Synthesis and Properties of Azide-Functionalized Ionic Liquids as Attractive Hypergolic Fuels

Article abstract of DOI:10.1002/asia.201900364

Hypergolic ionic liquids (ILs) have shown a great promise as viable replacements for toxic and volatile hydrazine derivatives used as propellant fuels, and hence, have attracted increasing interest over the last decade. To take advantage of the reactivity and high energy density of the azido group, a family of low-cost and easily prepared azide-functionalized cation-based ILs, including fuel-rich anions, such as nitrate, dicyanamide, and nitrocyanamide anions, were synthesized and characterized. All the dicyanamide- and nitrocyanamide-based ILs exhibited spontaneous combustion upon contact with 100 % HNO₃. The densities of these hypergolic ILs varied in the range 1.11–1.29 g cm^?3, and the density-specific impulse, predicted based on Gaussian 09 calculations, was between 289.9 and 344.9 s g cm^?3. The values of these two key physical properties are much higher than those of unsymmetrical dimethylhydrazine (UDMH). Among the studied compounds, compound IL-3b, that is, 1-(2-azidoethyl)-1-methylpyrrolidin-1-ium dicyanamide, shows excellent integrated properties including the lowest viscosity (30.9 M Pa s), wide liquid operating range (?70 to 205 °C), shortest ignition-delay time (7 ms) with 100 % HNO₃, and superior density specific impulse (302.5 s g cm^?3), suggesting promising applications with potential as bipropellant formulations.

Full text of DOI:10.1002/asia.201900364

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Accepted Article
Title: Synthesis and Properties of Azide-functionalized Ionic Liquids as
Attractive Hypergolic Fuels
Authors: Zhenyuan Wang, Guangxing Pan, Binshen Wang, Ling
Zhang, Weiwei Zhao, Xing Ma, Jichuan Zhang, and Jiaheng
Zhang
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Synthesis and Properties of Azide-functionalized Ionic Liquids as
Attractive Hypergolic Fuels
Zhenyuan Wang§[a,b], Guangxing Pan§[a,b], Binshen Wang[a,b], Ling Zhang[a,b], Weiwei Zhao[a,b], Xing
Ma[a,b], Jichuan Zhang*[a,b], and Jiaheng Zhang*[a,b,c]
Dedication ((optional))
Abstract: Hypergolic ionic liquids (ILs) have shown a great promise
as viable replacements for toxic and volatile hydrazine derivatives as
propellant fuels, and hence, have attracted increasing interest over
the last decade. To take advantage of the reactivity and high energy
density of the azido group, a family of low-cost and easily-prepared
azide-functionalized-cation-based ILs, including fuel-rich anions, such
as nitrate, dicyanamide, and nitrocyanamide anions, were
synthesized and characterized. All the dicyanamide and
nitrocyanamide based ILs exhibited spontaneous combustion upon
contact with 100% HNO3. The densities of these hypergolic ILs varied
in the range of 1.11-1.29 g·cm-3, and the density-specific impulse,
predicted based on Gaussian 09 calculations, was between 289.9 and
344.9 s·g·cm-3. Both these values of two physical properties are much
higher than those of unsymmetrical dimethylhydrazine (UDMH).
Among the studied compounds, compound IL-3b, i.e., 1-(2-
azidoethyl)-1-methylpyrrolidin-1-ium dicyanamide, shows excellent
integrated properties including the lowest viscosity (30.9 m·Pas), wide
liquid operating range (-70 to 205°C), shortest ignition-delay time (7
ms) with 100% HNO3, and superior density specific impulse (302.5
s·g·cm-3), suggesting promising application potential as bipropellant
formulations.
for more environment-friendly hypergolic fuels has become a
necessity for space science.[7, 8]
In recent years, a novel concept of hypergolic ionic liquids (ILs)
has emerged to address the challenges offered by hydrazine
derivatives. This is because of their enhanced physico-chemical
properties, including extremely low volatility, low toxicity, good
thermal stability, high loading density, wide liquid range, short
ignition delay, and good thrust control.[9-13] Moreover, the
properties of hypergolic ILs can be readily varied and tuned
through the modification of the cationic and/or anionic
components. Hence, the development of new hypergolic ILs for
use as aerospace propellants has attracted significant attention.
The hypergolicity of some dicyanamide-based ionic liquids were
first discovered by Schneider and co-workers in 2008.[14] Since
then, several families of hypergolic ILs with different structures
-
16]
have been reported, including azide (N3 ),[15,
dicyanamide
17-21]
23]
([N(CN)2]-),[6,
nitrocyanamide
([N(NO2)CN]-),[22,
borohydride,[24-38] etc.
