected the strength of
the interaction between the cation headgroup and the anion.
Density. The density values are reported in Table 4 together
with the standard deviations (σ). The effects of the alkyl chain
length of the cation and the anion on the density have been
investigated and shown as Figure 10 and Figure 11, respectively.
From the experimental densities for a constant anion as shown
in Figure 10, the density has been found to decrease with an
increase in the length of alkyl chain in the cation. In other words,
a decrease in the density and an increase in the formula weight
were observed with increasing number of carbon atoms in the
Figure 11. Density variation with the anion of ammonium-based ionic
+
+
+
+
liquid: □, [N2222] ; ■, [N ] ; Δ, [N ] ; ○, [N ] .
2224
2226
2228
increasing occupied volume of the cations resulted from the
increased interionic separation and the lowered packing
efficiency with the elongation of the alkyl chain length, resulting
43
in a lower overall density.
Figure 11 shows the anion nature dependency of density for
the ILs, and the density has clearly increased with an increase in
the bulkiness of the anion in the order SbF6− > PF6− > BF . It
−
4
can be noted that excellent agreement has been observed
alkyl chain. Taking [N222n]SbF as an example, the density
decreased from 1.7152 g·cm for [N2222]SbF to 1.7389 g·cm
for [N2228]SbF . Similar trends were consistent with the reported
observations in other IL tethered imidazolium, pyridinium, and
between our density data and the literature values by San
́
chez et
6
−3
−3
44
45
al. and Zhang et al. Taking into account the results, Fredlake
6
46
et al. have attributed the high densities of the ILs to the nature
of the anions, which can occupy nearby positions around the
cation. The obtained results of this study indicate that the
6
40−42
phosphonium cations.
This could be rationalized that the
G
dx.doi.org/10.1021/je300993n | J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Table 5. Experimental Refractive Index, n , of the System Ionic Liquids + TFSA for Different Mass Fractions w at Temperature T =
D
a
2
0 °C and Pressure p = 0.1 MPa
ILs
w = 0.00
w = 0.10
w = 0.20
w = 0.30
w = 0.40
w = 0.50
w = 0.60
[N2222]BF4
[N2222]PF6
[N2222]SbF6
[N2224]BF4
[N2224]PF6
[N2224]SbF6
[N2226]BF4
[N2226]PF6
[N2226]SbF6
[N2228]BF4
[N2228]PF6
[N2228]SbF6
1.3292
1.3292
1.3292
1.3292
1.3292
1.3292
1.3292
1.3292
1.3292
1.3292
1.3292
1.3292
1.3421
1.3398
1.3349
1.3451
1.3405
1.3382
1.3475
1.3421
1.3381
1.3493
1.3427
1.3381
1.3512
1.3499
1.3408
1.3543
1.3522
1.3449
1.3572
1.3549
1.3462
1.3583
1.3562
1.3484
1.3601
1.3602
1.3498
1.3636
1.3632
1.3532
1.3654
1.3667
1.3563
1.3672
1.3681
1.3578
1.3656
1.3691
1.3571
1.3698
1.3745
1.3611
1.3728
1.3762
1.3628
1.3745
1.3781
1.3668
1.3732
1.3641
1.3771
1.3786
1.3693
1.3782
1.3678
1.3857
1.3782
1.3745
1.3722
1.3811
1.3875
1.3760
1.3809
1.3866
a
w is the mass fraction of ionic liquid in the (ionic liquid + TFSA) systems. Standard uncertainties u are u(T) = 0.05 °C, u(p) = 0.01 MPa, u(w) =
.005 and the combined expanded uncertainty U is U (n ) = 0.005 (0.95 level of confidence).
0
c
c
D
densities of these ILs can be fine-tuned with judicious selection of
the cation and anions.
