Ultrafast igniting, imidazolium based hypergolic ionic liquids with enhanced hydrophobicity

Article abstract of DOI:10.1039/C6NJ03233H

Exploring ultrafast igniting and hydrolytically stable ionic liquids (ILs) has a wide scope in hypergolic rocket fuels. The hydrophobicities of ILs, induced by a smart change in alkyl substituents of imidazolium cations with energy-rich cyanoborohydride, [BH₃CN]^- and dicyanamide, [DCA]^- anions were examined for the first time. The physico-chemical properties and performance of ILs were also studied. Consequently, hydrolytic stability in terms of moisture study of all the ILs was thoroughly investigated under standard environmental conditions. The analysis indicates that the IL 14, 1-allyl-3-octyl imidazolium cyanoborohydride is most hydrophobic and hydrolytically stable. Studies evaluating the role of the cationic hydrocarbon chain and the nature of anions of ILs on the properties of hypergolic fuel were carried out. All the ILs are liquid at room temperature and exhibit a positive heat of formation. IL 10, 1,3-diallyl-imidazolium cyanoborohydride exhibited the shortest ignition delay of 1.9 ms with WFNA and IL 11, 1-allyl-3-ethyl imidazolium cyanoborohydride presented the lowest viscosity of 16.62 mPa s. Therefore, these ILs can be suggested as potential candidates for replacing acutely toxic and carcinogenic hydrazine and its methylated derivatives as hypergolic fuels.

Full text of DOI:10.1039/C6NJ03233H

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Ultrafast Igniting, Imidazolium based Hypergolic Ionic Liquids with
Enhanced Hydrophobicity
Vikas K. Bhosalea and Prashant S. Kulkarnia*
Received 00th January 20xx,
Accepted 00th January 20xx
Exploring ultrafast igniting and hydrolytically stable ionic liquids (ILs) has a wide scope in hypergolic rocket fuels.
DOI: 10.1039/x0xx00000x
Hydrophobicity of IL, induced by
a smart change in alkyl substituents of imidazolium cation with energy-rich
www.rsc.org/
cyanoborohydride, [BH3CN]- and dicyanamide, [DCA]- anions were examined for the first time. The physico-chemical
properties and performance evaluation of ILs were also studied. Consequently, hydrolytic stability in terms of moisture
study of all the ILs was thoroughly investigated under standard environmental conditions. The analysis indicates that the IL
14, 1-allyl-3-octyl imidazolium cyanoborohydride is most hydrophobic and hydrolytically stable. The studies evaluating the
role of cationic hydrocarbon chain and nature of anions of ILs on the properties of hypergolic fuel were carried out. All the
ILs are liquid at room temperature and have exhibited a positive heat of formation. IL 10, 1,3-diallyl-imidazolium
cyanoborohydride exhibited the shortest ignition delay of 1.8 ms with WFNA and IL 11, 1-allyl-3-ethyl imidazolium
cyanoborohydride presented the lowest viscosity of 16.62 mPa·s. Therefore, these ILs can be suggested as potential
candidates for replacing acutely toxic and carcinogenic, hydrazine and its methylated derivatives as hypergolic fuel.
