Nitrato-Functionalized Task-Specific Ionic Liquids as Attractive Hypergolic Rocket Fuels

Article abstract of DOI:10.1002/chem.201701804

Hypergolic ionic liquids (HILs) as potential replacements for hydrazine derivatives have attracted increasing interest over the last decade. Previous studies on HILs have mostly concentrated on the anionic innovations of ionic liquids to shorten the ignition delay (ID) time, but little attention has been paid to cationic modifications and their structure–property relationships. In this work, we present a new strategy of cationic functionalization by introducing the energetic nitrato group into the cationic units of HILs. Interestingly, the introduction of oxygen-rich nitrato groups into the cationic structure significantly improved the combustion performance of HILs with larger flame diameters and duration times. The density-specific impulse (ρI_sp) of these novel HILs are all above 279.0 s g cm^?3, much higher than that of UDMH (215.7 s g cm^?3). In addition, the densities of these HILs are in the range of 1.22–1.39 g cm^?3, which is much higher than that of UDMH (0.79 g cm^?3), showing their higher loading capacity than hydrazine-derived fuels in a propellant tank. This promising strategy of introducing nitrato groups into the cationic structures has provided a new platform for developing high-performing HILs with improved combustion properties.

Full text of DOI:10.1002/chem.201701804

A Journal of
Accepted Article
Title: Nitrato-Functionalized Task-Specific Ionic Liquids as Attractive
Hypergolic Rocket Fuels
Authors: Qinghua Zhang, Yi Wang, Shi Huang, Wenquan Zhang,
Tianlin Liu, and Xiujuan Qi
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Nitrato-Functionalized Task-Specific Ionic Liquids as Attractive
Hypergolic Rocket Fuels
Yi Wang,[a] Shi Huang,[a] Wenquan Zhang,[a] Tianlin Liu,[a] Xiujuan Qi,[b] and Qinghua Zhang*[a]
Abstract: Hypergolic ionic liquids (HILs) as potential replacements
for hydrazine derivatives have attracted increasing interest over the
last decade. Previous studies on HILs are mostly concentrated on
the anionic innovations of ionic liquids to shorten the ignition delay
(ID) time, but few attentions have been paid to the cationic
modifications and their structure-property relationships. In this work,
extremely low volatility and excellent hypergolic properties. In
the pioneering work, Schneider and co-workers firstly
demonstrated that some dicyanamide hypergolic ionic liquids
with the allyl and propargyl functionalities can spontaneously
ignite rapidly when they are mixed with white fuming nitric acid
(WFNA).[5] After that, a number of TSILs with hypergolic
properties have been developed and evaluated for potential
applications of liquid bipropellant fuels. Shreeve et al. have
reviewed on a variety of TSILs with different structures which
showed the potential applications for hypergolic fuels.[6] As
compared with traditional hydrazine-based fuels, these TSILs as
hypergolic fuels exhibit some promising advantages including
low vapor toxicity, high thermal stability, low fire hazard and
corrosivity, and good thrust control, etc.[7] In addition, unlike
traditional hydrazine derivatives, the structural designability of
we presented
a new strategy of cationic functionalization by
introducing the energetic nitrato group into the cationic units of HILs.
Interestingly, the introduction of oxygen-rich nitrato groups into the
cationic structure has obviously improved the combustion
performance of HILs with larger flame diameters and duration times.
The density-specific impulse (ρIsp) of these novel HILs are all
above 279.0 s•g•cm-3, much higher than that of UDMH (215.7
s•g•cm-3). In addition, the densities of these HILs are in range of
1.22-1.39 g•cm-3, which is much higher than that of UDMH (0.79
g•cm-3), showing their higher loading capacity than hydrazine-
derived fuels in propellant tank. This promising strategy of
introducing nitrato group into the cationic structures has provided a
new platform for developing high-performing HILs with improved
combustion properties.
TSILs have provided
a promising platform towards new
hypergolic fuels, in which the cationic or anionic components
can be independently designed or modified for a specific need.
Thus, the search for new HILs as the substitutes of hydrazine
derivatives has become
a
promising strategy for the
development of next-generation hypergolic fuels.
Introduction
In the field of aerospace and aviation technologies, propellants
have been the subject of intensive research over the past few
decades because they are the main power sources in the
propulsion systems of missiles and rockets.[1] Nowadays,
hydrazine and its methylated derivatives are still widely used as
the liquid hypergolic fuels in some bipropellant systems, but their
high volatility, toxicity and handling costs are already beyond the
need for the ever-growing environmental and safety issues.[2-4]
Against this background, there is an urgent and continuing need
for exploring new hypergolic fuels as replacements of hydrazine
derivatives.
