Dialkoxy functionalized quaternary ammonium ionic liquids as potential electrolytes and cellulose solvents

Article abstract of DOI:10.1039/c1nj20062c

A series of new ionic liquids, based on dialkoxy-functionalized quaternary ammonium cations {side chains: 1 = CH₃, 1O1 = CH₃OCH ₂, 1O2 = CH₃OC₂H₄, 2O2 = C ₂H₅OC₂H₄; cations: [N _11,1O1,1O2], [N_11,1O1,2O2], [N_11,1O2,1O2], [N_11,1O2,2O2] and [N_11,2O2,2O2]}, with BF₄ ^-, (CF₃SO₂)₂N^- (NTf ₂) and CH₃CO₂^- (OAc) as counteranions, have been prepared and characterized. Their basic properties, such as spectroscopic characteristics, melting point, glass transition temperature, thermal stability, electrochemical window, density, refractive index, viscosity and conductivity, were measured and comparatively studied. The incorporation of two flexible alkoxy chains makes the quaternary ammonium salts highly qualified to be low-viscous and high-conductive room temperature ILs, and even some of them have significantly better fluidity than the popular imidazolium ILs with a similar molecular weight, e.g. [N_11,1O1,2O2] BF₄ (151 cP and 2.11 mS cm^-1, M_w: 249) vs. [HMIm]BF₄ (220 cP and 1.2 mS cm^-1, M_w: 256) at 25 °C. The electrochemical windows of these ILs were evaluated up to 5.5 V. In addition, the dialkoxy OAc ILs were found to have excellent solvent power for cellulose under mild conditions, e.g. a solution of 18 wt% microcrystalline cellulose in [N_11,2O2,2O2]OAc at 80 °C. By precipitation with water, the dissolved cellulose (I crystal structure) was regenerated as nanosized cellulose II particles with increased surface area and decreased crystallinity, determined by FE-SEM and XRD.

Full text of DOI:10.1039/c1nj20062c

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PAPER
Dialkoxy functionalized quaternary ammonium ionic liquids as potential
electrolytes and cellulose solvents
Zhengjian Chen,ab Shimin Liu,a Zuopeng Li,a Qinghua Zhanga and
Youquan Deng*a
Received (in Montpellier, France) 25th January 2011, Accepted 18th April 2011
DOI: 10.1039/c1nj20062c
A series of new ionic liquids, based on dialkoxy-functionalized quaternary ammonium cations
{side chains: 1 = CH3, 1O1 = CH3OCH2, 1O2 = CH3OC2H4, 2O2 = C2H5OC2H4; cations:
[N11,1O1,1O2], [N11,1O1,2O2], [N11,1O2,1O2], [N11,1O2,2O2] and [N11,2O2,2O2]}, with BF4À, (CF3SO2)2NÀ
À
(NTf2) and CH3CO2 (OAc) as counteranions, have been prepared and characterized. Their basic
properties, such as spectroscopic characteristics, melting point, glass transition temperature,
thermal stability, electrochemical window, density, refractive index, viscosity and conductivity,
were measured and comparatively studied. The incorporation of two flexible alkoxy chains
makes the quaternary ammonium salts highly qualified to be low-viscous and high-conductive
room temperature ILs, and even some of them have significantly better fluidity than the
popular imidazolium ILs with a similar molecular weight, e.g. [N11,1O1,2O2]BF4
(151 cP and 2.11 mS cmÀ1, Mw: 249) vs. [HMIm]BF4 (220 cP and 1.2 mS cmÀ1, Mw: 256)
at 25 1C. The electrochemical windows of these ILs were evaluated up to 5.5 V. In addition,
the dialkoxy OAc ILs were found to have excellent solvent power for cellulose under mild
conditions, e.g. a solution of 18 wt% microcrystalline cellulose in [N11,2O2,2O2]OAc at 80 1C.
By precipitation with water, the dissolved cellulose (I crystal structure) was regenerated
as nanosized cellulose II particles with increased surface area and decreased crystallinity,
determined by FE-SEM and XRD.
ILs are usually much more viscous than their non-functionalized
Introduction
analogues, since the incorporation of any functional group will
Ionic liquids (ILs), a class of modern solvents, are of increasing
inevitably lead to an increase in both polarity and molecular
interest because of their versatile applications in various fields:
size. In many fields, the required viscosity of ILs should be
organic synthesis,1 transition metal catalysis,2 biotechnology,3
as low as possible, including heterogeneous catalysis and
separation techniques,4 ionothermal synthesis,5 nano-materials,6
supporting electrolytes, but except viscous lubricants.
lubricants,7 lithium-ion batteries,8 dye-sensitized solar cells,9
Among various types of functional groups, alkoxy groups
supercapacitors,10 sensors,11 light-emitting materials,12 magnetic
are exceptionally featured by their ability to form low-viscosity
fluids,13 and so on. The major advantage of ILs is their
ILs, since the highly flexible alkoxy chains do not pack as
superior physicochemical properties, including negligible vapor
efficiently as alkyl chains, according to molecular dynamics
simulations.20 Up to now, ether-functionalized ILs have included
pressure that offers the greatest environmental benefit, low
imidazolium cations,21–26 quaternary ammonium cations,20,27–31
flammability, wide liquid range, high conductivity and wide
phosphonium cations,32 cyclic quaternary ammonium cations,33,34
electrochemical window and so on. Additionally, the structures
guanidinium cations,35,36 sulfonium cations,37 as well as
and properties of ILs can be custom-tailored to fit each specific
perfluoroalkylborate anions.38 As a matter of fact, the alkoxy
purpose, through incorporating anions or cations with functional
groups, such as –NH2,14 –COOH,15 –SO3H,16 –OH,17 –CN,18
chain effect on viscosity strongly depends on the IL structure,
and –SH,19 etc., which provides more promising perspectives in
and for example prefers to be present in quaternary ammonium
ILs rather than in imidazolium ILs.21,27 The poor alkoxy chain
both fundamental and practical aspects. However, functionalized
effect on viscosity reduction of imidazolium ILs may be related
a Centre for Green Chemistry and Catalysis,
Lanzhou Institute of Chemical Physics,
Chinese Academy of Sciences, Lanzhou, 730000, China.