formation (∆Hf), low molecular weight exhaust products, high
specific heat and storage stability, minimum toxicity and cost.5
Ionic liquids (ILs, melting below 100 °C6,7) are emerging very
Introduction
Hypergolic
fuels
(Monomethyl
Hydrazine,
MMH;
fast in the field of hypergolic fuel because of their unique
properties such as negligible vapor pressure, liquideous over
wide temperature range, low viscosity, high density, easy to
synthesize by exchanging anion and cation, and hypergolic
with storable oxidizers.8–11 Therefore, the research community
has immensely relied on ILs, which may have the potential to
replace the conventional, acutely toxic, carcinogenic and highly
volatile, MMH and UDMH fuels.12 Due to the continuous
endeavors by researchers, ILs are evolving as green hypergolic
fuel in space propulsion, by gaining the desirable properties of
rocket fuel. Theoretically, innumerable ILs are possible, but
only a few of them will exhibit the experimental ignition test
with storable oxidizers. Ignition is the redox (fuel as base and
oxidizer as acid) type of reaction of hypergolic fuel,13 which
gets augmented due to the incorporation of an energy-rich
group such as -C≡N, B-H, -N=N, -C=C-, -C≡C-, -C≡N, -NH2, etc. in
ILs. Therefore, it is essential to consider these groups, while
designing the architecture of new hypergolic ILs. Sengupta et
al. predicted the significance of energy-rich groups in
hypergolic ILs using theoretical calculations.14,15 Therefore the
resultant ILs have been developed by culminating various
energy-rich anions such as borohydride [BH4]-,16,17
Unsymmetrical Dimethyl Hydrazine, UDMH) present unique
importance as a liquid rocket fuel because it has a self-igniting
tendency while coming in contact with storable liquid
oxidizers. Due to the peculiar properties of the fuel, it
eliminates the external ignition source and thus, the system
becomes more reliable. In addition, the hypergolic system
offers the start-restart capability to the engine during multiple
operations. Hypergolicity is measured in terms of ignition
dealy (ID), which is a time lapse between physical contact of
fuel and an oxidizer, and the appearance of the flame.1,2
A
shorter ID (< 50 ms) was in scope for the safe and reliable
functioning of the liquid rocket engine.3 However, hunting for
the novel alternatives of MMH and UDMH, having ID < 5 ms
(i.e Ultrafast ID) with attractive properties has great demand in
space propulsion.4 The ultrafast ID has significance as it lowers
the risk of detonation and avoids an accident that may occur
during space mission.
While developing the new rocket fuel for space propulsion,
it is essential to consider the desirable properties such as liquid
over temperature range -40 to +60 °C, large density, low
viscosity and ignition delay, preferably positive heat of
cyanoborohydride
[BH3CN]-,18–20
dicyanoboro-hydride
[BH2(CN)2]-,21,22 nitrate [NO3]-,22,23 azide [N3]-,24–26 dicyanamide
[DCA]-,4,18,22,25–32 nitrocynamide [NCA]-,22,25–28,33 aluminium
borohydride
bis(borono)hypophosphite
[Al(BH4)4]-,34
[BH3(CN)BH2(CN)]-,35
with various
[PH2(BH3)2],36
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saturated, unsaturated heterocyclic aromatic and non-
aromatic cations.
It was found that the ILs containing hydrogen rich anions
such as [BH4]-, [BH3CN]-, [BH2(CN)2]-, [Al(BH4)4]-, and
[BH3(CN)BH2(CN)]- manifested ultrafast ignition delay due to
easy combustion with storable oxidizers. However, the
hydrolytic stability of hypergolic ILs is of great concern in order
to choose as a rocket fuel, because it gives poor water
stability, while exposed in the environment.21,35 Hence, it
affects the physico-chemical properties of the IL, which is
essential for liquid rocket fuel and consequently, hampers the
performance of rocket engine. Therefore, development of ILs
having ultrafast ignition delay with enhanced hydrophobicity is
a very challenging task to accomplish. Overall, it is observed
that the anions [BH2(CN)2]-, [BH3(CN)BH2(CN)]- and [PH2(BH3)2]-
are found to be hydrolytically stable than corresponding ILs of
[BH4]-, [BH3CN]- anion.21,35 However, synthesis of [BH2(CN)2]-,
[BH3(CN)BH2(CN)]-, requires toxic HgCl2 and increases the
synthesis complication rendering low yield whereas,
[PH2(BH3)2]- exhibits low thermal stability and also, associated
with synthesis complication. Therefore, the advancement in
the area of probing new ultrafast igniting, hydrolytically stable
ILs fuel has taken renewed interest.35 The objective of this
investigation also falls in the similar category.
Scheme 1. Synthesis of new imidazolium-based ILs
.
of purified salts 1-7 with sodium cyanoborohydride or silver
dicyanamide (Scheme 1). The synthesis of cyanoborohydride
based ILs 8-14 was efficiently carried out in DMF. ILs
were colorless and 11-14 were faint yellow liquids with good
yields (78-92%). ILs 15-17 were obtained by metathesis of
salts with silver dicyanamide in methanol for 24 h in dark
8 & 9
3, 6
&
7
light (to avoid oxidation in presence of light). The reaction gave
brown colored liquids with reasonable yields (78-83%). All the
ILs were purified and stored in a capped vial under N2
atmosphere. ILs 8-17 were soluble in acetonitrile,
dichloromethane, chloroform, methanol, ethanol, acetone and
insoluble in less polar solvents like diethyl ether and hexane.