Since 2008, the technical innovations in the field of ionic
liquids (ILs) have been attempted for developing safer and more
environment-friendly bipropellant fuels. Pioneering studies have
demonstrated that using task-specific ionic liquids (TSILs) as
replacements for hydrazine derivatives is a viable option for the
explorations of new-generation liquid rocket fuels due to their
Figure 1. (A) The typical cations for known HILs; (B) the representative anions
for known HILs; (C) the evolution of the ignition delay (ID) times of some
typical HILs with white fuming nitric acid (WFNA).
[a]
Dr. Y. Wang, Dr. S. Huang, Dr. W. Zhang, Dr. T. Liu, Prof. Q. Zhang.
Research Center of Energetic Material Genome Science, Institute of
Chemical Materials, China Academy of Engineering Physics (CAEP).
Mianyang, 621900 (China).
By systematically analyzing the cationic and anionic
structures of previously reported HILs,[8-26] we have found that
the typical cations of known HILs are mainly concentrated on
some quaternary ammonium cations (e.g., 1-alkyl-3-methyl-
imidazolium,1-alkyl-pyridinium, N-methyl-N-alkyl-pyrrolidinium,
N-alkyl-trimethyl-ammonium in Figure 1A), while the anions have
undergone a rapid evolution from traditional dicyanamide anion
([N(CN)2]-)7 to strongly reducing borohydride-rich anions (e.g.,
[BH4]-, [BH3CN]-, [Al(BH4)4]-, [BH2(CN)2]-, [BH3(CN)BH2(CN)]- and
[H2P(BH3)2]-).[17-25] As a result, the ignition delay (ID) times of
E-mail: qinghuazhang@caep.cn
[b]
Dr. X. Qi.
School of Material Science and Engineering, Southwest University of
Science and Technology, Mianyang, 621900 (China).
Supporting information for this article is given via a link at the end of
the document.
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some known HILs with the common oxidizer of white fuming
nitric acid (WFNA) have a great breakthrough and the present
shortest ID time has reached 1 ms (Figure 1C), [24] which is much
shorter than that of unsymmetric dimethyl hydrazine (UDMH)
(4.8 ms) under same conditions.[20] These previous studies are
mainly focused on the design and synthesis of new hypergolic
anions with the aim of shortening the ID times of HILs , however,
the cationic modifications have been ignored for a long time. In
fact, as an important component of HILs, cationic structure has
also very important effect on the combustion properties.
Unfortunately, the knowledge of how cationic structure (e.g.,
quaternary ammonium type substituted functional groups)
affecting the hypergolic properties of HILs is still very limited. In
this context, studies on the relationships between cationic
structures and combustion properties should be strengthened.
It is well known that the essence of spontaneous combustion
between HILs and oxidizers is an oxidation-reduction reaction, in
effects on the subsequent igniting process of HILs and thereby
improve the ignition and combustion efficiency. In addition, a
series of traditional dicyanamide-based HILs as control fuels are
synthesized and comparatively studied. In this work, tremendous
efforts were made to study three series of TSILs based on
nitrato- and hydroxyl-functionalized cations as shown in Scheme
1 (Series I-III), and the detailed comparison of their differences
in thermal properties, densities, viscosities, ignition delay times,
and combustion duration were particularly emphasized.
Results and Discussion
First, we synthesized three series of dicyanamide-based HILs
according to typical metathesis reactions (Series I-III, Scheme 1).
The precursors of HIL-1 to HIL-4 were synthesized by literature
[23, 29]
procedures
and the others were purchased from
commercial sources. Except HIL-1 and HIL-12 (m.p. 60.0 oC and
96.3oC, respectively), other HILs are viscous liquids at room
temperature. Next, the structural identification of all HILs was
performed by 1H and 13C NMR spectroscopy and high-resolution
mass spectrometer (HRMS). The characterization data
supported our structural assignment of as-synthesized TSILs
(see ESI). The main physicochemical properties of these TSILs
including their thermal properties (Tm and Td), densities (ρ),
viscosities (η), heats of formation (ΔfH), ignition delay (ID) time,
specific impulse (Isp) and density-specific impulse (ρIsp), were
measured or calculated (Table 1).
which the anionic reaction with oxidizer induces the ignition
[27-28]
process of HILs.
In general, those HILs with fuel-richer
anions can exhibit shorter ID times (Figure 1C). In turn, for a
given hypergolic anion, it is not clear whether combustion
properties of HILs (e.g., ID times, combustion duration) could be
significantly influenced by changing oxidation-reduction activity
of cations. Herein, we are interested in designing high-
performing HILs is by introducing oxygen-rich nitrato group to
cations to functionalize their oxidation-reduction activities, which
may result in an improved combustion properties.