E-mail: ydeng@licp.cas.cn; Fax: +86-931-4968116
b Graduate School of the Chinese Academy of Sciences,
Beijing 100039, China
to the hydrogen bonds between alkoxy oxygen atoms and
imidazolium ring hydrogen atoms,22 leading to reduced flexibility
of alkoxy groups. In contrast, the alkoxy chains in quaternary
ammonium ILs can be flexible enough to remarkably reduce
viscosity, relative to isoelectronic alkyl chains.20 As a result,
c
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1596 New J. Chem., 2011, 35, 1596–1606

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from the alkylation reaction between HN(CH3)2 and
BrC2H4OCH3 or BrC2H4OC2H5. Then the tertiary amines
were quaternized with different alkoxyalkyl halides to yield
ammonium halide salts. In the final step, anion exchange
reaction afforded the desired ILs. For comparisons, monoalkoxy
[N112,1O2] ILs were also prepared. These newly prepared
compounds were confirmed by 1H-NMR, FT-IR, ESI-MS
(positive ion mode), ion-selective electrodes and moisture
meter. The observed spectral data are well consistent with
the expected structure and composition. The residual halide
ion concentrations in halide-free salts were evaluated to be less
than 0.26%. The water content could be reduced to 40–70 ppm
for NTf2 ILs, 80–120 ppm for BF4 ILs, and 200–300 ppm
for OAc ILs, after vacuum drying for 24 h at 80 1C and
10À2–10À3 mbar. This vacuum drying process would be carried
out again, prior to each property test.
Scheme 1 Structures and abbreviations of the cations and anions
used in this work.
the alkoxy-functionalized quaternary ammonium ILs have
been used as high-conductivity electrolytes in electrochemical
devices.28,29
The thermal properties, such as melting point, glass transition
temperature, crystallization temperature, solid–solid transition
temperature and thermal stability of these (di)alkoxy Ils, were
investigated using differential scanning calorimetry (DSC) and
thermogravimetry (DTA). For BF4- and NTf2-based salts,
which are liquids at room temperature, their basic properties,
such as density, refractive index, electrochemical window,
viscosity and conductivity, were measured and comparatively
studied. Besides, the dialkoxy OAc ILs were found to have
excellent performance in dissolving cellulose. The regenerated
cellulose samples were characterized by field emission scanning
electron microscopy (FE-SEM) and X-ray diffraction (XRD).
Recently, ILs have also shown significant promise in biomass
processing, in particular cellulose.39–41 Cellulose is the most
abundant renewable natural polymer that can be converted
into value-added fuels and chemicals.39–43 However, one of the
main barriers to the use of cellulose is its poor solubility in
organic solvents, due to its strong hydrogen-bonding network
and high crystallinity.44,45 Recently, ILs were reported to be
effective in dissolving cellulose, especially for those with strong
À 48
hydrogen-bond acceptingÀan50ions, such as ClÀ,46,47 Me2PO4
,
À 49
HCO2
,
and CH3CO2
,
etc. These anions can interact
Phase transition and melting point
with hydroxyl groups and disrupt the hydrogen bonds in
cellulose.51,52 For example, solutions of up to 25 wt% cellulose
were prepared in [BMIm]Cl under microwave heating.46
Moreover, the methods to convert cellulose, such as direct
hydrolysis to hydroxymethylfurfural,53 hydrogenation to
hexitols,54 and acetylation of cellulose,55 etc., have been
successfully performed in ILs. Another alternative to the use
of ILs for cellulose is pretreatment, since the dissolved cellulose
can be easily regenerated by precipitation with water or
organic solvents.56,57 The regenerated cellulose is less crystalline
than the native ones, providing greater accessibility to chemical
and enzymatic transformations.57,58
The solid–liquid phase transition of these ILs was determined
by differential scanning calorimetry (DSC). The corresponding
data for glass-transition temperature (Tg), crystallization
temperature (Tc), melting point (Tm) and solid–solid transition
temperature (Ts–s) are summarized in Table 1.
In general, four types of phase transition behaviors could
be observed for all of these ILs during a cooling and heating
Table 1 Thermal properties of the (di)alkoxy ILsa
c
Entry IL
Tgb/1C Tc /1C Tmd/1C Ts–se/1C
Although ether groups have been shown to exert a favourable
effect on increasing the fluidity of ILs, there is little information
about di- or multi-ether functionalized ILs, except for dialkoxy
imidazolium ILs23 and poly(ethylene glycol)-ammonium
ILs.31 Therefore, it is still unknown whether the alkoxy chain
effect on fluidity persists in ILs with more than one ether group
attached, and how other properties of these ILs will be. To
answer these questions, this paper introduces and characterizes
a new class of dialkoxy functionalized quaternary ammonium
ILs (Scheme 1).
1
2
3
4
5
6
7
8
[N112,1O2]BF4
[N11,1O1,1O2]BF4 À91.0
À94.0f (À43.5)g 7.8h
[N11,1O1,2O2]BF4 À89.6
[N11,1O2,1O2]BF4
À32.8
35.9
[N11,1O2,2O2]BF4 À79.0 (À18.8) 6.4
[N11,2O2,2O2]BF4 À80.5
[N112,1O2]NTf2
À95.6i
[N11,1O1,1O2]NTf2 À90.4
[N11,1O1,2O2]NTf2 À89.8
[N11,1O2,1O2]NTf2 À82.3
[N11,1O2,2O2]NTf2 À84.3
[N11,2O2,2O2]NTf2 À84.8
[N112,1O2]OAc
9
10
11
12
13
14
15
16
22.0
27.1
36.1
39.7
[N11,1O2,1O2]OAc
[N11,1O2,2O2]OAc À58.8 (À19.5) 31.1
3.4, 20.3
À26.9, À8.9, 23.2
[N11,2O2,2O2]OAc 3.2 34.3
Results and discussion
a
At a scanning rate of 10 1C minÀ1, except for OAc-based salts at 5 1C
Synthesis and characterization
minÀ1
upon cooling (upon heating). Melting point. Solid–solid transition
.
b
c
Glass-transition temperature. Crystallization temperature
d
e
Scheme 1 outlines the three-step synthesis of the new dialkoxy-
functionalized quaternary ammonium ILs. First, tertiary amines
(Me2NC2H4OCH3 and Me2NC2H4OC2H5) were synthesized
temperature. Literature value: À97.27 g Literature value: À26.27
f
Literature value: 4.27 i Literature value: À96.27
h
c
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New J. Chem., 2011, 35, 1596–1606 1597

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Fig. 2 TGA traces of five ILs.
Fig. 1 Different phase transition behaviors of the (di)alkoxy ILs.
