Synthesis and hypergolic activity evaluation of some new ammonium-imidazolium based ionic liquids

Article abstract of DOI:10.1039/c6ra01364c

In this study, a new class of dicyanamide-based ionic liquids containing mono and dicationic counterparts was synthesized and characterized. Results demonstrate that the physical properties of these salts, including thermal stability, melting point, density and viscosity, are highly dependent on the structure of cation and type of anion. It was observed that the thermal stability of dicationic ionic liquids is higher for the monocationic ones. Furthermore, droplet tests were used to investigate the hypergolic properties of these ionic liquids. All of the synthesized compounds showed hypergolic activity upon contact with white fuming nitric acid, suggesting that some may have potential for application as bipropellants or energetic additives.

Full text of DOI:10.1039/c6ra01364c

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Synthesis and hypergolic activity evaluation of
some new ammonium-imidazolium based ionic
liquids†
Cite this: RSC Adv., 2016, 6, 26386
*
Reza Fareghi-Alamdari, Faezeh Ghorbani-Zamani and Negar Zekri
In this study, a new class of dicyanamide-based ionic liquids containing mono and dicationic counterparts
was synthesized and characterized. Results demonstrate that the physical properties of these salts,
including thermal stability, melting point, density and viscosity, are highly dependent on the structure of
cation and type of anion. It was observed that the thermal stability of dicationic ionic liquids is higher for
the monocationic ones. Furthermore, droplet tests were used to investigate the hypergolic properties of
these ionic liquids. All of the synthesized compounds showed hypergolic activity upon contact with
white fuming nitric acid, suggesting that some may have potential for application as bipropellants or
energetic additives.
Received 16th January 2016
Accepted 29th February 2016
DOI: 10.1039/c6ra01364c
www.rsc.org/advances
greater thermal stability, lower volatility, and more exibility in
tuning/varying their physicochemical properties.
Introduction
The tunability of dianionic ILs has been enhanced by
cationic moieties.20 Although many dianionic ILs exhibit
melting points higher than 100 C, they are still referred to as
“ionic liquids” for convenience20,21 since they are liquids at
temperatures at which they are employed.
On the basis of the advantages of the dianionic ILs, some
applications of these compounds have been explored, such as
high temperature lubricants,22,23 solvents in high temperature
reactions,22–24 additives in dye-sensitized solar cells,25 extraction
liquids26 in chromatography27–29 and mass spectroscopy,30 and
as electrolytes for secondary batteries.22
A hypergolic rocket propellant is a propellant that spontane-
ously ignites when a fuel and an oxidizer come into contact with
each other.1,2 Traditional hypergolic propellants (such as
hydrazine and its derivatives) are extremely toxic and/or carci-
nogenic.3 They also have less energy density than such cryo-
genic propellant combinations as liquid hydrogen/liquid
oxygen.4 As a result, a launch vehicle, which uses a hypergolic
propellant, must carry a greater mass of fuel than one using
these cryogenic fuels. Hence, there is considerable interest in
the development of suitable alternatives for hydrazine family of
hypergolic fuels.5–7
ꢀ
Despite much effort, most of the research work has thus far
focused on monocationic type energetic ionic liquids. To gain
further understanding of structure-property relations and extend
the applications of ILs as energetic and hypergolic components,
it is incumbent to explore new dicationic IL structures.
In the current study and in continuation of our previous
study on ILs we would like to introduce the synthesis of
unsymmetrical monocationic salts have been synthesized based
on imidazolium and ammonium cations. Furthermore, asym-
metrical dicationic compounds have been synthesized by
combination of imidazolium and ammonium cations and their
properties have been compared with those of the new synthe-
sized monocationic ILs (Scheme 1).
Regarding the selection of the energetic anion, dicyanamide
anion provides a combustible organic structure and gives ILs
with low viscosities and wide liquid ranges.8 On the other hand,
the introduction of fuel-rich amine groups as well as vinyl and
allyl substitutents into the cations can increase the energy
density and hypergolic reactivity of the resultant energetic ILs.10
Therefore, open-chain amines, vinyl and allyl substitutents, and
Recently, ionic liquids (ILs) have received considerable
attentions as energetic materials for propellant applications.8–12
Hypergolic ILs have become paragons of environmental
friendliness, green chemistry and low vapour toxicity materials,
which exhibit energetic performances comparable to hydrazine.
They have shown great potential to meet the requirements of
developing non-toxic, high performance propellant formula-
tions; especially for space propulsion applications.13–16
Such liquid salts tend to have low volatilities and viscosities
and high thermal and chemical stabilities, and oen exhibit
wide liquid ranges, all of which allow their application as
bipropellant fuels in a variety of conditions.