Thermal Properties, Densities, and Viscosities
As one of the most important physicochemical properties,
thermal properties of twelve HILs were first investigated. As
shown in Table 1, except that HIL-1 and HIL-12 are solids at
room temperature, other ten HILs show the low melting points
(Tm) of < -40 oC, as determined by differential scanning
calorimeter (DSC) analysis. By analyzing the melting points of
common HILs (HILs 9-11) and nitrato-/ hydroxyl-functionalized
HILs (HILs 2-8), we found that the introduction of nitrato or
hydroxyl groups into the HILs’ cations has no obvious effects on
the melting points of these dicyanamide-based HILs (Table 1),
and no definable correlations between functional groups and
melting points are observed.
The thermal stabilities of these HILs were also evaluated. For
non-functional HILs (HILs 9-12), they all exhibited high thermal
stabilities with the decomposition temperatures (Td) higher than
267 oC. As for these cation-functionalized HILs (HILs 1-8), the
introduction of hydroxyl group into the cationic units has no
obvious effect on decomposition temperatures (Td: 248-280 oC
for HILs 5-8, Table 1), however, the incorporation of energetic
nitrato group gives rise to an obvious decrease of decomposition
temperatures (around 200 oC for HILs 1-4, Table 1) (Figure S1).
In addition to thermal properties, density is another important
parameter for HILs, which directly determines loading capacity
of liquid fuels in the propellant tank. In general, a higher density
means that more fuels can be packed into rocket fuel tank,
thereby affording a higher energy and combustion contribution to
Scheme 1. Chemical structures and synthetic route of studied HILs.
To verify the feasibility of our designing ideas, here, we first
select traditional dicyanamide as the anion, and then try to
introduce oxygen-rich nitrato groups into four types of
quaternary ammonium cations by two different approaches. One
is the direct introduction of nitrato group to form nitrato-
functionalized TSILs (Series I in Scheme 1). The other is indirect
introduction of nitrato group by hydroxyl-functionalized TSILs
(Series II in Scheme 1). Since the hydroxyl group can react with
the WFNA to in-situ yield the nitrato-functionalized TSILs with an
obvious exothermic process, which may also have positive
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Table 1. Physicochemical properties of these twelve ionic liquids.
[a]
[b]
[h]
[j]
Entry
Tm
Td
(oC)
186.6
OB[c]
(%)
-71.5
-67.2
-89.6
-78.1
ρ[d]
(g•cm-3)
1.39
1.30
1.22
η[e]
(mPa•s)
-
133.0
130.3
303.5
ΔfH[f]
(KJ•mol-1)
237.0
169.1
68.3
ID[g]
(ms)
26
41
53
Isp
(s)
ρIsp
(oC)
60.0
-50.2
-49.9
-50.1
(s•g•cm-3)
325.0
294.5
296.6
285.6
HIL-1
HIL-2
HIL-3
HIL-4
233.8
226.5
243.1
234.1
186.8
194.1
200.9
1.22
50.1
58
HIL-5
HIL-6
HIL-7
HIL-8
HIL-9
HIL-10
-48.9
-50.0
-49.8
-49.9
-46.1
-50.7
248.2
260.8
269.1
280.8
267.8
316.3
-109.5
-103.6
-130.6
-122.4
-128.7
-122.0
1.21
1.19
1.13
1.09
1.12
1.11
70.1
51.5
49.1
51.3
19.7
10.9
115.9
64.4
-54.7
-60.2
264.3
215.7
61
51
61
62
33
30
252.6
246.7
260.4
256.5
259.9
256.9
305.6
293.6
294.3
279.6
291.1
285.2
HIL-11
HIL-12
-50.2
96.3
308.6
306.2
64[i]
-151.1
-145.5
1.05
1.05
27.2
-
89.7
53.2
47
-
262.1
262.5
275.2
275.6
UDMH
-57
-159.7
0.79
0.51
53.3
4.8
273.0
215.7
[a] Melting point. [b] Thermal decomposition temperature. [c] Oxygen balance (based on CO) for CaHbOcNd: 1600(c-a-
b/2)/Mw; Mw = molecular weight. [d] Density at 25 oC. [e] Viscosity at 25 oC. [f] Heat of formation. [g] Ignition-delay (ID) time
with WFNA. [h] specific impulse (Explo5 v6.02. IL/WFNA = 24/76, w/w; isobaric conditions, equilibrium expansion, 7.0 MPa
chamber pressure). [i] Boiling point. [j] density-specific impulse.