Table 2 Thermal decomposition temperature (Td) of nine ILs
cycle (Fig. 1). The first type shows only liquid–glass transition
without freezing and melting (Fig. 1a, entries 2, 3 and 6–12
in Table 1). In addition to some BF4-based salts, all of the
NTf2 salts engage in this behavior. In our opinion, the highly
flexible alkoxy groups and the NTf2 anion that has two
co-conformations59 reduce the packing efficiency of ILs and
allow them to form an amorphous glass rather than an
ordered crystalline solid at low temperature. The second type
is characteristic of a fast crystallization process upon cooling,
followed by a melting point upon heating (Fig. 1b, entries 4, 13
and 14 in Table 1). Two dialkoxy ILs engaging in this behavior
are both based on the symmetrical [N11,1O2,1O2] cation (with BF4
and OAc), arguing for the promotion function of the cation
symmetry on the crystallization of ILs. The third type is
represented by two low-symmetry salts, such as [N11,1O2,2O2]BF4
that shows cold crystallization between its glass transition
point and melting point (Fig. 1c, entries 1 and 5, Table 1). The
fourth type is characteristic of polymorphic transitions (Fig. 1d,
entries 15 and 16 in Table 1). For example, [N11,1O2,2O2]OAc
underwent two exothermic solid–solid transitions at 3.4 and
20.3 1C to the stable state prior to final melting at 31.1 1C.
Besides, a large enthalpy gain, generally referred to as a plastic
crystal,60 was observed at 23.2 1C for [N11,2O2,2O2]OAc.
Seven of the 16 ILs were shown to have melting points (Tms)
below 39.7 1C (entries 1, 4, 5 and 13–16 in Table 1). The
melting point reflects the strength of a crystal lattice that is
governed by three major factors: intermolecular force, molecular
symmetry and the degree of conformational freedom.61 For a
given anion (BF4 or OAc), the Tm values of ILs decrease in the
order: [N11,1O2,1O2] 4 [N112,1O2] 4 [N11,1O2,2O2], for example,
[N11,1O2,1O2]BF4 (35.9 1C), [N112,1O2]BF4 (7.8 1C) and
[N11,1O2,2O2]BF4 (6.4 1C). In this order, the lowest Tm value
is expected for [N11,1O2,2O2]-containing salts, because
[N11,1O2,2O2] is less symmetrical than [N11,1O2,1O2] and is richer
in side-chain conformations than [N112,1O2]. Consequently, the
second alkoxy chain was found to be able to decrease the
melting points of quaternary ammonium ILs, though only in
the case of different chain lengths. Generally, BF4-based salts
exhibit lower melting points than their OAc counterparts. The
negative charge delocalization of the BF4 anion over the four
Entry
ILs
Tda/1C
1
2
3
4
5
6
7
8
9
[N112,1O2]BF4
376b
245
243
339
387c
266
351
160
151
[N11,1O1,1O2]BF4
[N11,1O1,2O2]BF4
[N11,1O2,2O2]BF4
[N112,1O2]NTf2
[N11,1O1,1O2]NTf2
[N11,1O2,1O2]NTf2
[N112,1O2]OAc
[N11,2O2,2O2]OAc
a
b
Temperature at 5% mass loss. Literature value 377 1C.27 c Literature
value 388 1C.27
electronegative F atoms should be responsible for the lower
melting points of BF4 ILs, through weakening cation–anion
interaction.1
Thermal stability
The thermal stability of nine ILs, which cover all kinds of ions
used in this paper, was determined by thermogravimetry
(TGA). Fig. 2 shows five typical TGA curves, and Table 2
presents the corresponding thermal decomposition tempera-
ture (Td), ranging from 151 to 387 1C. The observed DTA
results reflect that the thermal stability of these salts depends
on the substituent group electronegativity and the anion
nucleophilicity, referring to the nucleophilic N+-dealkylation
mechanism for IL thermolysis.62
On the one hand, incorporating a second ether group
exerts a negative effect on the thermal stability of ILs, in
particular a 1O1 group with a short spacer length of only one
methylene between the alkoxy oxygen atom and the quaternary
nitrogen atom, for example (Td), [N112,1O2]BF4 (376 1C) 4
[N11,1O2,2O2]BF4 (339 1C) c [N11,1O1,1O2]BF4 (245 1C). As
could be deduced from the 1H-NMR spectra (d ppm, see
Experimental section), where (e.g., in Fig. 3) the proton
chemical shifts of –OCH3 (3.38 and 3.67 ppm) are greater
than that of –N+CH3 (3.12 ppm), the alkoxy oxygen atom
tends to be a bit more electronegative than the quaternary
nitrogen atom. Thus, the electron-withdrawing nature of alkoxy
c
1598 New J. Chem., 2011, 35, 1596–1606
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Table
3
Cathodic and anodic limits (Ecathodic and Eanodic vs.
Ag/AgCl) and electrochemical windows (EW) for the (di)alkoxy ILs
based on BF4 and NTf2, determined on a glassy carbon electrode at
room temperature
Entry
ILs
Ecathodic/V
Eanodic/V
EW/V
Fig. 3 The proton chemical shifts (d ppm) of [N11,1O1,1O2]BF4 in
CCl3D.
1
2
3
4
5
6
7
8
[N112,1O2]BF4
À2.94
À2.95
À3.11
À2.90
À2.91
À2.98
À2.95
À3.07
À3.08
À2.95
À3.03
À2.95
2.45
2.35
2.30
2.43
2.42
2.36
2.59
2.49
2.49
2.52
2.45
2.48
5.39
5.30
5.41
5.33
5.33
5.34
5.54b
5.56
5.57
5.47
5.48
5.43
[N11,1O1,1O2]BF4
[N11,1O1,2O2]BF4a
[N11,1O2,1O2]BF4
[N11,1O2,2O2]BF4
[N11,2O2,2O2]BF4
[N112,1O2]NTf2
[N11,1O1,1O2]NTf2
[N11,1O1,2O2]NTf2
[N11,1O2,1O2]NTf2
[N11,1O2,2O2]NTf2
[N11,2O2,2O2]NTf2
groups should be considered in making the cation more
sensitive to the nucleophilic attack of anion on heating, via
increasing the positive charge density on the cation core.
This proposition was also supported by the oxygen-induced
d increase in the –N+CH2R moiety, which denotes the electron-
withdrawing ability of ether groups. For example, the d
values in the N+-bound methylene: N+CH2CH3 (3.55 ppm,
in [N112,1O2]BF4) o N+CH2CH2OCH2CH3 (3.65 ppm, in
[N11,1O2,2O2]BF4) { N+CH2OCH3 (4.64 ppm, in [N11,1O1,1O2]BF4),
correlate well with the above Td order, in turn from 376 1C to
339 1C and to 245 1C.
On the other hand, the anion species also shows a significant
effect on the thermal stability of ILs. As might be expected, the
ILs with the weakly nucleophilic BF4 and NTf2 anions62
exhibit much better thermal stability than OAc ILs. For
example, the Td values of the three [N112,1O2] derivatives are,
respectively, 376 1C for BF4, 387 1C for NTf2 and only 160 1C
for OAc.