In recent years, our research group also involved in finding
alternatives to the existing liquid propellant fuels by testing
various combinations of ILs.18,37 In this article, development of
new ultrafast igniting, hydrophobic ILs containing fuel rich
[BH3CN]- and [DCA]- with modified imidazolium cations have
been explored. The [BH3CN]- anion is known to improve ID by
undergoing facile decomposition of B-H bond with a storable
oxidizer and imidazolium-based salts are most likely result in a
thermodynamically favorable liquid at room temperature.
Additionally, these ILs are easy to synthesize with higher yield.
Subsequently, the hydrolytic stability of ILs was improved by
employing the unsaturated and long hydrocarbon side chain to
imidazolium cation. The hydrophobic behavior of new ILs was
examined by subjecting them to moisture analysis test. An
attempt was also made to achieve the short ID of 5 ms with
WFNA. Indeed, the impact of the side chain of imidazolium
cation brings the change in physico-chemical and combustion
properties of ILs. The developed hypergolic ILs are exhibiting
reasonable fuel performance and properties including low
viscosity, high density and thermal stability, low melting
temperature, ultra short ignition delay with WFNA, positive
∆Hf, comparable specific impulse (Isp) and high-density specific
impulse (ρIsp).Therefore, it could demonstrate great interest in
the field of hypergolic rocket fuels.
Characterisation
ILs 8-10, 13, 14 & 17 were water insoluble and ILs 11, 12, 15 &
16 were water soluble as confirmed by visual test at 30 °C. The
structural identification of ILs was carried out by using FT-IR,
1H & 13C-NMR and HR-MS (Supporting information, SI-A). The
physico-chemical properties of ILs 8-17 including viscosity (Ƞ),
density (ρ), melting temperature (Tm), decomposition
temperature (Td), the heat of combustion (∆Hc) and heat of
formation (∆Hf) were calculated. The performance evaluation
parameters
such,
ID, Isp
and
ρIsp were
also
measured/computed.
Hydrolytic stability
Hydrolytic stability of hypergolic ILs has vital importance as
they are salts, being hygroscopic absorbs a significant amount
of moisture, though they are known to be hydrophobic.38 This
impurity lowers the quality of ILs that are being utilized for
various applications.39,40 In bipropellant, quality of fuel has key
significance as it affects the performance of rocket engine. The
handling and storage of ILs for a longer duration could absorb
the moisture present in the environment. Longer exposure of
some of the borane based compounds in moisture
environment formed white precipitation.41 1H-NMR study of
one of the hydrophilic IL, 11 in D2O revealed the chemical
change when the acidic C2-H of imidazolium cation is replaced
by Deuterium (Supporting Information fig. SI-A35). It confirms
Results and discussion
Synthesis
The imidazolium salts 1-7 were obtained by quaternization of
allyl or methyl imidazole using n-alkyl or allyl halide. They were
purified with ethyl acetate, dried and analyzed prior to the
second step. ILs 8-17 were synthesized by metathesis reactions
2 | J. Name., 2012, 00, 1-3
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1.00
12
11
10
9
ν1
ν2
IL 8
IL 9
0.95
0.90
0.85
0.80
0.75
IL10
IL 11
IL 12
IL 13
IL 14
IL 15
IL 16
IL 17
IL8
IL9
8
IL10
IL11
IL12
IL13
IL14
IL15
IL16
IL17
7
6
5
4
[EMIM][BH CN]
3
3
[EMIM][BH3CN]
2
1
24
48
72
Time (h)
96
120
144
3800 3700
3600 3500
3400
3300
-1
3200
3100 3000
Wavenumber (cm
)
Fig. 1 Moisture content (%) of ILs 8-17and [EMIM][BH3CN]
Fig. 2 FT-IR study of ILs 8-17 and [EMIM][BH3CN]
the instability of imidazolium cation in water. Therefore
understanding the water stability of IL has special worth in
liquid hypergolic fuel.
Overall, it is important to note that only ILs 15
& 16 have
shown increasing trend of moisture content analysis while the
others have tried to attain the equilibrium.