The heat of formation (HOF) is one of the crucial characteristics
of hypergolic ILs, which is directly related to the number of
nitrogen-nitrogen bonds in the molecule. Thus, searching for ILs
with a high number of nitrogen-nitrogen bonds should be a
promising direction toward the development of new hypergolic
39]
ILs.[10,
Besides, the design of hypergolic ILs is based on
Introduction
energetic materials. As often as possible, the structures of these
materials contain high-energy fragments such as a nitrogen-
nitrogen bond, an azido group, or an imidazole or triazole ring,
often with unsaturated substituents. It has been found using
theoretical calculations that the azido group is highly energetic,
adding approximately 280 kJ·mol−1 to the energy content of a
In the technology-intensive aviation and aerospace fields,
hypergolic liquid propellants have been a subject of in-depth
research over the past decades because of their high specific
impulse, excellent ignition with a low ignition-delay (ID) time, and
superb thrust control.[1, 2] The current choices of hypergolic liquid
propellants are primarily focused on hydrazine and its derivatives,
such as monomethyl hydrazine (MMH) and unsymmetrical
dimethylhydrazine (UDMH). However, these hydrazine-based
fuels are highly toxic due to their volatility and carcinogenic
properties, which leads to expensive storage and handling
procedures and costly safety precautions.[3-6] Hence, the search
molecule, which is highly favorable for the design of novel
40]
hypergolic ILs.[16,
In 2008, Schneider et al. reported the
formation of several imidazolium-based azides with saturated and
unsaturated side chains.[15] However, when treated with red-
fuming nitric acid and N2O4, all the liquid azide salts reacted
vigorously with copious production of red fumes, but without
ignition. Shortly thereafter, Shreeve et al. designed and
synthesized some azide-functionalized hypergolic ILs that contain
the
2-azido-N,N,N-trimethylethylammonium
cation
with
nitrocyanamide, dicyanamide, dinitramide or azide anion by
metathesis reactions.[16] Among these, four hypergolic ILs
exhibited hypergolic activity with WFNA, with high values of HOFs
ranging from 380 to 894 kJ·mol-1. Interestingly, HOFs of the bis-
azido-substitued ILs were nearly twice that of the mono-azido-
substituted ones with the same anion, showing significant
contribution of the azido group for the HOF of ILs. Besides, the
[a]
Z. Wang, G. Pan, B. Wang, L. Zhang, J. Zhang, W. Zhao, X. Ma, J.
Zhang, State Key Laboratory of Advanced Welding and Joining,
Harbin Institute of Technology, Shenzhen, 518055, China.
E-mail: jiahengzhang@hit.edu.cn
Z. Wang, G. Pan, B. Wang, L. Zhang, J. Zhang, W. Zhao, X. Ma, J.
Zhang, Research Centre of Flexible Printed Electronic Technology,
Harbin Institute of Technology, Shenzhen, 518055, China
J. Zhang, Zhuhai Institute of Advanced Technology Chinese
Academy of Sciences, Zhuhai, 519000, China
[b]
[c]
2-azido-N,N,N-trimethylethylammonium
nitrocyanamide
exhibited the shortest ID of 8 ms, which is the same as that
exhibited by monomethyl-hydrazine. However, 2-azido-N,N,N-
trimethylethylammonium nitrocyanamide stayed in the solid state
Supporting information for this article is given via a link at the end of
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in a wide range of room temperatures, which may limit its
application scope.
structures and purities of whom were in good agreement with that
expected.
To take advantage of the reactivity and high energy density of
the azido group, a series of low-cost, easily prepared, and
environment-friendly azide-functionalized cations were designed
and synthesized from commercially available materials.
Meanwhile, the above mentioned cations were combined with
anions that have been previously employed in ionic-liquid-fuel-
based hypergolic mixtures, i.e., nitrate, dicyanamide and
Crystal structure
Single crystals of IL-5a were obtained through slow
recrystallization from an ethanol solution, and a suitable crystal
was selected for XRD analysis. The structure is shown in Figure
1 and the crystallographic data are summarized in Table S4 in the
Supporting Information. The X-ray crystallographic analysis
revealed that IL-5a crystallized in the monoclinic Pbca space
group with the lattice parameters of a=12.2781(7), b=12.6945(7),
and c=13.2741(6) Å; α=90°, β=90°, and γ=90°; ρ=1.498 g·cm-3
and Z=8. In the structure of the cation, the azide group was
connected to the nitrogen atom of morpholine ring through an
ethyl group (-CH2-CH2-). The cations and anions interacted with
each other through electrostatic interactions to form the
supramolecular structure of IL-5a.
nitrocyanamide anions, to form
a novel family of azide-
functionalized hypergolic ILs. These hypergolic ILs exhibited a
wide liquid operating range, superb density specific impulse, high
density, low viscosity, and simultaneously fast ID times (as short
as 7 ms).