9
10
11
12
a
b
Being supercooled for about 0.5 h at 25 1C. Literature value 5.40 V.27
Table 4 Some basic properties of the (di)alkoxy ILs based on BF4
and NTf2 at 25 1C
Entry IL
rb/g cmÀ3 nc
Zd/cP ke/mS cmÀ1
1
2
3
4
5
6
7
8
[N112,1O2]BF4
[N11,1O1,1O2]BF4
[N11,1O1,2O2]BF4a 1.1894
[N11,1O2,1O2]BF4
[N11,1O2,2O2]BF4
[N11,2O2,2O2]BF4
[N112,1O2]NTf2
[N11,1O1,1O2]NTf2 1.4466
[N11,1O1,2O2]NTf2 1.4054
[N11,1O2,1O2]NTf2 1.4215
[N11,1O2,2O2]NTf2 1.3819
[N11,2O2,2O2]NTf2 1.3475
1.1952f
1.2321
1.3980 308g
1.3985 171
1.78h
2.20
2.11
1.03
1.04
1.02
1.4007 151
1.4086 338
1.4094 274
1.4096 223
1.2115
1.1730
1.1418
1.4318i
1.4104 61.2j 3.14k
Electrochemical window
1.4098
1.4095
1.4147
1.4143
1.4139
49.5 3.46
47.9 3.21
66.9 2.45
63.4 2.30
58.6 2.18
9
A wide electrochemical window is required for ILs to be used
as electrolytes in electrochemical devices, like capacitor or
battery systems with high energy and power density.63 One
of the most attractive properties of quaternary ammonium ILs
is their high electrochemical stability in comparison with
imidazolium ILs.27 The electrochemical windows of the room-
temperature ILs used in this paper were evaluated by cyclic
voltammetry on a glassy-carbon electrode, as illustrated in
Fig. 4. Table 3 shows the test results for the respective cathodic
(Ecathodic) and anodic (Eanodic) limits, and electrochemical
windows (EW = Eanodic À Ecathodic). Evidently, all of the
measured ILs possess wide electrochemical windows, ranging
up to 5.30–5.57 V. Unlike the first alkoxy group (sometimes
causes more than 0.5 V reduction in EW),27,34 the second
alkoxy group shows no significant negative effect on the
electrochemical windows of the dialkoxy ILs. Consequently,
the electrochemical stability of these ILs is comparable to that
of tetraalkylammonium ILs (B5.6 V)1,27,38 and pyrrolidinium
ILs (B5.5 V),34,63,64 and is larger than imidazolium ILs
(B4.5 V)21,65 and protic ILs (B4.0 V).66 Sometimes, the
10
11
12
a
b
c
Being supercooled for about 0.5 h at 25 1C. Density. Refractive
index. Viscosity. Conductivity. Literature value 1.21.27 g Literature
value 335.27 h Literature value 1.7.27 i Literature value 1.45.27 j Litera-
ture value 60.27 k Literature value 3.1.27
d
e
f
second alkoxy group can even enhance the cathodic limit, for
example, [N112,1O2]BF4 (Ecathodic = À2.94 V) vs. [N11,1O1,2O2]BF4
(Ecathodic = À3.11 V). In the surroundings of the dialkoxy
cations, both BF4 and NTf2 have high anodic stability with
E
anodic = 2.45 Æ 0.15 V.
Density and refractive index
Physicochemical properties, including density, refractive index,
viscosity and conductivity of these (di)alkoxy ILs based on
BF4 and NTf2 at 25 1C were investigated, as summarized in
Table 4. Fig. 5 presents the obtained density (r, Table 4)
values at 25 1C, ranging from 1.1418 to 1.4466 g mLÀ1. In
comparison to [N112,1O2] ILs, [N11,1O1,1O2] analogues are a bit
more dense, as a result of incorporating an additional alkoxy
oxygen unit. For the dialkoxy ILs, their density shows a
general decrease with lengthening of the alkoxy chains, i.e.
[N11,1O1,1O2] 4 {[N11,1O2,1O2] 4 [N11,1O1,2O2]} 4 [N11,1O2,2O2] 4
[N11,2O2,2O2].
However,
although
[N11,1O2,1O2
]
and
[N11,1O1,2O2] are structurally similar isomers, the former ILs
are comparatively more dense (about 0.02 g mLÀ1) than the
latter ILs, e.g., [N11,1O2,1O2]BF4 (1.2115
[N11,1O1,2O2]BF4 (1.1894 g mLÀ1) at 25 1C. This finding can
g ) vs.
mLÀ1
Fig. 4 Typical cyclic voltammetry of [N11,1O2,1O2]NTf2.
c
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electron clouds of the perfluorinated anions (BF4 and NTf2),
and lead the ILs to show low refractive index (o1.415). In
Fig. 6, only a small change in refractive index occurs during
gradually lengthening the alkoxy chains. Interestingly, there
is a comparatively large difference in the refractive index
between the isomeric ILs. For a given anion, the refractive
index of [N11,1O2,1O2] ILs is larger than that of [N11,1O1,2O2
]
ILs, which results from the above-mentioned fact that the
former are more dense than the latter.
Viscosity
Viscosity is an important parameter for ILs in determining
their potential for applications, because low viscosity is required
for high conductivity and a fast mass transport rate, and to
facilitate operations, like filtration, extraction and decantation.
Relative to imidazolium ILs, quaternary ammonium ILs are
usually rather viscous, which limits their applications, especially
in the field of electrochemistry, in spite of their high electro-
chemical stability. Although many quaternary ammonium ILs
with a flexible ether bond have been shown to own low
viscosity, they are still more viscous than their imidazolium
analogues.34 One of the purposes in this work is to test the
feasibility of providing lower-viscous quaternary ammonium
ILs, through incorporating more than one alkoxy group.
The viscosity (Z, Table 4) values of these (di)alkoxy ILs
(BF4 and NTf2) were plotted in Fig. 7, ranging from 47.9 to
338 cP at 25 1C. The evidence derived from the Z values
confirms that the second alkoxy group can further markedly
decrease the viscosity of quaternary ammonium ILs, for
example, [N11,1O1,1O2]BF4 (171 cP) is only half as viscous as
[N112,1O2]BF4 (308 cP) at 25 1C. In addition, some of these
dialkoxy quaternary ammonium ILs are even evidently less
viscous than imidazolium ILs with a similar molecular weight,
e.g., [N11,1O1,2O2]BF4 (151 cP, Mw: 249) vs. 1-hexyl-3-methyl-
imidazolium [HMIm]BF4 (220 cP, Mw: 256)21 at 25 1C. For a
constant anion, the viscosity of these dialkoxy ILs decreases in
Fig. 5 Density of the (di)alkoxy ILs based on BF4 and NTf2 at 25 1C.
be extended to account for the great difference in refractive
index, viscosity and conductivity between these isomeric ILs
(see below). Besides, NTf2-based ILs (B1.4 g mLÀ1) are much
more dense than BF4 ILs (B1.2 g mLÀ1), mainly because of
the heavy sulfur in NTf2.