In comparison with monocationic ILs, dianionic ILs have the
potential to exhibit superior physical properties17–20 in terms of
Faculty of Chemistry and Chemical Engineering, Malek-Ashtar University of
Technology, Tehran, Iran. E-mail: reza_fareghi@yahoo.com
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra01364c
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Scheme 1 Multistep synthetic route of ILs.
dicyanamide anion were chosen for the preparation of a new
series of monocationic and dicationic ILs. Thermophysical and
hypergolic properties of the newly synthesized compounds were
investigated to evaluate their potential use in bipropellant
systems.
Experimental section
Reagents and instruments
All the solvents and chemicals were purchased from Sigma-
Aldrich Chemical Company and used without further purica-
tion. All the synthesized compounds were characterized by H
1
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and 13C NMR spectroscopies using a Bruker DRX–300 instru-
ment. Chemical shis were reported relative to TMS as the
internal standard. FTIR spectra were obtained on a Nicolet 800
spectrophotometer. Elemental analyses were obtained using
a Heraeus CHN analyzer (Germany) instrument. Thermal
gravimetric analyses (TGA) were recorded using a thermal
analysis Perkin Elmer (Pyris Diamond model) instrument, with
Æ0.1 ꢀC and Æ0.2 mg precisions in temperature and mass
measurement, respectively.
General procedure for synthesis of ammonium-imidazolium
based compounds (10a–13a). 1-Methylimidazole or 1-vinyl-
imidazole (30 mmol) was slowly added into a stirred solution of
halide-functionalized ammonium salt (8b or 9b) (35 mmol) in
acetonitrile (30 mL). The reaction mixture was heated to reux
at 110 ꢀC for 24 hours to give a semi-crystalline liquid. The
product was washed with a mixture of acetonitrile and ethyl
acetate (50 : 50) and dried under vacuum at 80 ꢀC to give
a yellow viscous (10a and 11a) or dark red (12a and 13a), semi-
crystalline liquid in approximately 90% yield.
General procedure for synthesis of dicyanamide-based ILs.
Silver dicyanamide was prepared by mixing equal molar
amounts of silver nitrate and sodium dicyanamide in aqueous
solutions followed by ltration. The dicyanamide-based ILs
were prepared by metathesis of corresponding salts with silver
dicyanamide according to a literature method.31
Densities, r, were measured using an Anton Paar DMA 5000
vibrating tube densimeter and viscosities, h, were measured
using an Anton Paar MCR 300 rheometer.
Synthesis procedures
General procedure for synthesis of imidazolium-based
compounds (1a and 2a). To a vigorously stirred solution of 1-
methylimidazole or 1-vinylimidazole (30 mmol) in toluene (20
Results and discussion
ꢀ
mL) at 0 C was added 3-chloro-1-propene (2.85 mL, 35 mmol).
ꢀ
The solution was heated to reux at 110 C for 24 hours, aer
In this study, the ammonium and imidazolium mono and
dicationic based ionic liquids were synthesized using a multi-
step synthetic route (Scheme 1).
The structures of these ionic liquids were elucidated using
NMR spectroscopy, FTIR spectrometries and elemental anal-
ysis. The compounds 1b, 2b, and 4b were previously reported in
the literature.32–34 However, these salts were synthesized and
their properties were studied under the same experimental
conditions for the sake of comparison.
The energetic performance of ILs in practical applications is
determined by their properties such as melting or decomposi-
tion point, density, viscosity, detonation properties or even
sensitivity, which vary by different substituents and energy-
containing groups in their structures. Thus, some important
properties of synthesized DCHILs were measured and the
results are discussed below.
which the toluene was decanted and the remaining viscous liquid
washed with n-hexane and dried under vacuum to give a yellow or
red viscous liquid (1a or 2a) in approximately 90% yield.
General procedure for synthesis of ammonium-based
compounds (4a–7a). 2-Azido-N,N-dimethylethyl amine (DMAZ)
was prepared according to a literature method in which 2-
chloro-N,N-dimethyl ethanol amine hydrochloride was reacted
with sodium azide in water, followed by ether extraction to get
a good yield.10
Ammonium-based salts (4a–7a) were prepared by alkylation of
DMAZ (3) with alkyl or allyl halides such as methyl, ethyl, butyl
iodide and 3-chloro-1-propene according to the following
method.
To a vigorously stirred ꢀsolution of DMAZ (3.42 g, 30 mmol) in
acetonitrile (20 mL) at 0 C was added alkyl or allyl halide (35
mmol). The solution was heated to reux at 60 ꢀC for 24 hours,
aer which the precipitate was ltered, washed with n-hexane
and dried under vacuum to give a white or cream oil/semi-
crystalline product (93% yield).