Heats of formation (HOF, ΔfH°) of ionic liquids were calculated
based on a Born–Haber energy cycle (Scheme 2).
the propulsion process. Of these ionic liquids, the nitrato-
functionalized HILs (HILs 1-4) have relatively higher densities
(ranging 1.22 to 1.39 g•cm-3),approximately 15.8~24.1 % higher
than non-functional HILs (HILs 9-12) (Table 1). In fact, the
densities of four nitrato-functionalized HILs (HILs 1-4, Table 1)
are obviously higher than most known dicyanamide-based ionic
liquids (around 0.9-1.1 g•cm-3) and unsymmetric dimethyl
hydrazine (UDMH) (0.79 g•cm-3),[20] which indicates that nitrato-
functionalized HILs have higher loading capacity than traditional
hydrazine-based fuels. The high densities of nitrato-
Scheme 2. Born–Haber cycle for the formation of ionic liquids; the number of
functionalized HILs may result from their multiple hydrogen
moles of the respective products are given by a, b, c, and d.
bonds C-H…O between aromatic/aliphatic C-H bonds and nitrato
group.[30] With respect to hydroxyl-functionalized HILs (HILs 5-8),
For all the ionic liquids, calculation of the HOFs was simplified
by using Equation (1),[33] in which ΔHL is the lattice energy of the
ionic salts.
because of only one oxygen atom in the molecule, their limited
C-H…O hydrogen bonds result in their slightly higher densities
than the non-functional HILs (HILs 9-12, Table 1). Due to the
additional C-H…O hydrogen bonds in the molecules, these
nitrato- and hydroxyl-functionalized HILs (HILs 1-8) exhibited
higher viscosities than those non-functional HILs (HILs 9-12). Of
these new HILs, these nitrato-functionalized HILs (HILs 1-4)
showed the highest viscosities ranging from 130-330 mPa•s
owing to their multiple C-H…O hydrogen bonds (Table 1).
ΔfH° (ionic liquids, 298K) = ΣΔfH°(cation, 298K)-ΣΔfH°(anion,
298K)-ΔHL (1)
For 1:1 salts, considering the nonlinear nature of the cations
and anion used, ΔHL (in kJ/mol) was predicted by using Equation
(2), as suggested by Jenkins et al.,[34] in which nM and nX
depended on the nature of ions Mp+ and Xq–, respectively, and
had a value of 6 for nonlinear polyatomic ions.
Theoretical study
ΔHL = UPOT + [p(nM/2 – 2) + q(nx/2-2)]RT
(2)
Theoretical calculations were performed by using the Gaussian
09 suite of programs.[31] For these new HILs, geometric
optimization and frequency analyses were completed by using
the B3LYP functional with the 6-31+G** basis set.[32] Single
energy points were calculated at the MP2/6-311++G** level of
theory. For all of as-synthesized compounds, the optimized
structures were characterized to be true local energy minima on
the potential-energy surface without imaginary frequencies.
The lattice-potential energy (UPOT) was calculated according to
Equation (3),[35] in which ρm is the density (g•cm-3) and Mm is the
chemical formula mass of the ionic material.
UPOT (kJ/mol) = 1981.2(ρm/Mm)1/3 + 103.8 (3)
ΔfH(g)C+° = ΔfH(g)C° + IEC
ΔfH(g)A-° = ΔfH (g)A° + EAA
(4)
(5)
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The heats of formation (HOFs) of the ionic salts were obtained
by computing the component cations and anions. Specifically,
the computation of HOFs for both the cations and anions was
performed according to literature methods,[36-37] that is, the gas-
phase HOFs of the ions were determined by using Equations (4)
and (5) (IE=ionization energy; EA=electron affinity). In Equations
(4) and (5), additional calculations for the corresponding neutral
molecules (ΔfH(g)C° and ΔfH(g)A° were performed for the
atomization reaction CaHbNcOd → aC(g) + bH(g) + cN(g) + dO(g)
by using G2 theory. Based on the results from Equation (4), the
HOFs of the cations were obtained by using isodesmic reactions.
The isodesmic reactions for different cations are shown in
Scheme 3.
Explo5 v6.02 software. As shown in Table 1, though the specific
impulse (Isp) of nitrato-functionalized HILs (HILs 1-4) are slightly
lower than hydroxyl-functionalized HILs (HIL 5-8) and common
ones (HILs 9-12), their density-specific impulse (ρIsp) are
obviously higher than the corresponding hydroxyl-functionalized
and common ones owing to their relatively high density.
Especially for HIL-1, it has the highest density-specific impulse
(ρIsp) value of 325.0 s•g•cm-3, much higher than that of UDMH
(215.7 s•g•cm-3), which indicates that HIL-1 has excellent
application potential.