In this investigation, the hydrolytic stability of all the
synthesized ILs 8-17 and the proven ultrafast igniting IL,
[EMIM][BH3CN]20 were evaluated by exposing them in Orbital
Incubator Shaker (SAII-OS-S25, India) containing 50.89 (±5) %
moisture content and 32 (±2) °C temperature for several days
at 100 rpm. Finally, the amount of moisture content absorbed
was estimated by using UniBloc Moisture Analyzer (Shimadzu,
MOC63u, halogen heater). Three readings of each experiment
were recorded and the average mean value was noted. As
evident from Fig. 1, IL 15 absorbs the highest (11%) whereas IL
14 absorbs the lowest amount of water content (1.5%). The ILs
8, 10, 12 & 17 exhibited nearly same affinity towards water.
Additionaly, ILs 8-17 & [EMIM][BH3CN] were analyzed by FT-IR
spectrometer42 with Zn-Se ATR crystal and gold coated mirror
(Fig. 2). The moisture content absorbed by ILs was observed by
the transmittance of the H2O in FT-IR. Lower the transmittance
of O-H bond, higher the moisture content present in ILs. It is
found that ILs 8-14 and [EMIM][BH3CN] exhibited the IR band
at ʋ1 (3570-3556 cm-1 of asymmetric stretching) & ʋ2 (3430-
3420 cm-1 of symmetric stretching) and ILs 15-17 display only
one broad band at 3425-3420 cm-1 due to the strong coupling
of ʋ1 & ʋ2 frequency band for O-H bond of water. It is due to
the stronger interaction of [DCA]- and weaker interaction of
[BH3CN]- with H2O. These two techniques were satisfying in
predicting the trend of moisture content absorbed by ILs,
except IL 17. In moisture analysis study, IL 17 exhibits lower
water content, however, IR study shows maximum
absorbance. Perhaps, the bulkiness of the molecule due to its
long octyl chain and the large voids between the anion and
cation must have retained the water molecule which might
have resulted different trend in Moisture Analyzer. Thus, the
Moisture Analyzer and FTIR data interpret that the ILs that
absorb low moisture content are having enhanced
hydrophobicity than other ILs. The reported IL, [EMIM][BH3CN]
absorbs high moisture content than ILs 8-14 and hence, exhibit
lower hydrolytic stability.
Hydrophobicity of the ILs was improved with the smart
choice of hydrocarbon chain length of cation with the same
anion. ILs
9 & 14 having octyl chain length of cation are
hydrophobic and exhibit high moisture stability. It is due to the
high molar volume of ILs reduces the interaction with water.43
Nevertheless, IL 10 also exhibits hydrophobicity; though having
shorter hydrocarbon chain. In this case, unsaturated
hydrocarbon chain may diminish the hydrogen bond
interactions with water and results in the hydrophobicity. The
[DCA]- based ILs were found to have high moisture affinity
(except IL 17) than the [BH3CN]- based ILs of the same cation.
The synthesized [BH3CN]- based ILs have showed better
moisture stability than the reported IL, [EMIM][BH3CN]. No
doubt, new family of hydrolytically stable hydrophobic ILs
based on [BH3CN] anion has emerged in present findings.
Thermal Properties
The thermal properties of ILs such as glass transition
temperature (Tg) and decomposition temperature (Td), are
important to measure the operational temperature range of
hypergolic ILs. A liquid over wide temperature range ensures
that IL can be safely stored. The Tg of all the synthesized ILs
were found to be < -50 °C except IL
9 (Tm = -38.22 °C).
Therefore, it fulfills the desirable property (Tm ≥ -40 °C) of
liquid rocket fuel.5 The Td of ILs 8-17 was analyzed by
Differential Thermal Analysis (DTA) in a nitrogen atmosphere
at a heating rate of 20 °C min-1 (Table 1). The IL 10 exhibited
the lowest Td (242 °C) and IL 17 shows highest Td (326 °C). The
results depicts that the thermal stability of ILs is directly linked
with the type of anion and hydrocarbon chain of the cation.