Results and Discussion
Synthesis
Some readily available and relatively inexpensive starting
materials were used for the synthesis of the ILs via
a
straightforward three-step procedure as illustrated in Scheme 1.
In the first step, the reactions of five types of tertiary amine
hydrochlorides (i.e., N,N-diethyl-2-chloroethylanine hydrochloride,
N,N-dimethyl-3-chloropropylamine
hydrochloride,
2-
pyrrolidinoethyl chloride hydrochloride, 2-piperidinoethyl chloride
hydrochloride, and 2-morpholinoethyl chloride hydrochloride) with
sodium azide complex in water were conducted for the formation
of azide substituted tertiary amine. Subsequently, quaternization
reactions of azide substituted tertiary amines with MeI led to the
formation of azide substituted quaternary ammonium iodides.
Finally, fifteen azide-functionalized ILs were generated via
metathesis reactions of azide-substituted quaternary ammonium
iodides with silver nitrate, silver dicyanamide or silver
nitrocyanamide. 1H and 13C NMR spectroscopy, X-ray
crystallographic analysis, infrared spectroscopy and elemental
analysis were conducted for the characterization of resulting ILs,
Figure 1. ORTEP diagram of IL-5a.
Scheme 1. Synthesis of the studied ILs.
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Besides, X-ray diffraction analysis of three iodide precursors
with the same cations as IL-1, IL-2, and IL-3 was also conducted.
The structures and crystallographic data are summarized in
Figure S2 in the Supporting Information.
the other hand, in the case of the same anion, the densities of ILs
did not exhibit a similar trend.
Thermal properties
The phase transition temperature (Tg/Tm) and thermal
decomposition temperature (Td), determine the liquid operating
range, and hence, are the vital parameters for propellant fuels.
The values of Tg/Tm and Td were determined by means of
differential scanning calorimetric (DSC) measurements at the
heating rate of 5°C ·min-1. As shown in Table 1, all the fifteen salts
were ionic liquids based on the definition of ionic liquids as the
ones having a melting point below 100°C . To become a fuel
component of a hypergolic mixture, it is desirable for a compound
to exhibit a wide liquid-phase range in order to permit application
of the fuel at extreme temperatures. Table 1 shows that most of
the ILs (except IL-1a, IL-2b and IL-5a) were liquids at room
temperature and their liquid operating ranges were relatively wide
(>270°C ), which are superior to those of unsymmetrical dimethyl
Figure 2. Densities of the studied ILs.
hydrazine
(UDMH,
121°C )
and
2-azido-N,N,N-
trimethylethylammonium nitrocyanamide (217°C ).[16] Much the
same as most traditional ILs, all of the hypergolic ILs have no
boiling point and only undergo an exothermic decomposition upon
heating at high temperature.[10] Hence the decomposition
temperature (Td) directly decides the maximum allowable
operating temperature of hypergolic ILs. Generally speaking, an
ideal Td value of a hypergolic IL is expected to be over 200°C.[27]
It can be seen from Table 1 that Td values of all these ILs lay within
the range of 200.5-211.2°C, showing that they had a high thermal
stability. Moreover, the anions were found to have a significant
effect on Td. In the case of the same cation, Td values of
dicyandiamine-based ILs (IL-b series) were the lowest, while the
nitrocyanamide-based ILs (IL-c series) possessed the highest
values of Td. In contrast, no definite correlations were observed
between cation/anion and the phase transition temperature
(Tg/Tm).
Scheme 2. Born-Haber cycle for the formation of ILs; the number of moles of
the respective products are given by a, b, c, and d.