Refractive Index. In the past few years, numerous
2
4,47−49
papers
have been published about refractive indices of
ILs. However, investigation on the refractive index of binary
mixtures of ILs and acid, which maybe directly acid-catalyzed
guided reactions, has been scarcely reported. In this study, the
refractive indices (n ) of the binary mixture of ILs and TFSA at
D
different mass fraction (w, w = mass of ionic liquid/(mass of ionic
liquid + mass of acid)) ranging from 0.00 to 0.60 in the
temperature of 20 °C at atmospheric pressure are summarized in
Table 5.
The refractive indices of the ILs studied in this work were
plotted against mass fraction in Figures 12, 13, and 14. As can be
Figure 13. Refractive index of binary mixture of PF anion-based ILs and
6
acid variation with the w: □, [N2222]PF ; ■, [N ]PF ; Δ, [N2226]PF6;
6
2224
6
○
, [N2228]PF6.
Figure 12. Refractive index of binary mixture of BF anion-based ILs and
4
acid variation with the w: □, [N2222]BF ; ■, [N ]BF ; Δ, [N2226]BF4;
4
2224
4
○
, [N2228]BF4.
Figure 14. Refractive index of binary mixture of SbF anion-based ILs
6
and acid variation with the w: □, [N2222]SbF ; ■, [N ]SbF ; Δ,
6
2224
6
seen, the refractive index, n , increased pseudolinearly with
[N2226]SbF ; ○, [N2228]SbF6.
6
D
increasing the mass fraction of ILs over the studied w range.
Furthermore, taking into account of the studied [N222n] (for n =
Miscibility. Variable solubility of ILs in organic solvents is
2
, 4, 6, and 8)-based ILs with a constant anion, like PF , BF , or
another interesting physicochemical property to their applica-
6
4
50
SbF , it was observed that the elongation of alkyl chain on the
tion. The solubility of the studied ILs in various organic
solvents were then investigated at room temperature and the
corresponding results are presented in Table 6.
6
cation had a positive effect on the n of binary mixture of TFSA
D
and ILs with fixed w. Taking the [N222n]SbF as an example, the
6
nD increased from 1.368 for the TFSA/[N2222]SbF to 1.387 for
As is known, the cation combined with BF anion results in
hydrophilic ILs, whereas the same cation associated with PF6
6
4
the TFSA/ [N2228]SbF with w = 0.6.
6
H
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a
Table 6. Miscibility of Ionic Liquids in Different Solvents
entry
ILs
H O
DMSO
CHCl3
EtOAc
EtOH
Et O
MeCN
Me CO
CH Cl
2
2
2
2
nm
nm
m
2
1
2
3
4
5
6
7
8
9
[N2222]BF4
[N2222]PF6
[N2222]SbF6
[N2224]BF4
[N2224]PF6
[N2224]SbF6
[N2226]BF4
[N2226]PF6
[N2226]SbF6
[N2228]BF4
[N2228]PF6
[N2228]SbF6
m
nm
m
m
m
m
m
m
m
m
m
m
m
nm
nm
nm
m
nm
nm
nm
nm
m
nm
nm
nm
m
nm
nm
nm
nm
nm
nm
nm
nm
nm
nm
nm
nm
m
m
m
m
m
m
m
m
m
m
m
m
pm
m
m
m
m
m
m
m
m
m
m
m
nm
pm
m
m
nm
nm
m
pm
m
pm
pm
m
nm
m
m
m
m
m
nm
nm
m
pm
m
m
pm
pm
m
pm
m
m
1
1
1
0
1
2
m
m
m
nm
nm
m
m
m
m
m
m
m
m
a
Determined at room temperature: m, miscible; nm, nonmiscible; pm, partially miscible. Dimethyl sulfoxide, DMSO; ethyl acetate, EtOAc; ethanol,
EtOH; ethyl ether, Et O; acetonitrile, MeCN; acetone, Me CO.