Density and viscosity of the synthesized dicyanamide-based
ILs
General procedure for synthesis of halide-functionalized The rst part of this work consisted of density and viscosity
ammonium-based salts (8b and 9b). To a stirred solution of 2- measurements at atmospheric pressure. The densities and
(dimethylamino) ethanol (3.00 mL, 30 mmol) in acetonitrile (20 viscosities of all the synthesized dicyanamide-based ILs were
mL) at 0 ꢀC was added methyl iodide (1.86 mL, 35 mmol) or 3- measured under the same experimental condition and the
chloro-1-propene (2.85 mL, 35 mmol). The solution was heated results are presented in Table 1.
ꢀ
to reux at 60 C for 12 hours, aer which the precipitate was
Density reects the energy performance of energetic mate-
ltered, washed with n-hexane and dried under vacuum to give rials. Generally, compounds with higher density have higher
a white or cream solid product (8a and 9a, 93% yield). energy contribution due to a higher mole number that can be
Compounds 8b and 9b were prepared by reaction of 8a and 9a packed into a limited volume. Moreover, the cationic and
with thionyl chloride. A solution of the ammonium-based salt (8a anionic structures of the energetic ILs have direct effects on the
or 9a) (30 mmol) in chloroform (20 mL) was cooled in an ice- density of the salts. The density of ILs containing the same
water bath and a solution of thionyl chloride (4.41 mL, 37.5 anion structure depends on three different aspects of cation
mmol) in chloroform (20 mL) was added dropwise with stirring. structure:35 (i) the length of the substituted alkyl chain, (ii) the
Aer the mixture was le at reux for 3–4 hours, the precipitate functionalization of the chain with hydrophilic groups, and (iii)
was ltered. The crude product was puried by dissolving it in the head group of the cation. In addition, the presence of the
boiling ethanol and then the impurities were separated by energetic groups such as –NO2, –N3, and –CN increase strong H-
ltration. The product was dried under vacuum to give a yellow bond interactions in molecules and as a result the density of the
(8b, 86% yield) or light brown (9b, yield: 81%) solid.
compound can increases.
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Table 1 Properties of the synthesized hypergolic ionic liquids
Tonset
Entry Compound (ꢀC)
r
h
ID
(g cmÀ3
)
(cP)
(ms) Ref.
1
2
3
4
5
6
7
8
9
1b
2b
4b
5b
6b
7b
10b
11b
12b
13b
211.12 1.13
208.23 1.15
182.20 1.19
182.34 1.21
157.96 1.03
200.60 1.13
252.32 1.23
235.78 1.25
226.19 1.17
217.66 1.18
112.9 16
32
33
30.3
81
13
35
73
51
43
52
63
41
36
This work
34
93
101
71
This work
This work
This work
This work
This work
This work
180
—
570
368
10
In agreement with this approach, the azide-functionalized
ILs (entries 3–6) show higher density compared to imidazo-
lium based salts (entries 1 and 2). As can be seen from Table 1,
the densities decreases with increasing length of the hydro-
carbon linkage chain (entry 5). It is also clear from table that the
density for the dicationic ILs is higher than that for mono-
cationic ILs when the dicationic ILs are compared to the mon-
ocationic ILs with the identical anions (entries 7–10). It is
depended on the differences in van der Waals volumes for
dicationic and monocationic ILs.
The viscosity of energetic ILs is one of the important issues
inuencing the performance of the compounds. The high
viscosity may be caused by the presence of a strong H-bonded
network as well as strong interactive forces such as electrostatic
or van der Waals interactions. Hence, the viscosities of all the
synthesized compounds were measured. The results show that
imidazolium-based ILs have lower viscosities compared to azide-
functionalized ones. On the other hand, when the dicationic ILs
are compared to monocationic ones having the same anions, the
viscosity in the dicationic ILs is much larger than that in the
monocationic ones (Table 1, entries 7–10). This may be due to the
stronger interionic and interamolecular interactions as well as
more bulky cations in the dicationic ILs than monocationic ones,
which certainly gives low mobility and high viscosity.34
Fig. 1 TGA curves for the synthesized ILs: (A) imidazolium-based ILs,
(B) ammonium-based ILs, (C) ammonium-imidazolium-based ILs.