Hypergolic Test
As potential liquid propellant fuels, evaluating their hypergolic
reactivity with propellant oxidizers is very important since it
determines whether these fuels are suitable for practical
applications. In general, hypergolic reactivity is estimated by
recording the ignition delay (ID) times, which represents the time
interval between the initial fuel/oxidizer contact and the start of
combustion. Herein, a droplet test with newly prepared 100%
HNO3 (WFNA) as the oxidizer was employed to measure the ID
times of these HILs. As an example, a series of high-speed
camera (2000 fps) photos of HIL-3 are shown in Figure 2. After
the liquid drop of HIL-3 contacted the pool surface of WFNA, it
underwent a complex and gradual process, involving a rapid
spreading on pool surface of WFNA, the violent reaction with
WFNA, spontaneous ignition, the combustion duration and
finally a gradual flame-quenching (Figure 2).
NH3
+
+
+ 2NH3
+
CH3NH2
+
+
2NH3
+
+ CH3NH2
NH3
NH3
+
+
+
+
+
+
+
2NH3
+
CH3NH2
CH3NH2
+
+
2NH3
NH3
+
+
+
NH3
+
+
+
+ CH3NH2
+
2NH3
2NH3
NH3
+
+
CH3NH2
+
+
Scheme 3. Isodesmic reactions for calculating the HOFs of these twelve
cations.
The enthalpy of reaction (ΔrHo298) is obtained by combining
the MP2/6-311++G** energy difference for the reaction, the
scaled zero-point energies, and other thermal factors. As a
result, the heats of formation of all ionic liquids could be readily
extracted.
Figure 2. High-speed camera photos that show a spatially resolved ignition
From Table 1, the ΔfH values of these HILs ranged from -60.2
to 237.0 kJ•mol-1, and the ΔfH values of nitrato- and hydroxyl-
functionalized HILs (HILs 1-8) are generally lower than those of
non-functional HILs (HILs 9-12). By using the calculated heats of
formation and the experimentally measured densities, the
specific impulse (Isp) of these HILs was calculated by virtue of
event for a droplet of HIL-3 falling into WFNA.
Except for HIL-12, all other HILs (HILs 1-11) can
spontaneously ignite upon contact with WFNA, and their ID
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times are in the range of 26-61 ms (Table 1). Essentially, the
spontaneous combustion between hypergolic ionic liquids (HILs)
and oxidizers is an oxidation-reduction reaction. Therefore, the
reductive activity of anions in HILs usually plays a decisive effect
on the ignition delay times (IDs). For most dicyanamide anion-
based HILs (Figure S2), their ID times with the oxidizer of WFAN
are always longer than 20 ms due to the weak reduction activity
of dicyanamide anion. In this work, these nitrato-functionalized
dicyanamide HILs also fall within a normal range (26-61 ms). For
nitrato-functionalized HILs (HILs 1-4), except that HIL-1 is a solid
at room temperature exhibiting an ID time of 26 ms, other HILs
as liquid fuels showed a little longer ID times (41-58 ms) than
those non-functional HILs (33-47 ms for HILs 9-11), which are
probably due to their relatively higher viscosities (Table 1). But
for hydroxy-functionalized HILs (HILs 5-8), they did not exhibit
the expected shorter ID times by heat-accumulating from the
reaction between hydroxy group and WFNA. The main reason is
that hydroxy-functionalized HILs could not well contact with
WFNA in a very short time due to their high viscosities, and
consequently, the heat-accumulating from their reaction is
limited (Figure S3). On the whole, the relatively high viscosities
of nitrato- and hydroxyl-functionalized HILs (HILs 2-8) make their
ID times a little longer than non-functional HILs (HILs 9-11)
(Table 1), although the introduction of nitrato and hydroxyl
groups increases the oxygen-rich character and even oxygen
balance of these ionic liquid fuels (Table 2).
than their hydroxyl-functionalized and non-functional analogues
(HIL-2 vs HIL-6/HIL-10, HIL-3 vs HIL-7/HIL-11, HIL-4 vs HIL-8)
(Table 2). In addition, under same conditions of droplet test, the
combustion durations of nitrato-functionalized HILs (except the
solid HIL-1) are also longer than that of other eight HILs (HIL-2
vs HIL-6/HIL-10, HIL-3 vs HIL-7/HIL-11, HIL-4 vs HIL-8) (Table
2). The obvious improvements in both flame size and
combustion time can be attributed to the high oxygen-balance
character of nitrato-functionalized cations in these HILs (Table 1),
thereby promoting their combustion properties including larger
brilliant whitish flames and longer combustion duration time.
Table 2. Hypergolic properties of twelve HILs.
Diameters of violent-burning
Violent combustion
duration(ms)[a]
Entry
largest brilliant whitish flame
(cm)
2
HIL-1
HIL-2
HIL-3
HIL-4
HIL-5
HIL-6
HIL-7
HIL-8
HIL-9
HIL-10
HIL-11
HIL-12
18
94
118
68
110
67
92
63
56
72
90
-
21
18
16
16
7
5
7
15
7
6
-
HIL-6
HIL-10
HIL-2
[a]Violent combustion duration (detailed calculation see Figure S4).