2
2
anion produces strongly hydrophobic compounds. This
density change also observed for the ILs with the fixed cation
corresponded to the anion nature; for example, the density
increased with an increase in the size of anion. To the best of our
knowledge, this was the first report that the refractive index of a
binary mixture of IL and acid was measured at different mass
fractions. The refractive index versus mass fraction pointed out
the occurrence of a trend that refractive index increased
pseudolinearly with increasing w values. In addition, the
phenomenon of solubility of ILs in different solvents might be
interpreted as the principle of “like dissolves like”.
−
−
conclusion that the anion nature (BF4 or PF ) determined
6
the hydroscopicity of ILs also obtained from the Table 6.
Without exception, all the investigated ILs were fully soluble in
highly polar MeCN solvent and absolutely insoluble in poorly
polar Et O solvent. The polarity of the solvents studied here
2
decrease in the order as H O > DMSO > MeCN > Me CO >
2
2
CHCl > EtOAc > EtOH > CH Cl > Et O. On the other hand,
3
2
2
2
polar solvents of DMSO and Me CO could dissolve all of the
2
studied ILs except [N2222]BF . Unexpectedly, the solubility of ILs
4
in these solvents mentioned above seemed not to be affected by
5
1
the length of the alkyl chain on the cation.
ASSOCIATED CONTENT
■
However, the solubility of these ILs fixed with the anion in
other low polar solvents, such as CHCl , EtOAc, EtOH, and
*
S
Supporting Information
3
Synthetic process of investigated ionic liquids; FT-IR spectrum
and NMR spectrum of selected precursor and data of all
synthesized precursors (Figure S1, Figure S5, Figure S6, and
Table S1); FT-IR spectrum and NMR spectrum of selected ionic
liquid and data of all synthesized ionic liquids (Figure S2, Figure
S7, Figure S8 and Table S2). This material is available free of
charge via the Internet at http://pubs.acs.org..
CH Cl , changed from insoluble or partially soluble to fully
2
2
soluble with elongation of the alkyl chain on the cation (Table 6,
entries 1, 4, 7 and 10; 2, 5, 8 and 11; 3, 6, 9 and 12). These
phenomenon helped to elucidate that the polarities of the
ammonium cation containing ILs decrease with an increase in the
52−54
alkyl chain length of the cation,
thus corresponding to an
increase of the solubility in low polar solvents. This conclusion
was supposed that the degree of aggregation of ILs decreased
with an increase in the alkyl chain length of cations, which
promoted the organic cation dispersing better in the low polar
AUTHOR INFORMATION
■
Corresponding Author
solvents. The solubility in these solvents (CHCl , EtOAc, EtOH,
3
*E-mail: fangfangliu@hebust.edu.cn; sjzhang@home.ipe.ac.cn.
and CH Cl ) confirmed the coexistence of apolar and polar
2
2
Funding
domains, arising from the alkyl chain and polar moieties,
4
5
This work was financially supported by the National Basic
Research Program of China (973 Program, Grant No. 2009-
CB219901), National Natural Science Foundation of China
(Grant No. 51004093) and Beijing Natural Science Foundation
(Grant No. 2122052).
respectively.
CONCLUSIONS
■
−
In this work, series of MF -based ILs have been synthesized and
m
characterized. The experimental data for the thermal temper-
ature, density, and refractive index were discussed. Meanwhile,
their miscibility of the investigated ILs in solvents also has been
measured. We found a general trend in the influence of the alkyl
chain length of the cation and the nature of the anion on the
physicochemical properties of ILs: with the same anions, ILs
containing elongation of the alkyl chain of ammonium cation
exhibited lower decomposition temperature, lower density, but
higher solubility in low polar solvents than those in polar
solvents. When the cation tethered with ethyl, butyl, or hexyl was
considered, an increase of the anion size led to a decrease on the
melting point to ILs. However, when the number of carbon
atoms at the alkyl chain was higher than 6, there was an inversion
of melting point on the anion size sequence. The pattern in
Notes
The authors declare no competing financial interest.
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