The ILs 8-14 exhibited low thermal stability than ILs 15-17
because [BH3CN]- undergoes easy decomposition as compared
to the [DCA]-. Furthermore, the allyl-substituted cation has
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Table 1. Physical and chemical properties, and performance parameters of ILs
a
b
h
i
ILs
Tm
Td
ρc
(g.cm-3)
ηd
ΔHce
(kJ.mol-1)
ΔHff
(kJ.mol-1)
IDg
Isp
ρIsp
(°C)
(°C)
(mPa·s)
(ms)
(s)
(s.g/cm3)
8
<-50
-36.22
<-50
<-50
<-50
<-50
<-50
<-50
<-50
<-50
<-50
-57.2
275
283
247
265
256
264
265
325
324
326
221
62.3j
0.9251
0.9056
0.9582
0.9533
0.9403
0.9310
0.9078
1.0861
1.0473
0.9976
0.9611
0.7860
38.17
85.90
21.68
16.62
25.56
32.62
74.34
20.00
35.48
47.72
18.00
0.492
-7207 (1.5)
-9267 (3.5)
-6885 (2.5)
-6350 (3)
-7037 (1)
-7749 (1.5)
-10460 (2.5)
-6325 (2.5)
-7284 (2)
-9983 (3.5)
-5169
217
371
4.7 (0.25)
31.2 (2.0)
1.9 (0.1)
3.1 (0.1)
3.4 (0.15)
4.0 (0.05)
32.6 (4.6)
16.9 (1.3)
41.1 (3.1)
272.0 (38)
1.5
234
236
233
232
233
234
234
227
230
233
230
239
300
301
299
296
301
302
299
304
305
302
295
292
9
10
379
11
237
12
289
13
366
14
536
15
425
16
17
507
665
[EMIM][BH3CN]20,37
UDMH44
85.47
51.63
-1989
4.816
a Melting temperature , bDecomposition temperature was observed using DTA at heating rate 20 ˚C.min-1, cDensity at 25 ˚C, dViscosity at 25˚C, eThe average heat of
combustion measured using ELECTRO CERAMICS Bomb Calorimeter BC 8000 with standard deviation, fHeat of formation. gIgnition delay measured with WFNA
(98.40 % HNO3) along with standard deviation, hSpecific impulse measured using NASA-CEC-71 code, iDensity specific impulse, jBoiling Point
lower thermal stability than alkyl substituted cation with the combustion.1 Amongst the ILs 8-17, the IL 11 exhibited lowest
same anion. It was also observed that the size of the alkyl viscosity of 16.62 mPa·s and it is lesser than the proven IL,
chain length of cation does not influence on the Td. The Td of [EMIM][BH3CN]. Table 1, it can be seen that the viscosity varies
BH3CN based ILs was found to improve in comparison with with hydrocarbon chain length of cation as ethyl < allyl <
proven IL, [EMIM][BH3CN].
propyl < butyl < pentyl < octyl. The increase in the chain length
of cation obviously increases the viscosity of IL, it may be due
to the increase in the hydrogen bonding and van der Waals’
interaction of bulky cation. In addition, the type of anion also
affects the viscosity of IL. The [DCA]- based ILs 15-17 possess
Density and viscosity
Density (ρ) and viscosity (Ƞ) are significant physical properties
of liquid rocket fuel. It directly reflects on the propulsion
performance of fuel. A high-density fuel offers an efficient
loading of fuel with low space and therefore, it minimizes the
dead weight of storage tank and possibly increases the range
of space vehicle. As evident from Table 1, it is clear that the
[BH3CN]- based ILs 8-14 have density ≥ 0.91 g.cm-3 which is
comparable with reported [BH2(CN)2]-, [BH3(CN)BH2(CN)]-
based ILs.20,35 ILs with dicyanamide anion 15-17 have density ≥
lower viscosity than [BH3CN]- based ILs (10
same cation. Most of the ILs 10 13 15-16, have viscosity <
40 mPa·s except the ILs 14 17. The high viscosity may be
, 13 & 14) with the
8
,
-
&
9,
&
attributed to the long hydrocarbon chain length of a cation
which lowers the mobility of IL.
Heat of formation and heat of combustion
1.0 g.cm-3, which is higher than the [BH3CN]- based ILs. Heat of formation (∆Hf) and heat of combustion (∆Hc) are
Amongst the ILs 8-17, the density varies with hydrocarbon noteworthy properties of rocket fuel, and higher the value
chain length of cation as allyl > ethyl > propyl > butyl > pentyl > more efficient is the fuel for space propulsion.5,46 The
octyl, which is ascribed to the increasing molecular size of the experimental ∆Hc of ILs 8-17 was estimated using adiabatic
cation. The low density of IL can be attributed interstices Electro Ceramics Bomb calorimeter. The experiment was
between cation and anion. A bulky cation cannot to the pack performed in triplicate and the average values were noted. As
closely with anion.45 Moreover, the densities of new ILs are depicted in Table 1, it is observed that ILs have ∆Hc > -6300
comparable with known hypergolic ILs, and are much higher kJ.mol-1, which is higher than IL, [EMIM][BH3CN] (-5169 kJ/mol)
than the hydrazine-based fuel (e.g. UDMH, 0.79 g.cm-3).44 It and UDMH (-1989 kJ.mol-1).44 The ∆Hc is related to the
indicates high loading capacity of ILs in rocket fuel tank.