Viscosity and specific impulse
In a bipropellant system, low viscosities can facilitate rapid
mass and heat transfer between the fuel and oxidizer in the mixing
process; thereby, favoring the inducement of spontaneous
initiation and combustion reactions. Therefore, a low viscosity of
hypergolic ILs is one of the vital properties that meets the practical
application criteria required for well-performing fuels. The
viscosities of these azide-substituted ILs are listed in Table 1. The
anions also significantly influenced viscosities, and a similar trend
the one for densities was found (viscosities: nitrate-
Density
In addition to thermal properties, density is another vital
parameter for evaluating energy level of the propellant fuels. In
general, a fuel with higher density not only allows more fuels to
be packed into a given volume of the fuel tank, but also increases
the performance through higher Isp values, thereby contributing
higher energy and combustion contribution to the propulsion
process. Figure 2 and Table 1 show that the densities of all ILs
were greater than 1.10 g·cm-3, which are superior to the traditional
UDMH (0.79 g·cm-3); thus, suggesting that these new ILs also
possess higher loading capacity than the traditional hydrazine-
based fuels. Besides, Table 1 shows that anions have a
significant effect on the densities. In particular, densities of
dicyandiamine-based ILs were in the range of 1.11 to 1.20g·cm-3,
which were significantly lower than that for nitrocyanamide-based
one (1.19 to 1.29 g·cm-3), while the nitrate-based ILs exhibited the
highest densities of all the studied ones (1.20 to 1.31 g·cm-3). On
based>nitrocyanamide-based>dicyandiamine-based).
Among
these, IL-3b depicted the lowest viscosity of 30.9 mPa·s, which is
lower than those for known azide-functionalized ILs.[10, 21]
The theoretical calculations were performed using the
Gaussian 09 suite of programs.[42] Heats of formation (HOF, ΔfH)
of these newly synthesized ILs were calculated based on the
Born-Haber energy cycle (Scheme 2) and isodesmic reactions.
The computational processes and details are provided in the
Supporting Information, while the corresponding results are listed
in Table S1-S3. By using the calculated heats of formation and
the experimentally measured densities, the values of the specific
impulse (Isp) and the density impulse (ρIsp) were calculated with
Explo5 v6.02 software.[43] Table 1 shows that the density impulse
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of all the azide-functionalized ILs were greater than 289 s·g·cm-3,
and much higher than that of UDMH (215.7 s·g·cm-3),[20] indicating
that these newly synthesized azide-functionalized ILs have
excellent application potential.
Table 1. Physicochemical properties of 15 ILs.
[a]
[b]
[f]
[g]
[h]
Tg/Tm
(°C)
Td
η[c]
ρ[d]
ID[e]
ms
ΔfHsalt
Isp
ρIsp
ILs
(°C)
mPa·s
g·cm-3
kJ·mol-1
s
s·g·cm-3
1a
1b
1c
2a
2b
2c
3a
3b
3c
4a
4b
4c
5a
5b
5c
26.8
<-70
<-70
<-70
3.5
204.1
200.5
211.2
206.5
204.2
210.5
206.2
205.0
210.2
208.2
209.0
210.5
207.0
205.7
210.9
-
1.20
1.11
1.19
1.22
1.11
1.20
1.25
1.16
1.23
1.21
1.15
1.20
1.31
1.22
1.30
-
15
95
-
113.03
309.96
406.66
59.49
268.3
262.6
267.8
267.4
261.1
267.0
266.4
260.8
266.1
266.0
261.0
265.8
265.5
260.0
265.3
322.0
291.5
318.7
326.2
289.8
320.4
333.0
302.5
327.3
321.9
300.2
316.3
347.8
317.2
344.9
59.2
93.4
279.9
42.8
71.8
334.1
30.9
62.1
518.4
84.5
93.1
-
12
65
-
258.79
355.19
110.54
307.31
405.38
91.52
<-70
<-70
<-70
<-70
<-70
<-70
<-70
95.8
<-70
<-70
7
201
-
25
75
-
285.17
383.46
2.25
310.8
404.6
17
80
198.39
295.24
[a] Glass transition temperature (Tg)/melting point (Tm). [b] Decomposition temperature (onset). [c] Viscosity at 25°C . [d] Density at 25°C . [e] Ignition-delay (ID) time
with 100% HNO3. [f] Heat of formation. [g] Specific impulse (Explo5 v6.02) at the optimum oxidizer (100% HNO3)-to-fuel ratio; isobaric conditions, equilibrium
expansion, 7.0 MPa chamber pressure. [h] Density impulse.
Hypergolic test
process performed on IL-3b. Following the contact of IL-3b with
100% HNO3, a drastic oxidation-reduction reaction occurred
immediately, and a hypergolic ignition with a flame kernel was
clearly observed after 7 ms.
Figure 3. Droplet test recorded for IL-3b with 100% HNO3.