Conclusions
7 cm
7 cm
7 cm
7 cm
7 cm
HIL-7
HIL-11
HIL-3
In summary, a series of nitrato- and hydroxyl-functionalized HILs
were designed and synthesized. Their structures of these TSILs
were thoroughly characterized by NMR spectroscopy,
electrospray ionization mass spectroscopy (ESI-MS), and
elemental analysis. Their physicochemical and hypergolic
properties for these HILs including thermal properties, densities,
viscosities, heats of formation, specific impulse, ignition delay
times, and combustion behavior, were investigated in detail. Of
these HILs, the nitrato-functionalized HILs exhibited acceptable
ID times (ranging from 26 ms to 58 ms) with the oxidizer of
WFNA and obviously improved combustion performance
including larger brilliant whitish flames and longer combustion
duration times. The nitrato-functionalized HILs exhibit higher
density-specific impulse (ρIsp) than those of common HILs and
UDMH. In addition, the densities of these nitrato-functionalized
HILs are obviously higher than those of non-functional HILs and
traditional UDMH. These promising properties and combustion
performance of nitrato-functionalized HILs make them a new
class of potential candidates for replacements of hydrazine
derivatives in the liquid bipropellant formulations. Our studies
have demonstrated that the incorporation of oxygen-rich groups
7 cm
Figure 3. Comparisons of the violent-burning largest brilliant whitish flames of
three kinds of HILs.
From the viewpoint of practical applications, the combustion
flame and combustion duration times of HILs are also very
important aspects, which have long been ignored in previous
studies. In this work we are also interested in evaluating the
combustion behavior of these HILs. As shown in Figure 3, under
same conditions of droplet test, the violent-burning largest
brilliant whitish flames of nitrato-functionalized HILs 2 and 3 are
obviously larger than those of their analogues, indicating that
they can violently burn more efficiently. Further analysis showed
that the diameters of largest brilliant whitish flame of those
nitrato-functionalized liquid salts (except HIL-1) are much bigger
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1
HIL-4: pale yellow liquid, 73 % yield; H NMR (600 MHz, DMSO-
d6): δ(TMS, ppm): 4.98 (2H, t), 3.82 (2H, t), 3.14 (9H, s); 13C
NMR (151 MHz, DMSO-d6): δ(TMS, ppm): 119.66, 67.34, 62.48,
53.41. ESI-HRMS: m/z calcd. for cation C5H13N2O3 [M]+:
149.0921; found: 149.0913; anion calcd. for C2N3 [M]-: 66.0098,
found: 66.0107; elemental analysis calcd (%) for C7H13N5O3
(215.1018): C 39.07, H 6.09, N 32.54, found: C 39.30, H 6.46, N
32.03.
into the cationic structures of HILs can open a new avenue to
develop high-performing hypergolic fuels.
Experimental Section
Chemicals
These dicyanamide-based hypergolic ionic liquids were
synthesized according to typical metathesis reaction (as shown
in Scheme 1). Taking HIL-1 as an example, the general
HIL-5: pale yellow liquid, 80 % yield; 1H NMR (600 MHz, DMSO-
d6): δ(TMS, ppm): 8.96 (2H, d, J = 6.3), 8.56 (1H, t), 8.10 (2H, t),
5.19 (1H, s), 4.65 (2H, t), 3.86 (2H, q); 13C NMR (151 MHz,
DMSO-d6): δ(TMS, ppm):146.24, 145.77, 128.40, 119.72, 60.04,
60.74. ESI-HRMS: m/z calcd. for cation C7H10NO [M]+: 124.0757;
found: 124.0750; anion calcd. for C2N3 [M]-: 66.0098, found:
66.0101; elemental analysis calcd (%) for C9H10N4O (190.0855):
C 56.83, H 5.30, N 29.46, found: C 55.68, H 5.39, N 29.52.
synthetic
procedure
is
shown
as
follow:
1-(2-
(nitrooxy)ethyl)pyridin-1-ium bromide (6.07 g, 24.4 mmol) was
dissolved in distilled water (60 mL) and freshly prepared
AgN(CN)2 salt (5.19 g, 30.0 mmol) was added[38]. The resulting
suspension was stirred at RT for 12 h. After simple filtration and
removal of water by rotary evaporation, acetonitrile (50 mL) was
added to dilute the resulting crude ionic liquid. This acetonitrile
solution was cooled in refrigerator for several hours to form
flocculent precipitates, then flocculent precipitates were filtrated
out and the resulting acetonitrile solution was dried by
anhydrous magnesium sulfate. After simple filtration and
removal of acetonitrile by rotary evaporation, ionic liquid HIL-1
was obtained as a pale yellow solid in high yield (83 %).