The refractive index (n, Table 4) of these (di)alkoxy ILs
(BF4 and NTf2) at 25 1C lies in a range of 1.3980–1.4147. As seen
in Fig. 6, the anion species (BF4 and NTf2), the incorporation
of an alkoxy oxygen atom {[N112,1O2] vs. [N11,1O1,1O2]}, and the
alkoxy chain length (1O1, 1O2 and 2O2) all have but little
impact (o Æ0.01) on the refractive index. According to the
Lorenz–Lorentz equation:
(n2 À 1)/(n2 À 1) = 4prNa/3M
where r is the density, N is a constant, a is the polarizability
and M is the molar mass, respectively, the refractive index
generally increases with increasing polarizability and density.67
High polarizability implies great electron-cloud deformability.
The strong electron-withdrawing fluorine atoms ‘‘lock up’’ the
the following order: {[N11,1O2,1O2
] 4 [N11,1O2,2O2] 4
[N11,2O2,2O2] 4 [N11,1O1,1O2] 4 [N11,1O1,2O2]}. In this order,
the Z values fail to increase with lengthening of side chain,
Fig. 6 Refractive index of the (di)alkoxy ILs based on BF4 and NTf2
at 25 1C.
Fig. 7 Viscosity of the (di)alkoxy ILs based on BF4 and NTf2 at 25 1C.
c
1600 New J. Chem., 2011, 35, 1596–1606
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Table 5 Temperature-dependent viscosity and Arrhenius parameters
Z/cP
1C\IL
[N11,1O2,2O2]OAc
[N11,1O2,2O2]BF4
[N11,1O1,2O2]NTf2
[N112,1O2]NTf2
25
30
40
50
60
70
80
930
661
315
158
89.0
49.1
32.2
274
193
112
71.1
46.0
31.4
23.3
47.9
39.4
27.1
19.4
14.5
11.6
9.2
61.2
53.1
37.0
25.1
18.4
14.0
11.1
ln Z = ln A + EZ/RT (T in K, R = 8.314 J molÀ1 KÀ1
)
EZa/kJ molÀ1
A/cP
54.6
2.6 Â 10À7
39.2
3.5 Â 10À5
26.4
1.1 Â 10À3
28.0
7.8 Â 10À4
R2 b
0.9993
0.9980
0.9983
0.9990
a
b
Activation energy. Linear fitting parameter.
as usual.68 In brief, the contribution of each alkoxy group
to viscosity follows the order of (1O2 4 2O2 4 1O1). The
(1O2 4 2O2) relation on viscosity argues that the 2O2 group is
longer but more flexible than the 1O2 group, and flexibility
dominates over length in reducing the viscosity of the ether
derivatives. Another interesting feature in Fig. 7 is that among
the ILs with a common anion, the most and the least viscous
ones are, respectively, based on [N11,1O2,1O2] and [N11,1O1,2O2],
although they are isomers of each other. A possible explanation
for the distinct difference in viscosity between the isomeric ILs
is their unequal densities, as mentioned above. In detail, the
less dense [N11,1O1,2O2] ILs, relative to [N11,1O2,1O2] ILs, pack
loosely with more interstices to promote mass transport,
by referring to the hole theory.69 Besides, the anion species
plays an important role in determining the viscosity of ILs,
and generally follows the order of OAc 4 BF4 4 NTf2, in
agreement with the previous literature.68 For example, three
[N11,1O2,2O2]-based salts are, respectively, 930 cP for OAc,
274 cP for BF4 and 63.4 cP for NTf2, at 25 1C.
Fig. 8 Arrhenius plots of viscosity for four ILs.
The temperature dependence of viscosity was investigated
for four ILs over a temperature range from 25 to 80 1C, as
listed in Table 5. The viscosity of the four ILs decreases with
increasing temperature, as usual.70 However, there is a great
difference in the decreasing rate of viscosity among these ILs.
Generally, the more viscous ILs decrease more rapidly in viscosity.
For example, the viscosity of [N11,1O2,2O2]OAc could be decreased
28.9 times from 930 to 32.2 cP, and yet [N11,1O1,2O2]NTf2 was
decreased only 5.2 times from 47.9 to 9.2 cP, when heated from
25 to 80 1C. The Arrhenius plots of viscosity were also constructed
according to the equation:
therefore show lower viscosity and slower decreasing rate of
viscosity.
Conductivity
Low viscosity allows high conductivity for ILs to be applied as
electrolytes in electrochemical devices. Another major factor
on conductivity of ILs is the ionic volume. Fig. 9 summarizes
the conductivity (k, Table 4) of the (di)alkoxy ILs based
on BF4 and NTf2 at 25 1C, varying in a range from 1.02 to
3.46 mS cmÀ1
. In comparison to [N112,1O2]-based ILs,
although [N11,1O1,1O2] ILs are larger with an additional alkoxy
oxygen atom, they still display higher conductivity, owing to
their lower viscosity, for example, [N112,1O2]BF4 (1.78 mS cmÀ1
)
ln Z = ln A + EZ/RT
vs. [N11,1O1,1O2]BF4 (2.20 mS cmÀ1) at 25 1C. Additionally,
some of the dialkoxy ILs can be even more conductive
than imidazolium ILs of a similar molecular weight, for
example, [N11,1O1,2O2]BF4 (2.11 mS cmÀ1) vs. [HMIm]BF4
as illustrated in Fig. 8. The values of EZ, A, and the linear fitting
parameter (R2) for the ILs were calculated and listed in Table 5.
According to the values of R2 (40.99), these ILs were well fit with
the Arrhenius model from 25 to 80 1C. EZ is the activation energy
for viscous flow, which is always regarded as the energy barrier to
be overcome by ion transport. Among the four ILs, the EZ value
decreases in the order: [N11,1O2,2O2]OAc (54.6 kJ molÀ1) 4
(1.2 mS cmÀ1 21
at 25 1C. Keeping the anion constant, these
)
ILs {e.g., [N11,1O1,1O2]NTf2: 3.46 mS cmÀ1, 25 1C} are also
more conductive than tetraalkylammonium, pyrrolidinium
and protic ILs, such as [N1116]NTf2 (0.43 mS cmÀ1, 25 1C),71
[P14]NTf2 (2.2 mS cmÀ1, 25 1C),64 and triethylammonium NTf2
(3.2 mS cmÀ1, 130 1C),72 but less conductive than [ethyl-MIm]-
based ILs, such as [EMIm]NTf2 (8.8 mS cmÀ1, 20 1C)65 and
[N11,1O2,2O2]BF4 (39.2 kJ molÀ1
)
4
[N112,1O2]NTf2
(28.0 kJ molÀ1) 4 [N11,1O1,2O2]NTf2 (26.4 kJ molÀ1). This evidence
indicates that the NTf2 ILs have a lower energy barrier and
c
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Fig. 10 FE-SEM images of the original and IL-treated cellulose
samples.