As potential hypergolic fuels, evaluating the hypergolic
reactivity with bipropellant oxidizers is crucial in determining
whether the fuel itself is suitable for practical propellant
applications. In general, hypergolic reactivity is estimated by
recording the ID time, which is defined as the time interval
between the instant when the fuel droplet comes into contact with
the oxidizer liquid pool surface and the instant when a flame
emerges. The shorter ID times can contribute positively towards
higher combustion efficiency in the propellant fuels. To investigate
the ID times of ILs, a droplet test with newly prepared 100% HNO3
as oxidizer was performed and the visible ignition was captured
using high-speed camera at 2000 fps (each sample was tested
three times). Figure 3 shows a representation of the droplet test
Figure 4. Ignition delay (ID) times of the studied ILs.
The ID times of the newly synthesized ILs are shown in Figure
4 and Table 1 without the nitrate-based ILs (IL-a series), which
can not be ignited. In principal, an acceptable time for ignition
delay in the real life applications is less than 50 ms.[19] Compared
to the traditional hypergolic ILs, such as [BMIm]N(CN)2 (ID=46
ms), five of the synthesized dicyandiamine-based ILs (IL-b series)
showed shorter ID times (7-25 ms). Meanwhile, to the best of our
knowledge, IL-3b demonstrated the shortest ID time (7 ms) of all
the azide-functionalized ILs that have ever been reported. Studies
have shown that the fuel-rich anion of an IL plays a decisive role
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(3V, 80 mL), dried over Na2SO4, filtered, and concentrated in a rotary
evaporator under vacuum. 2-azido-N,N-diethylethan-1-amine was
obtained with a yield of 96.48% (1.37 g).
2-Azido-N,N-diethyl-N-methylethan-1-aminium iodide: An acetonitrile
solution of iodomethane (1.5 mol·L-1, 20 mL, 30 mmol) was added
dropwise to the ice-cold acetonitrile solution of 2-azido-N,N-diethylethan-
1-amine. Then the mixture was stirred and reacted at 0°C for 24 h.
Afterwards, the solvent and excess iodomethane were removed under
reduced pressure, which afforded 2-Azido-N,N-diethyl-N-methylethan-1-
aminium iodide in excellent yield (2.63g, 96.13%).
in inducing hypergolicity, while the cation plays a secondary role,
i.e., subtly affecting the ID time. By analyzing the factors that
might affect the ID time during the hypergolic test, viscosity of the
hypergolic IL is one of the most-important factors. The shorter ID
times of dicyandiamine-based ILs (IL-b series) may be partly
attributed to their relatively lower viscosities than the
nitrocyanamide-based ILs (IL-c series), which makes the rapid
mixing and an oxidation-reduction reaction between the IL and
100% HNO3 relatively much easier and faster.
2-Azido-N,N-diethyl-N-methylethan-1-aminium
dicyanamide:
The
AgN(CN)2 (1.76g, 10 mmol) was added into an aqueous solution of 2-
azido-N,N-diethyl-N-methylethan-1-aminium iodide, and stirred at room
Conclusions
temperature for 24
h without light emission. After filtration and
concentration, colorless liquid of IL-1b (2-azido-N,N-diethyl-N-
a
In summary, 15 novel azide-functionalized-cation-based ILs
were designed and synthesized, and 10 of the dicyanamide-
based and nitrocyanamide-based ILs exhibited decent hypergolic
properties upon contact with 100% HNO3. The hypergolic ILs
displayed excellent physiochemical properties including low
melting points, high thermal stability, low viscosities, and unique
hypergolic reactivity. Besides, compound IL-3b showed the best
integrated properties including the lowest viscosity and the
shortest ignition-delay time with 100% HNO3 among all the azide-
functionalized ILs that have been reported to the best of our
knowledge. Furthermore, the cost-effective and scale-up
advantages of IL-3b highlights its promising potential as a
replacement for toxic and volatile hydrazine derivatives.
methylethan-1-aminium dicyanamide) was formed (1.98g, 95.60%). The
other ILs were also prepared following the same procedure.
IL-1a: white solid; 88.20% yield; 1H NMR (400 MHz, D2O) δ 3.85 (t, J=5.8
Hz, 2H), 3.52-3.41 (m, 2H), 3.36 (q, J=7.3 Hz, 4H), 2.98 (s, 3H), 1.27 (t,
J=7.3 Hz, 6H); 13C NMR (101 MHz, D2O) δ 58.31 (s), 57.18 (s), 47.32 (s),
44.18 (s), 7.04 (s); IR (KBr): 3425, 2984, 2396, 2104, 1762, 1384, 826,
553; elemental analysis calcd (%) for C7H17N5O3: C 38.35, H 7.82, N 31.94;
found: C 38.28, H 7.89, N 31.90.