1
HIL-6: pale yellow liquid, 85 % yield; H NMR (600 MHz, DMSO-
d6): δ(TMS, ppm): 9.03 (1H, s), 7.66 (1H, s), 7.62 (1H, s), 5.13
(1H, s), 4.20 (2H, t), 3.86 (3H, s), 3.73 (2H, t); 13C NMR (151
MHz, DMSO-d6): δ(TMS, ppm):137.45, 123.97, 123.30, 119.71,
60.01, 52.40, 36.34. ESI-HRMS: m/z calcd. for cation C6H11N2O
[M]+: 127.0866; found: 127.0858; anion calcd. for C2N3 [M]-:
66.0098, found: 66.0100; elemental analysis calcd (%) for
C8H11N5O (193.0964): C 49.73, H 5.74, N 36.25, found: C 49.52,
H 6.07, N 35.73.
Synthesis of hypergolic ionic liquids
1
HIL-1: pale yellow solid, 83 % yield; H NMR (600 MHz, DMSO-
HIL-7: white liquid, 72 % yield; 1H NMR (600 MHz, DMSO-d6):
δ(TMS, ppm): 5.21 (1H, s), 3.83 (2H, s), 3.50-3.49 (4H, m), 3.42-
3.40 (2H, m), 3.03 (3H, s), 2.09 (4H, s); 13C NMR (151 MHz,
DMSO-d6): δ(TMS, ppm):119.72, 65.34, 65.04, 56.18, 48.65,
21.60. ESI-HRMS: m/z calcd. for cation C7H16NO [M]+: 130.1226;
found: 130.1211; anion calcd. for C2N3 [M]-: 66.0098, found:
66.0103; elemental analysis calcd (%) for C9H16N4O (196.1324):
C 55.08, H 8.22, N 28.55, found: C 54.45, H 8.39, N 28.06.
d6): δ(TMS, ppm): 9.12 (2H, d, J = 6.5), 8.67 (1H, t), 8.22 (2H, t),
5.09-5.04 (4H, m); 13C NMR (151 MHz, DMSO-d6): δ(TMS, ppm):
146.78, 145.93, 128.63, 119.59, 71.8, 58.22. ESI-HRMS: m/z
calcd. for cation C7H9N2O3 [M]+: 169.0608; found: 169.0613;
anion calcd. for C2N3 [M]-: 66.0098, found: 66.0102; elemental
analysis calcd (%) for C9H9N5O3 (235.0705): C 45.96, H 3.86, N
29.78 found: C 46.47, H 4.43, N 29.02.
1
HIL-2: pale yellow liquid, 78 % yield; H NMR (600 MHz, DMSO-
HIL-8: white liquid, 78 % yield; 1H NMR (600 MHz, DMSO-d6):
δ(TMS, ppm): 5.24 (1H, s), 3.82 (2H, s), 3.38-3.87 (2H, m), 3.10
(9H, s); 13C NMR (151 MHz, DMSO-d6): δ(TMS, ppm):119.69,
67.63, 55.73, 53.83, 53.81, 53.79. ESI-HRMS: m/z calcd. for
cation C5H14NO [M]+: 104.1070; found: 104.1087; anion calcd.
for C2N3 [M]-: 66.0098, found: 66.0100; elemental analysis calcd
(%) for C7H14N4O (170.1168): C 49.39, H 8.29, N 32.92, found:
C 48.98, H 8.65, N 32.62.
d6): δ(TMS, ppm): 9.16 (1H, s), 7.78 (1H, s), 7.70 (1H, s), 4.92
(2H, t), 4.62 (2H, t), 3.89 (3H, s); 13C NMR (151 MHz, DMSO-d6):
δ(TMS, ppm): 137.69, 124.22, 123.24, 119.61, 71.63, 46.76,
36.39. ESI-HRMS: m/z calcd. for cation C6H10N3O3 [M]+:
172.0717; found: 172.0713; anion calcd. for C2N3 [M]-: 66.0098,
found: 66.0103; elemental analysis calcd (%) for C8H10N6O3
(238.0814): C 40.34, H 4.23, N 35.28 found: C 40.42, H 4.53, N
35.27.
1
1
HIL-9: pale yellow liquid, 75 % yield; H NMR (600 MHz, DMSO-
HIL-3: yellow liquid, 70 % yield; H NMR (600 MHz, DMSO-d6):
d6): δ(TMS, ppm): 9.10 (2H, d, J = 5.7 Hz), 8.60 (1H, t), 8.16 (2H,
t), 4.63 (2H, q), 1.55 (3H, t); 13C NMR (151 MHz, DMSO-d6):
δ(TMS, ppm):145.84, 144.99, 128.54, 119.58, 56.88, 16.73. ESI-
HRMS: m/z calcd. for cation C7H10N [M]+: 108.0808; found:
108.0799; anion calcd. for C2N3 [M]-: 66.0098, found: 66.0102;
elemental analysis calcd (%) for C9H10N4 (174.0905): C 62.05, H
5.79, N 32.16, found: C 61.82, H 5.76, N 31.78.