molecular structure of IL, it increases with hydrocarbon chain
Low viscosity of ILs is an eminent property for choosing the length of cation as ethyl < allyl < propyl < butyl < pentyl < octyl
fuel for bipropellant formulation. The low viscous fuel aids for the same anion. Within the same imidazolium cation, the
facile transport of fuel from the tank to the combustion [BH3CN]- based ILs gives higher ∆Hc than the [DCA]- based ILs. It
chamber, thus, favoring rapid mixing with oxidizer for instant is because the [BH3CN] based ILs liberates an additional
4 | J. Name., 2012, 00, 1-3
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Combustion reactions
ILs
10 CO2 + 10 H2O + 0.5 B2O3 + 1.5 N2
13 CO2 + 13 H2O + 0.5 B2O3 + 1.5 N2
C10H20 BN3 15.75 O
+
8.
2
20.25 O
C13H26 BN3
C10H16 BN3
+
9.
2
10CO2 + 8H2O + 0.5 B2O3 + 1.5N2
9CO2 + 8H2O + 0.5 B2O3 + 1.5N2
10CO2 + 9H2O + 0.5 B2O3 + 1.5N2
14.75 O2
13.75 O2
15.25 O2
+
10.
11.
12.
C9H16 BN3
C10H18 BN3
+
+
11 CO2 + 10 H2O + 0.5 B2O3 + 1.5 N2
13.
14.
15.
16.
C11H20 BN3
16.75 O2
+
Fig. 3.Hypergolic test of IL10.
15 CO2 + 114 H2O + 0.5 B2O3 + 1.5 N2
11 CO2 + 6.5 H2O + 2.5 N2
C15H28 BN3
C11H13 N5
22.75 O2
14.25 O2
+
+
Ignition delay (ID) is another significant property for measuring
the efficiency of liquid rocket fuel. A shorter ID is very
desirable for the safe and reliable functioning of the liquid
rocket engine.2 On the contrary, a very high ID can cause the
hard start of the engine and increases the possible risk of
detonation. Therefore, the ID < 50 ms is the acceptable limit of
liquid fuel in bipropellant engines.3 To measure the precise
ignition time of ILs an open cup drop test was performed at
atmospheric pressure. Prior to the test, ILs 8-17 were dried in
vacuum oven for 3 to 4 days at -760 mmHg & 80 °C. A 500 µl
Hamilton syringe was used to drop 10 µl of IL fuel into the pool
of cylindrical glass cuvette containing 1000 µl of oxidizer WFNA
(HNO3, 98.40%). The distance between the needle point
(where from fuel drop) and oxidizer were kept constant during
the ignition test. The sequence of images containing 2500
frame per second (fps) were recorded using Nikon 3700 fps
high-speed camera (Fig. 3). The average speed and size of IL
drop were measured and found to be 0.6 m/s and 3 mm
respectively. The time lapse between initial contact of fuel
drop to oxidizer and development of flame was measured. The
ID was measured in triplicate and average ID was noted (Table
1).
12 CO2 + 8.5 H2O + 2.5 N2
16 CO2 + 12.5 H2O + 2.5 N2
C12H17 N5
C16H25 N5
+
+
16.25 O2
22.25 O2
17.
Scheme 2.Combustion equations of ILs
product B2O3 as confirmed by the powdered XRD analysis
(JCPDS 00-006-0297) of combustion residue of IL 11
(Supporting Information, SI-C). Amongst all the ILs, the IL 14
gave highest ∆Hc (-10241 kJ.mol-1) and IL 15 gave lowest ∆Hc (-
6325 kJ.mol-1). Thereafter, the ∆Hc values were used to
determine the ∆Hf of ILs using combustion reactions (Scheme
2). The combustion products such as CO2 (∆Hf = -393.5 kJ.mol-
1), H2O (∆Hf = -241.8 kJ.mol-1), N2 (∆Hf = 0 kJ.mol-1), and B2O3
(∆Hf = -1273.5 kJ.mol-1) were considered for the calculation.47
The ∆Hf is mainly dependent on the molecular structure of IL.