IL-1b: colorless liquid; 88.66% yield; 1H NMR (400 MHz, D2O) δ 3.92 (t,
J=5.7 Hz, 2H), 3.56-3.48 (m, 2H), 3.43 (q, J=7.3 Hz, 4H), 3.05 (s, 3H), 1.35
(t, J=7.2 Hz, 6H); 13C NMR (101 MHz, D2O) δ 120.07 (s), 58.41 (s), 57.29
(s), 47.43 (s), 44.27 (s), 7.16 (s); IR (KBr): 3439, 2235, 2138, 1637, 1460,
1312, 526; elemental analysis calcd (%) for C9H17N7: C 48.41, H 7.67, N
43.91; found: C 48.36, H 7.77, N 43.87.
IL-1c: colorless liquid; 87.96% yield; 1H NMR (400 MHz, D2O) δ 3.88 (t,
J=5.8 Hz, 2H), 3.50-3.43 (m, 2H), 3.38 (q, J=7.3 Hz, 4H), 3.00 (s, 3H), 1.30
(t, J=7.3 Hz, 6H); 13C NMR (101 MHz, D2O) δ 116.20 (s), 58.34 (s), 57.22
(s), 47.37 (s), 44.21 (s), 7.07 (s); IR (KBr): 3434, 2171, 2107, 1635, 1427,
1266, 1154, 548; elemental analysis calcd (%) for C8H17N7O: C 42.28, H
7.54, N 43.14; found: C 42.20, H 7.61, N 43.09.
Experimental Section
Materials
IL-2a: colorless liquid; 88.07% yield; 1H NMR (400 MHz, D2O) δ 3.50 (t,
J=6.4 Hz, 2H), 3.46-3.38 (m, 2H), 3.14 (s, 9H), 2.16-2.03 (m, 2H); 13C NMR
(101 MHz, D2O) δ 64.08 (s), 53.14-52.72 (m), 47.83 (s), 22.28 (s); IR (KBr):
3427, 2955, 2401, 2102, 1763, 1379, 825, 554; elemental analysis calcd
(%) for C6H15N5O3: C 35.12, H 7.37, N 34.13; found: C 35.09, H 7.46, N
34.07.
IL-2b: colorless liquid; 88.02% yield; 1H NMR (400 MHz, D2O) δ 3.51 (t,
J=6.4 Hz, 2H), 3.48-3.40 (m, 2H), 3.15 (s, 9H), 2.17-2.05 (m, 2H); 13C NMR
(101 MHz, D2O) δ 120.08 (s), 64.12 (s), 53.18-52.77 (m), 47.84 (s), 22.32
(s); IR (KBr): 3454, 2241, 2135, 1644, 1485, 1310, 526; elemental analysis
calcd (%) for C8H15N7: C 45.92, H 7.23, N 46.86; found: C 45.82, H 7.28,
N 46.90.
IL-2c: colorless liquid; 87.89% yield; 1H NMR (400 MHz, D2O) δ 3.51 (t,
J=6.4 Hz, 2H), 3.48-3.40 (m, 2H), 3.15 (s, 9H), 2.18-2.04 (m, 2H); 13C NMR
(101 MHz, D2O) δ 116.21 (s), 64.05 (s), 53.12-52.70 (m), 47.79 (s), 22.26
(s); IR (KBr): 3424, 2172, 2108, 1631, 1427, 1270, 1153, 547; elemental
analysis calcd (%) for C7H15N7O: C 39.43, H 7.09, N 45.98; found: C 39.32,
H 7.17, N 45.88.
IL-3a: colorless liquid; 88.28% yield; 1H NMR (400 MHz, D2O) δ 3.94 (t,
J=5.5 Hz, 2H), 3.66-3.45 (m, 6H), 3.09 (s, 3H), 2.22 (s, 4H); 13C NMR (101
MHz, D2O) δ 65.18 (s), 61.99 (s), 48.17 (s), 45.31 (s), 21.07 (s); IR (KBr):
3433, 2983, 2398, 2138, 1762, 1371, 826, 555; elemental analysis calcd
(%) for C7H15N5O3: C 38.70, H 6.96, N 32.24; found: C 38.64, H 7.03, N
32.20.