Fig. 9 Conductivity of the (di)alkoxy ILs based on BF4 and NTf2 at
25 1C.
that the ether group is helpful for ILs to dissolve cellulose. Among
the three dialkoxy ILs, [N11,2O2,2O2]OAc (18 wt%) is the most
efficient solvent for cellulose, followed by [N11,1O2,2O2]OAc
(17 wt%) and [N11,1O2,1O2]OAc (15 wt%). Generally, high anion
concentration can make for high solvent power for cellulose.52
However, the OAc concentration order of {[N11,2O2,2O2]OAc o
[N11,1O2,2O2]OAc o [N11,1O2,1O2]OAc} goes against their solvent
power for cellulose, in turn from 18 wt% to 17 wt% to 15 wt%.
This finding might be better understood in conjunction with the
spatial heterogeneity in IL with an amphiphilic cation due to the
aggregation of alkyl tails.74,75 The hydrophobic tail aggregation
implies a trend to make other parts more hydrophilic and polar.
Hence, an appropriate increase in alkoxy chains leads ILs to be
more powerful cellulose solvents, through increasing the spatial
heterogeneity. The ILs with BF4 and NTf2 (entries 6 and 7,
Table 6) are poor solvents for MCC, consistent with previous
reports.52
[EMIm]BF4 (13.6 mS cmÀ1, 25 1C).21 For a common anion, the
conductivity of the dialkoxy ILs generally decreases in the
order of {[N11,1O1,1O2] Z [N11,1O1,2O2] 4 [N11,1O2,1O2] Z
[N11,1O2,2O2] Z [N11,2O2,2O2]}, which can be briefed as group
contribution: (1O1 4 1O2 Z 2O2). This order is determined
by two major factors: alkoxy chain length (2O2 4 1O2 4 1O1)
and the above-mentioned viscosity order (1O2 4 2O2 4 1O1).
For the isomeric ILs, viscosity is the only decisive factor on
their conductivity. As a result, [N11,1O1,2O2]BF4 (2.11 mS cmÀ1
and 151 cP) can be twice more conductive than [N11,1O2,1O2]BF4
(1.03 mS cmÀ1 and 338 cP) at 25 1C.
Dissolution of microcrystalline cellulose
These dialkoxy ILs ought to be cellulose-philic, in virtue of the
hydrogen bond acceptor at the alkoxy oxygen unit. Thereby,
we tested their performance in dissolving microcrystalline
cellulose (MCC with a degree of polymerization: 215–240).
The solubility (wt%, Table 6) of MCC in ILs was determined
by addition of MCC in small batches at 80 1C. As is evident
from the obtained results, the solvent power of the dialkoxy
OAc ILs (15–18 wt%) for MCC is powerful, comparing
favourably with [EMIm]OAc (17 wt%), and other effective
IL solvents for cellulose, such as [butyl-MIm]Cl (10 wt%,
100 1C, DP = 1000;46 18 wt%, 83 1C, DP = 286)73 and
[allyl-MIm]Cl (14.5 wt%, 80 1C, DP = 225).47 The relatively
low solubility of MCC in [N112,1O2]OAc (13 wt%) indicates
In order to evaluate the influence of IL treatment on the
cellulose structure, 5 wt% MCC/IL solutions were employed
to regenerate MCC by precipitation with distilled water and
drying in vacuo. The regenerated MCCs were characterized
using field emission scanning electron microscopy (FE-SEM)
and X-ray diffraction (XRD). As shown in Fig. 10, the IL-treated
MCCs have different surface morphologies from the original
sample, where the linear fibrils reflect the nature of the linear
cellulose molecules.76 The MCC regenerated from [EMIm]OAc
also shows linear textures, similar to the original MCC but
smaller in size. Meanwhile, the alkoxy quaternary ammonium
ILs transform the dissolved MCC into nanosized cellulose
particles. Relative to the [N112,1O2]OAc-treated MCC, the
[N11,2O2,2O2]OAc-treated MCC seems smaller-sized in dimension,
implying a larger accessible surface area for reagents to react
with.77 Fig. 11 shows XRD patterns of different cellulose
samples. The original MCC displays a typical cellulose I
structure with five diffraction peaks at 14.91, 16.61, 20.61,
Table 6 Solubility of MCC in ILs at 80 1C
Solubility of MCC
at 80 1C (wt%)
Entry
IL
1
2
3
4
5
6
7
[N11,1O2,1O2]OAc
[N11,1O2,2O2]OAc
[N11,2O2,2O2]OAc
[N112,1O2]OAc
15
17
18
13
17
No sol.b
No sol.b
ꢀ
22.81 and 34.51, assigned respectively to (101), (101), (021),
[EMIm]OAca
(002) and (040).45 After IL treatment, the cellulose II pattern
[N11,1O2,1O2]BF4
[N11,1O2,2O2]NTf2
was obtained, where three peaks at 12.11, 20.21 and 21.51 can
ꢀ
be indexed as the (101), (101) and (002) reflections of the
a
b
1-Ethyl-3-methylimidazolium OAc. Solubility o 1 wt%.
cellulose II structure.78 The integral area of the XRD curves
c
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Experimental section
1H-NMR, FT-IR, ESI-MS and GC/MS
1H-NMR spectra (d, ppm) were determined on a Mercury Plus
spectrometer (400 MHz). FT-IR spectra (cmÀ1) were obtained
in KBr discs using a Nicolet 5700 FT-IR spectrophotometer.
ESI-MS spectra (positive ion mode, m/z) were measured in
methanol on a Bruker Daltonics APEX II 47e FTMS. The
GC/MS (m/z) analyses were conducted with an Agilent 5973
mass selective detector/6890 GC/data system.
Water content and halide content
After vacuum drying at 80 1C and 10À2–10À3 mbar for 24 h, the
water content of each IL sample was determined by Karl-Fischer
titration, by using a Metrohm 831 KF Coulometer. The residual
halide content in halide-free ILs was evaluated by ion-selective
electrodes (ClÀ and BrÀ).
Phase transition and thermal stability
Fig. 11 XRD patterns of the original and IL-treated cellulose
samples.
The solid–liquid phase transition was investigated on a Mettler
Toledo differential scanning calorimeter model DSC822e, on a
cooling and heating cycle at a scanning rate of 10 1C minÀ1
,
was calculated to be 1.62, 1.41, 1.15 for the MCCs from
[EMIm]OAc, [N112,1O2]OAc and [N11,2O2,2O2]OAc, respectively.