δ(TMS, ppm): 4.99-4.98 (2H, m), 3.88-3.87 (2H, m), 3.61-3.58
(2H, m), 3.52-3.48 (2H, m), 3.08 (3H, s), 2.11 (4H, s); 13C NMR
(151 MHz, DMSO-d6): δ(TMS, ppm): 119.61, 67.99, 64.59, 60.00,
48.24, 21.37. ESI-HRMS: m/z calcd. for cation C7H15N2O3 [M]+:
175.1077; found: 175.1070; anion calcd. for C2N3 [M]-: 66.0098,
found: 66.0107; elemental analysis calcd (%) for C9H15N5O3
(241.1175): C 44.81, H 6.27, N 29.03, found: C 45.40, H 6.26, N
29.53.
HIL-10: pale yellow liquid, 82 % yield; 1H NMR (600 MHz,
DMSO-d6): δ(TMS, ppm): 9.09 (1H, s), 7.73 (1H, t), 7.64 (1H, t),
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4.19 (2H, q), 3.85 (3H, s), 1.42 (3H, t); 13C NMR (151 MHz,
DMSO-d6): δ(TMS, ppm):136.84, 124.10, 122.50, 119.70, 44.88,
36.30, 15.58. ESI-HRMS: m/z calcd. for cation C6H11N2 [M]+:
111.0917; found: 111.0907; anion calcd. for C2N3 [M]-: 66.0098,
found: 66.0100; elemental analysis calcd (%) for C8H11N5
(177.1014): C 54.22, H 6.26, N 39.52, found: C 54.03, H 6.80, N
39.38.
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HIL-11: colorless liquid, 80 % yield; H NMR (600 MHz, DMSO-
d6): δ(TMS, ppm): 3.46-3.36 (6H, m), 2.97 (3H, s), 2.09 (4H, m),
1.28 (3H, t); 13C NMR (151 MHz, DMSO-d6): δ(TMS,
ppm):119.16, 63.04, 58.53, 46.97, 21.18, 8.56. ESI-HRMS: m/z
calcd. for cation C7H16N [M]+: 114.1277; found: 114.1267; anion
calcd. for C2N3 [M]-: 66.0098, found: 66.0103; elemental analysis
calcd (%) for C9H16N4 (177.1014): C 59.97, H 8.95, N 31.08,
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HIL-12: white solid, 83 % yield; H NMR (600 MHz, DMSO-d6):
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δ(TMS, ppm): 3.34 (2H, q), 3.02 (9H, s), 1.26 (3H, t); 13C NMR
(151 MHz, DMSO-d6): δ(TMS, ppm):119.58, 61.39, 52.08, 52.06,
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88.1121; found: 88.1111; anion calcd. for C2N3 [M]-: 66.0098,
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1H and 13C NMR spectra were recorded on Bruker 600 AVANCE
spectrometer (600 and 151 MHz, respectively) with internal
standard (1H NMR: DMSO at 2.50 ppm; 13C NMR: DMSO at
39.52 ppm). High resolution mass spectra were performed on
Shimadzu LCMS-IT-TOF mass spectrometer using electrospray
ionization (ESI). Elemental analysis was performed on Flash EA-
1112 elemental analyzer. Thermal property measurements were
performed on TGA/DSC1 and DSC1 Mettler Toledo calorimeter
equipped with auto cool accessory. Densities were measured on
a Micromeritics Accupyc II 1340 gas pycnometer at 25 °C.
Viscosity measurements were performed on a Brook field
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Acknowledgements
The authors gratefully acknowledge support from the National
Postdoctoral Program for Innovative Talents (No. BX201600137).
and the National Natural Science Foundation of China
(No.11472251).
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Keywords: propellant fuel • hypergolic ionic liquids • nitrato-
functionalization • structure-property relationship • combustion
performance
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
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A
series of nitrato-functionalized
hypergolic ionic liquids were
synthesized, which exhibited improved
comprehensive performances
including higher densities, higher
density-specific impulse, larger
Yi Wang, Shi Huang, Wenquan Zhang,
Tianlin Liu, Xiujuan Qi, and Qinghua
Zhang*
Page No. – Page No.
Nitrato-Functionalized Task-Specific
Ionic Liquids as Attractive Hypergolic
Rocket Fuels
combustion flames and longer violent
combustion durations upon contact
with the oxidizer of WFNA.
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