N,N-diethyl-2-chloroethylanine hydrochloride (99%), N,N-dimethyl-3-
chloropropylamine hydrochloride (97%), 2-pyrrolidinoethyl chloride
hydrochloride (98%), 2-piperidinoethyl chloride hydrochloride (98%), 2-
morpholinoethyl chloride hydrochloride (99%), sodium dicyanamide (95%),
sodium azide (98%), silver nitrate (99%), and N-methyl-N-nitroso-N-
nitroguanidine (99%) were purchased from Aladdin. Silver nitrocyanamide
and silver dicyanamide were synthesized using methods reported in
previous literatures.[23, 41] All other chemicals and organic solvents were
obtained commercially as analytical grade materials and were used as
received.
Product characterization
1H and 13C NMR spectra were recorded with a 400 MHz (Bruker
AVANCE 400) spectrometer in D2O. The thermal property measurements
of ILs were performed using a TGA/DSC1 and a DSC1 Mettler Toledo
calorimeter at a heating rate of 5°C ·min-1. The density measurements were
performed at 25°C on a Mettler Toledo Densito 30PX densimeter. The
elemental analysis was conducted on a Flash EA-1112 elemental analyzer,
while the viscosity measurements were performed at 25°C on a Brook field
Rheometer DV3T. The ignition photographs of these newly synthesized
ILs with the oxidizer of 100% HNO3 were recorded on an Olympus i-speed
3 camera at 2000 fps.
IL-3b: colorless liquid; 88.30% yield; 1H NMR (400 MHz, D2O) δ 3.96 (t,
J=5.5 Hz, 2H), 3.69-3.49 (m, 6H), 3.11 (s, 3H), 2.24 (s, 4H); 13C NMR (101
MHz, D2O) δ 120.09 (s), 65.23 (s), 62.03 (s), 48.23 (s), 45.34 (s), 21.11
(s); IR (KBr): 3457, 2241, 2132, 1642, 1462, 1309, 933, 529; elemental
analysis calcd (%) for C9H15N7: C 48.85, H 6.83, N 44.31; found: C 48.79,
H 6.90, N 44.31.
IL-3c: colorless liquid; 88.12% yield; 1H NMR (400 MHz, D2O) δ 3.92 (t,
J=5.6 Hz, 2H), 3.64-3.41 (m, 6H), 3.08 (s, 3H), 2.20 (s, 4H); 13C NMR (101
MHz, D2O) δ 116.21 (s), 65.17 (s), 61.97 (s), 48.14 (s), 45.30 (s), 21.04
Synthesis of the ILs
2-Azido-N,N-diethylethan-1-amine: The sodium azide (1.95g, 30 mmol)
was added slowly into an aqueous solution of N,N-diethyl-2-
chloroethylanine hydrochloride (0.1 mol·L-1, 100 mL, 10 mmol) under
stirring at room temperature. The mixture was then heated to 80°C and
stirred overnight. After cooling to room temperature, NaOH was added until
the pH was equal to 10. Thereafter, the mixture was extracted with CH2Cl2
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(s); IR (KBr): 3424, 2172, 2111, 1635, 1434, 1274, 1155, 546; elemental
analysis calcd (%) for C8H15N7O: C 42.66, H 6.71, N 43.53; found: C 42.62,
H 6.78, N 43.49.
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Y. Ishikawa and M. J. McQuaid, J. Mol. Struct. THEOCHEM 2007, 818,
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W. Daimon, Y. Gotoh and I. Kimura, J. Propul. Power 1991, 7, 946-952.
O. d. Bonn, A. Hammerl, T. M. Klapötke, P. Mayer, H. Piotrowski and H.
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IL-4a: colorless liquid; 88.17% yield; 1H NMR (400 MHz, D2O) δ 3.95 (t,
J=5.5 Hz, 2H), 3.69-3.52 (m, 2H), 3.52-3.32 (m, 4H), 3.13 (s, 3H), 2.01-
1.80 (m, 4H), 1.79-1.57 (m, 2H); 13C NMR (101 MHz, D2O) δ 62.02 (s),
61.33 (s), 48.21 (s), 44.11 (s), 20.43 (s), 19.54 (s); IR (KBr): 3432, 2967,
2405, 2141, 1760, 1375, 822, 556; elemental analysis calcd (%) for
C8H17N5O3: C 41.55, H 7.41, N 30.28; found: C 41.49, H 7.44, N 30.25.