As a result, the [N11,2O2,2O2]OAc-treated MCC has the lowest
crystallinity, suggesting promise in cellulose pretreatment.
except for OAc-based ILs at 5 1C minÀ1. The thermal stability
study was recorded with 5 wt% of mass loss on a Pyris Diamond
Perkin-Elmer TG/DTA at a rate of 10 1C minÀ1 under an
N2 atmosphere.
Conclusions
Density, refractive index, viscosity and conductivity
This paper presents a new class of diether-substituted quaternary
ammonium ILs {side chains: 1 = CH3, 1O1 = CH3OCH2,
1O2 = CH3OC2H4, 2O2 = C2H5OC2H4; ions: [N11,1O1,1O2],
[N11,1O1,2O2], [N11,1O2,1O2], [N11,1O2,2O2], [N11,2O2,2O2]; counter-
The density was determined using a volume–mass method in a
10 mL volumetric flask calibrated with water. The refractive
index measurement was conducted on a WAY-2s Abbe refracto-
meter. The viscosity was determined on a Stabinger Viscosimeter
SVM 3000/GR. The conductivity was obtained using a Mettler-
Toledo Seven Multimeter. The temperature for the measurements
was maintained at 25 Æ 0.1 1C by means of an external tempera-
ture controller.
anions: BF4À, (CF3SO2)2NÀ (NTf2) and CH3CO2 (OAc)}.
À
Their basic physicochemical properties, such as spectroscopic
characteristics, crystallization temperature, melting point, glass
transition temperature, solid–solid transition temperature,
thermal stability, electrochemical window, density, refractive index,
viscosity and conductivity were measured and comparatively
studied with the monoalkoxy [N112,1O2]-based ILs. Most of the
(di)alkoxy quaternary ammonium salts are liquid at room
temperature. The ILs bearing two different alkoxy groups
tend to have a low or non-existent melting point (o31.1 1C).
The dialkoxy ILs exhibit lower thermal stability (151–351 1C)
than [N112,1O2] salts (160–387 1C), due to the electron-
withdrawing nature of alkoxy groups. The ILs based on BF4
and NTf2 have wide electrochemical windows of up to
5.30–5.57 V, suffering no significant negative impact from
the combination of a second alkoxy group. Due to the presence
of two flexible alkoxy chains, the dialkoxy quaternary ammonium
salts are well qualified to be low-viscous and high-conductive
ILs (minimum to 47.9 cP and maximum to 3.46 mS cmÀ1 at
25 1C), and even some of them are not only much more fluid
than [N112,1O2] ILs, but also more than imidazolium ILs with a
similar molecular weight. The dialkoxy OAc ILs are effective
in dissolving cellulose (up to 18 wt% at 80 1C), with the help of
the hydrogen accepting ether bonds. Hence, the dialkoxy
functionalized quaternary ammonium ILs could be promising
as both electrolytes and cellulose solvents.
Electrochemical window
The cyclic voltammetry testing was carried out on a CHI 660A
Electrochemical Work Station with a glassy carbon working
electrode and an Ag wire pseudo reference electrode at room
temperature.
Dissolution and regeneration of cellulose
The solubility of cellulose was determined by increasing the
cellulose concentration by 1.0 wt% increment. The mixture was
heated at 80 1C and stirred under a nitrogen atmosphere in a
glove box. Incremental cellulose was added until the solution
became optically transparent. The saturation point was con-
sidered to be reached when cellulose could not be dissolved
again within 5 h. The regeneration of cellulose was performed
on a 0.5 g scale. In a typical experiment, 50 mL distilled water
was added to a 5 wt% cellulose/IL solution (10 g) at room
temperature, and vigorously stirred for 2 h. The cellulose
suspension was filtered and washed with running distilled water
and acetone (5 Â 20 mL). Then the regenerated cellulose was
dried under vacuum for 24 h at room temperature.
c
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FE-SEM and XRD
3.66 (s, 3H), 3.52 (t, 2H), 3.39 (s, 3H), 3.10 (s, 6H); IR (cmÀ1):
2950, 2904, 2838, 1476, 1353, 1194, 1138, 1057; ESI-MS (m/z):
+
The surface morphologies of cellulose samples were determined
by field emission scanning electron microscopy (FE-SEM,
JSM-6701F). X-Ray diffraction (XRD) was measured using a
Siemens D/max-RB powder X-ray diffractometer. The diffraction
patterns were recorded with Cu Ka radiation (40 mA, 40 kV)
over a 2y range from 101 to 801 and a position-sentient detector.
148.1336 for [N11,1O1,1O2
]
with a calculated value of 148.13.
[N11,1O1,2O2]NTf2. Pale yellow liquid; yield: 90%; water
content: 63 ppm; chloride ion concentration: 0.03 wt%;
1H-NMR (CCl3D, 400 MHz, ppm): 4.60 (s, 2H), 3.81 (t, 2H),
3.66 (s, 3H), 3.60 (t, 2H), 3.54 (q, 2H), 3.11 (s, 6H), 1.21 (t, 3H);
IR (cmÀ1): 2981, 2938, 2881, 1474, 1352, 1190, 1135, 1055;
Synthesis of [N11,1O2]. CH3OC2H4Br (185.2 g, 1.33 mol)
was added dropwise to 33% aqueous dimethylamine solution
(546 g, 4.0 mol) and stirred for 24 h at 40 1C. After stirring, the
mixture was neutralized by 2 M aqueous NaOH (1.33 mol),
and then an appropriate amount of NaCl was dissolved to
make a saturated solution. Then the solution was azeotropically
distilled with the fraction collected between 80 and 90 1C. The
collected fraction was dried over KOH pellets, filtered and
washed with anhydrous diethyl ether (5 Â 100 mL). Subsequent
fractional distillation of the dried solution gave the product
as a colorless liquid (bp: 96–98 1C, 86 g, yield: 63%). GC/MS:
m/z = 15, 30, 42, 58, 72, 103.
+
ESI-MS (m/z): 162.1493 for [N11,1O1,2O2]
with a calculated
value of 162.15.
[N11,1O2,1O2]NTf2. Colorless liquid; yield: 89%; water content:
51 ppm; bromide ion concentration: 0.01 wt%; 1H-NMR
(CCl3D, 400 MHz, ppm): 3.79 (t, 4H), 3.62 (t, 4H), 3.38
(s, 6H), 3.20 (s, 6H); IR (cmÀ1): 2942, 2900, 2831, 1477, 1352,
1192, 1136, 1056; ESI-MS (m/z): 162.1492 for [N11,1O2,1O2
a calculated value of 162.15.