Thermal behaviour of the synthesized dicyanamide-based ILs
to anions. The dicationic ILs are also thermally more stable
than monocationic ones with identical anions.41,42 The higher
thermal stability of dicationic ILs is also associated with the
better positive charge dispersion balanced by two separated
monoanions, which gives them a stronger attraction to the
anions.41,42 In addition, the observed higher thermal stability of
dicationic ILs is attributed to the greater charge and intermo-
lecular interactions, their higher molecular weight, higher
density and smaller free volume. TGA results showed that allyl
or vinyl functionalized ILs (Table 1, entries 2, 6, 8 and 10)
exhibited slightly lower thermal stabilities than common
dicyanamide-based ones (Table 1, entries 1, 3–5, 7 and 9),
attributed to the rigidity of these groups.34
The thermogravimetric analysis (TGA) of all the synthesized
dicyanamide-based ILs was carried out in a nitrogen atmo-
sphere at a heating rate of 10 ꢀC minÀ1. The results of TGA are
depicted in Fig. 1 and the corresponding values are shown in
Table 1. Thermal stability of ILs is directly linked to the nature
of the anions and cations. Generally, ILs containing more
nucleophilic and coordinated anions decompose at lower
temperatures while ILs with poor proton withdrawing anions
are more stable to high-temperature decomposition.36–38 The
decomposition temperature of ILs also depends on the type of
the cations. Most studies have shown that imidazolium-based
ILs are thermally more stable than pyridinium and quaternary
ammonium-based ones.39,40 This may be due to the fact that the
imidazolium groups have better positive charge dispersion than
does the pyridine group, which give them a stronger attraction
The exact mechanism of thermal ionic liquid degradation is
not straightforward since multiple parameters must be taken
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Scheme 2 Proposed mechanisms of thermal decomposition of 12b.
into account. Thus, the decomposition mechanisms are subject
to multiple simultaneous initiation reactions (hemolytic
cleavage and proton transfer), autocatalytic effects and nally,
competing reaction pathways with different reaction rates
dependent on temperature and the heating rate.43,44 The sug-
gested decomposition mechanisms and thermal degradation
products of 12b are shown in Scheme 2. It seems that, at the rst
stage of decomposition, the linkages of dicyanamide anions
with corresponding cations are cleavage. Aerwards the cations
are dissociated to smaller fragments such as imidazole deriva-
tives, amines, cyanides, ethylene, acetylene and etc.
Fig. 2 The ID test processes of ammonium-imidazolium-based ILs
shown with high speed camera photos.
Conclusions
A series of ammonium and imidazolium-based ILs as well as
unsymmetrical ammonium-imidazolium based dicationic ILs
containing dicyanamide anion were successfully synthesized
and their chemical structures were characterized by NMR, FTIR
spectroscopies and elemental analysis. Dianionic ILs are more
thermally stable than monocationic ones. In addition, the
Hypergolic test
To evaluate the potential hypergolic characteristics of the melting points of dianionic ILs are substantially higher than
synthesized dicyanamide-based ILs and measure their actual those of monocationic compounds, which is probably due to
ignition delay (ID) time, the droplet test was used.2,45
the better positive charge dispersion in dianionic ILs giving
Using a pipette, the IL sample (10–50 mL) was transferred them a stronger attraction to the anions.
into a vial containing excess (1.5 mL) white fuming nitric acid
The synthesized ILs showed tremendous hypergolic activi-
(WFNA). A high speed camera was used to record the ignition ties with WFNA. In the context of our drop test conditions, the
process. The dicationic compounds are semi-crystalline liquids dianionic ILs represent longer ignition duration probably due
or solids at room temperature (Table 1, entries 7–10), hence, we to the higher energy content of these salts.
decided to subject them to drop test while these ILs were heated
The ammonium and imdazolium based ILs are not liquid at
until clear liquids were obtained. In reality, the melted salts are room temperature, but they were subjected to the drop tests as
not classied as hypergolic. Nevertheless, they can be used as heated liquids. These salts cannot be labelled as hypergolic fuels,
additives in hypergolic room temperature IL mixtures to nevertheless they can be used as additives in hypergolic IL
improve their properties. Fig. 2 shows typical observation of the mixtures.
synthesized dicyanamide-based ILs in contact with WFNA. The
ID values of the studied ILs are represented in Table 1. The
reaction of ammonium and imidazolium-based ILs (Table 1,
Acknowledgements
entries 1, 2 and 3–6) are quite similar to those observed in We gratefully acknowledge the nancial support by Malek-
ammonium-imidazolium based salts (Table 1, entries 7–10).
As previously reported, the unsaturated side chains initiate or
promote hypergolic ignition.9 Therefore, among the synthesized
ILs, compounds with allyl or vinyl unsaturated substitute show
lower ID times. In general, the dianionic ILs (Table 1, entries 7–
10) showed longer ignition durations than the monocationic
ones (Table 1, entries 1, 2 and 3–6), which probably demonstrates
the higher energy content of the corresponding salts.
Ashtar University of Technology (MUT).
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