Interestingly, ILs 8-17 have shown positive ∆Hf, and it is higher
than IL, [EMIM][BH3CN], which accomplishes the requirement
of rocket fuel.
Specific impulse
In this context, the pre-ignition reactions play a vital role. If
the amount of heat generated during the pre-ignition reaction
is sufficient for taking the fuel to its ignition temperature, then
only the ignition will take place. If the pre-ignition reaction fails
to generate the required heat then ignition may not take place
and only vigorous chemical reactions may be exhibited.
Therefore, ID of ILs 8-17 varies from 1.9 to 272 ms. Fig. 1
shows that the pre-ignition reaction of IL 10 commenced at 1.0
ms and shortest ignition at 1.8 ms. ID also depends upon the
chemical reactivity of fuel and oxidizer, the chamber pressure,
the oxidizer and fuel weight ratio, etc. Even though, same fuel
ID can be changed by the quality and purity of fuel and
oxidizer. Therefore, it is very difficult to compare with the
known literature data. IL 10 exhibited average shortest ID of
1.9 ms and IL 17 showed average longest ID of 272 ms. The ID
studies reveal that the nature of cation and anion, both play a
decisive role in determining the hypergolicity of IL. From Table
The performance of fuel is measured in terms of ‘specific
impulse (Isp)’, which is the total impulse per unit of the
propellant provided to the system. A high Isp indicates the
superior range of a space vehicle for its mission and therefore,
it has great importance in designing the rocket engine. The
experimental ∆Hf and density of ILs 8-17 were considered to
predict the Isp. The theoretical Isp was carried out at 2.4 X 106
Pa chamber pressure and 9.8 X 104 Pa atmospheric pressure
using NASA-CEC-71 code.48 The maximum Isp was noted by
employing the frozen flow condition at optimum oxidizer to
fuel ratio (O/F). The theoretical Isp values of ILs were found to
be ≥ 227 s. It is very difficult to compare these Isp values with
the literature values. It is because various research group used
different software codes and conditions for the measurement.
The IL
9 has exhibited the highest Isp of 236 s. The
demonstrated Isp is higher than the reported IL,
[EMIM][BH3CN] (230 s). Interestingly, the density specific
impulse (ρIsp) of ILs were found to be ≥ 296 s.g.cm-3, which is
superior than UDMH (292 s.g.cm-3) as given in Table 1.
1, it is seen that the [BH3CN] containing ILs (
9), showed relatively shorter ID than dicyanamide based ILs
15-17). It is due to the rapid oxidation of B-H bond with
8 & 11-14 except
(
WFNA. For the same anion, the ID varies with nature of
hydrocarbon chain of cation, too. The allyl substituted cation
with the same anion gives lower ID than methyl substituted
Ignition test
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cation. It is because the allyl group undergoes easy oxidation 49.11, 122.71, 123.97, 136.99 ppm; FT-IR (BRUKER ALPHA, Zn-
with WFNA than a methyl group. However, the chain length of Se ATR): ῡ = 3136, 3059, 2943, 2863, 1730, 1628, 1567, 1458,
alkyl group of cation lowers the ignition delay as allyl < ethyl < 1377, 1338, 1297, 1240, 1166, 1112, 1035, 841, 750, 617 cm-1.
propyl < butyl < pentyl < octyl. In summary, ILs
8 and 10-13
exhibit ultra short ID < 5 ms which is comparable with the 1-methyl-3-octyl-imidazolium chloride (2)
proven fuel UDMH.