]
+ with
[N11,1O2,2O2]NTf2. Colorless liquid; yield: 91%; water content:
46 ppm; bromide ion concentration: 0.01 wt%; 1H-NMR
(CCl3D, 400 MHz, ppm): 3.82 (m, 4H), 3.62 (m, 4H), 3.53
(q, 2H), 3.38 (s, 3H), 3.21 (s, 6H), 1.20 (t, 3H), 1.22 (m, 3H);
Synthesis of [N11,2O2]. The alkylation reaction, neutraliza-
tion and saturation followed the same procedure as described
in the synthesis of [N11,1O2], except that C2H5OC2H4Br was used
instead of CH3OC2H4Br. The saturated solution was extracted
with diethyl ether (5 Â 100 mL). The collected extracts were
dried with KOH pellets, filtered and rinsed with anhydrous
diethyl ether (3 Â 100 mL). Then, fractional distillation of
the dried solution gave the target product as a colorless liquid
(bp: 116–118 1C, 97 g, yield: 62%). GC/MS: m/z = 15, 30, 42,
58, 72, 117.
IR (cmÀ1): 2982, 2938, 2888, 2830, 1476, 1351, 1190, 1134, 1054;
+
ESI-MS (m/z): 176.1644 for [N11,1O2,2O2]
with a calculated
value of 176.16.
[N11,2O2,2O2]NTf2. Colorless liquid; yield: 89%; water content:
48 ppm; bromide ion concentration: 0.01 wt%; 1H-NMR
(CCl3D, 400 MHz, ppm): 3.84 (m, 4H), 3.62 (m, 4H), 3.53
(m, 4H), 3.21 (s, 6H), 1.20 (m, 3H), 1.20 (m, 6H); IR (cmÀ1):
2981, 2937, 2878, 1476, 1351, 1188, 1134, 1055; ESI-MS (m/z):
190.1806 for [N11,2O2,2O2]
+ with a calculated value of 190.18.
Synthesis of [N11,1O1,1O2]Cl and [N11,1O1,2O2]Cl. CH3OCH2Cl
(30.1 mL, 0.40 mol) was added dropwise to a mixture of
[N112,1O2]NTf2. Colorless liquid; yield: 92%; water content:
65 ppm; bromide ion concentration: 0.02 wt%; 1H-NMR
(CCl3D, 400 MHz, ppm): 3.79 (t, 2H), 3.52 (t, 2H), 3.48 (q, 2H),
3.38 (s, 3H), 3.10 (s, 6H), 1.40 (t, 3H); IR (cmÀ1): 2946, 2899, 2830,
1485, 1353, 1196, 1139, 1057.
60 mL anhydrous dichloromethane and [N11,1O2] or [N11,2O2
]
(0.40 mol equiv.) in a round bottom flask fitted with a drying
tube at 0 1C. After stirring for 4 h at 30 1C, the solvent was
evaporated under vacuum at 50 1C, affording the desired
product as a pale yellow viscous liquid (yield 4 99%).
Synthesis of BF4 ILs. The anion exchange was performed by
using triethyloxonium tetrafluoroborate {[Et3O]BF4}, which
was prepared following a published method.79 In a typical
procedure, an ammonium halide (0.10 mol) and equimolar
[Et3O]BF4 were added to 40 mL anhydrous dichloromethane
in a round bottom flask equipped with a drying tube. After stirring
for 2 h at room temperature, the mixture was concentrated
in vacuo at 80 1C to remove the solvent, yielding a colorless or
pale yellow liquid.
Synthesis of [N11,1O2,1O2]Br, [N11,1O2,2O2]Br, [N11,2O2,2O2]Br
and [N112,1O2]Br. In a typical procedure, a tertiary amine
{[N11,1O2] or [N11,2O2]} (0.40 mol) was put into acetonitrile
(80 mL) and the appropriate alkyl or alkoxyalkyl bromide
(0.44 mol) was added. After stirring for 48 h at 40 1C, the
solvent was removed under vacuum at 80 1C, leaving a pale
yellow solid. Recrystallization from a mixture of acetone and
anhydrous diethyl ether afforded a white solid (yield E 90%).
Synthesis of NTf2 ILs. In a typical procedure, an ammonium
halide (0.10 mol) and lithium bis(trifluoromethylsulfonyl)-
imide (LiNTf2) (0.12 mol) were added into 50 mL distilled
water and stirred for 4 h at room temperature. After the
reaction, the upper water phase was decanted off. The bottom
IL phase was washed three times with distilled water (20 mL)
in a separating funnel. Concentration under vacuum at 80 1C
yielded a colorless or pale yellow liquid.
[N11,1O1,1O2]BF4. Pale yellow liquid; yield 4 99%; water
content: 112 ppm; chloride ion concentration: 0.12 wt%;
1H-NMR (CCl3D, 400 MHz, ppm): 4.64 (s, 2H), 3.79
(t, 2H), 3.67 (s, 3H), 3.55 (t, 2H), 3.38 (s, 3H), 3.12 (s, 6H);
FT-IR (cmÀ1): 2951, 2907, 2837, 1636, 1479, 1410, 1286, 1205,
1120, 1055.
[N11,1O1,2O2]BF4. Pale yellow liquid; yield 4 99%; water
content: 106 ppm; chloride ion concentration: 0.12 wt%;
1H-NMR (CCl3D, 400 MHz, ppm): 4.66 (s, 2H), 3.82
(t, 2H), 3.67 (s, 3H), 3.54 (t, 2H), 3.53 (q, 2H), 3.13 (s, 6H),
[N11,1O1,1O2]NTf2. Pale yellow liquid; yield: 88%; water
content: 66 ppm; chloride ion concentration: 0.02 wt%;
1H-NMR (CCl3D, 400 MHz, ppm): 4.58 (s, 2H), 3.77 (t, 2H),
c
1604 New J. Chem., 2011, 35, 1596–1606
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1.20 (t, 3H); FT-IR (cmÀ1): 2979, 2937, 2879, 1637, 1480, 1389,
1203, 1119, 1055.
Acknowledgements
We acknowledge the National Natural Science Foundation of
China (No. 20533080) for financial support.
[N11,1O2,1O2]BF4. Colorless liquid that crystallized at ambient
temperature; yield 4 99%; water content: 95 ppm; bromide ion
concentration: 0.15 wt%; H-NMR (CCl3D, 400 MHz, ppm):
1
Notes and references
3.81 (t, 4H), 3.64 (t, 4H), 3.37 (s, 6H), 3.22 (s, 6H); FT-IR (cmÀ1):
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[N11,1O2,2O2]BF4. Colorless liquid; yield 4 99%; water
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Synthesis of OAc ILs. In a typical procedure, an ammonium
halide (0.15 mol) was dissolved in distilled water (300 mL) and
freshly prepared silver acetate (AgOAc) (0.18 mol) was added.
After vigorous stirring for 5 h at room temperature, the mixture
was filtrated and washed with distilled water (3 Â 100 mL). The
filtrate was collected and concentrated under vacuum at 80 1C,
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