IL-4b: colorless liquid; 88.19% yield; 1H NMR (400 MHz, D2O) δ 3.93 (t,
J=5.7 Hz, 2H), 3.62-3.52 (m, 2H), 3.50-3.31 (m, 4H), 3.11 (s, 3H), 1.98-
1.82 (m, 4H), 1.77-1.57 (m, 2H); 13C NMR (101 MHz, D2O) δ 120.04 (s),
62.06 (s), 61.36 (s), 48.29 (s), 44.14 (s), 20.46 (s), 19.58 (s); IR (KBr):
3464, 2233, 2138, 1638, 1465, 1304, 529; elemental analysis calcd (%)
for C10H17N7: C 51.05, H 7.28, N 41.67; found: C 51.01, H 7.37, N 41.62.
IL-4c: colorless liquid; 88.03% yield; 1H NMR (400 MHz, D2O) δ 3.92 (t,
J=5.6 Hz, 2H), 3.65-3.51 (m, 2H), 3.50-3.28 (m, 4H), 3.10 (s, 3H), 1.97-
1.76 (m, 4H), 1.75-1.50 (m, 2H); 13C NMR (101 MHz, D2O) δ 116.22 (s),
61.99 (s), 61.29 (s), 48.22 (s), 44.08 (s), 20.39 (s), 19.51 (s); IR (KBr):
3469, 2174, 2105, 1635, 1429, 1269, 1154, 547; elemental analysis calcd
(%) for C9H17N7O: C 45.18, H 7.16, N 40.98; found: C 45.12, H 7.23, N
40.93.
IL-5a: white solid; 88.11% yield; 1H NMR (400 MHz, D2O) δ 4.16-3.91 (m,
6H), 3.76-3.67 (m, 2H), 3.66-3.47 (m, 4H), 3.28 (s, 3H); 13C NMR (101
MHz, D2O) δ 62.91 (s), 60.34 (s), 47.49 (s), 43.97 (s); IR (KBr): 3429, 2959,
2400, 2117, 1764, 1382, 825, 555; elemental analysis calcd (%) for
C7H15N5O4: C 36.05, H 6.48, N 30.03; found: C 36.01, H 6.59, N 29.95.
IL-5b: colorless liquid; 88.04% yield; 1H NMR (400 MHz, D2O) δ 4.13-3.94
(m, 6H), 3.76-3.67 (m, 2H), 3.66-3.48 (m, 4H), 3.27 (s, 3H); 13C NMR (101
MHz, D2O) δ 120.08 (s), 62.94 (s), 60.37 (s), 47.58 (s), 44.01 (s); IR (KBr):
3455, 2237, 2137, 1638, 1473, 1309, 1128, 881, 529; elemental analysis
calcd (%) for C9H15N7O: C 45.56, H 6.37, N 41.32; found: C 45.53, H 6.46,
N 41.23.
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IL-5c: colorless liquid; 87.93% yield; 1H NMR (400 MHz, D2O) δ 4.12-3.91
(m, 6H), 3.73-3.66 (m, 2H), 3.64-3.46 (m, 4H), 3.25 (s, 3H); 13C NMR (101
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3489, 2174, 2115, 1633, 1438, 1271, 1128, 550; elemental analysis calcd
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This work was supported by the National Natural Science
Foundation of China (21703218), the Shenzhen Science and
Technology Innovation Committee (JCYJ20151013162733704),
Economic, Trade and Information Commission of Shenzhen
Municipality through the Graphene Manufacture Innovation
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Keywords: Hypergolic • Propellant • Azide-functionalized • Ionic
liquids • Ignition delay time
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§These authors contributed equally.
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Notes
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Entry for the Table of Contents (Please choose one layout)
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Text for Table of Contents
Zhenyuan Wang, Guangxing Pan,
Binshen Wang, Ling Zhang, Weiwei
Zhao, Xing Ma, Jichuan Zhang, and
Jiaheng Zhang*
Alternative hypergolic fuels:
A
family of novel azide-functionalized-
cation-based hypergolic ionic liquids
were designed and synthesized from
cost-effective materials. These ionic
Page No. – Page No.
((Insert TOC Graphic here: max.
width: 5.5 cm; max. height: 5.0 cm))
Synthesis and Properties of Azide-
functionalized Ionic Liquids as
Attractive Hypergolic Fuels
liquids displayed excellent properties
including low melting points, high
thermal stability, low viscosities, and
unique
hypergolic
reactivity,
demonstrating their promise as
potential alternative hypergolic fuels to
toxic and volatile hydrazine derivatives.
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