Methyl imidazole (0.10 mol, 8.76 g) loaded with octyl chloride
(0.11 mol, 17.45 g) at 80 °C for 60 h. A yellow coloured product
1
obtained with Yield 87 %; H NMR (500 MHz, [D6]DMSO, 25
Experimental
°C): δ = 0.72 (t, 3H, N-CH2-CH2-C5H10-CH3), 1.11- 1.55 (m, 10H, -
CH2-CH2-C5H10-CH3) 1.75 (m, 2H, N-CH2-CH2-C5H10-CH3) 3.92 (s,
3H, N-CH3) 4.25 (t, 2H, N-CH2-CH2-C5H10-CH3) 8.06 (s, 1H, CH)
Materials and Methods
The chemicals methyl imidazole (Aldrich), allyl imidazole (97%,
Across organics), allyl bromide (Aldrich), ethyl bromide (Sigma-
Aldrich), 1-chloropropane (98%, Sigma-Aldrich), butyl chloride
8.16 (s, 1H, CH) 10.00 (s, 1H ,CH)
[D6]DMSO, 25 °C): = 14.18, 22.47, 25.96, 28.86, 28.98, 30.06,
;
13C NMR (126 MHz,
δ
31.63, 36.12, 48.95, 122.77, 123.93, 137.33 ppm; FT-IR
(BRUKER ALPHA, Zn-Se ATR): ῡ= 3136, 3042, 2924, 2856, 2743,
1665, 1567, 1459, 1377, 1337, 1300, 1170, 1022, 879, 763,
653, 620 cm-1.
(Sigma-Aldrich),
pentyl
chloride
(Sigma-Aldrich),
1-
chlorobutane (Sigma-Aldrich), diethyl ether (ACS reagent ≥
99.8%, Sigma-Aldrich), sodium dicyanamide (Aldrich), sodium
cyanoborohydride (95%, Alfa Aesar), silver nitrate (Alfa Aesar),
ethyl acetate (≥ 99.5%, Merck), methanol (HPLC grade, Merk),
acetonitrile and N’N-dimethylformamide (99.8%, Sigma-
Aldrich) were used during the synthesis of ILs. All chemicals
were used as received without further purification. FT-IR
spectra were recorded on model BRUKER ALPHA FT-IR
spectrometer with Zn-Se ATR crystal and gold coated mirror.
1H and 13C spectra were recorded in DMSO-d6 solvent on a
Varian Mercury 500 MHz NMR spectrometer operating at 500
and 126 MHz. Chemical shifts are reported relative to the
Me4Si. The mass spectra were measured using HRMS-ESI
(Advion, Model no. 14850) at positive and negative mode
(range 10 to 1000 amu).The density and viscosity of ILs were
measured by using Anton Paar densitometer and Lovis 2000 M
Microviscometer at 25 °C. Hypergolic test of ILs was assessed
by Y3-S1 Camera, 3700 fps, Nikon. DSC of ILs was measured by
using Q20TA instrument 5, INDIUM as a reference with a
heating rate of 10 ˚C min-1 and the Td ILs were measured using
Perkin Elmer- simultaneous analyser (STA) 6000 at heating rate
1,3-diallyl-imidazolium Bromide (3)
Allyl imidazole (0.80 mol, 8.68 g) charged with allyl bromide
(0.088 mol, 10.66 g) at 60 °C for 24 h. A brown coloured
1
viscous product was obtained with Yield: 88%; H NMR (500
MHz, [D6]DMSO, 25 °C): δ = 5.01 (d, 4H; N-CH2-CH=CH2), 5.25-
5.26 (m, 2H; N-CH2-CH=CH2), 5.27-5.30 (m, 2H; N-CH2-CH=CH2),
5.99-6.07 (m, 2H; N-CH2-CH=CH2), 8.02 (d, 2H; CH), 9.86 ppm
(s, 1H, CH); 13C NMR (126 MHz, [D6]DMSO, 25 °C): δ = 51.23,
120.92, 123.0, 132.20, 136.51 ppm; FT-IR (BRUKER ALPHA, Zn-
Se ATR): ῡ = 3126, 3058, 2970, 2853, 1644, 1558, 1422, 1332,
1294, 1157, 993, 938, 833, 747, 625, 558 cm-1.
1-allyl-3-ethyl-imidazolium bromide (4)
Allyl imidazole (0.067 mol, 7.31 g) loaded with ethyl bromide
(0.074mol, 8.10 g) at 70 °C for 30 h. A yellow coloured product
1
was obtained with Yield. 90 %; H NMR (500 MHz, [D6]DMSO,
25 °C):
δ = 1.38 (t, 3H; -CH2-CH3) 4.27 (q, 2H; N-CH2-CH3) 4.95
of 20 °C.min-1.
Moisture analysis was carried out by using
(t, 2H; N-CH2-CH=CH2), 5.98-6.06 (m, 2H; N-CH2-CH=CH2), 5.26-
5.31 (m, 2H; N-CH2-CH=CH2) 5.98-6.08 (m, 1H; N-CH2-CH=CH2),
7.90 (d, 1H; CH), 8.04 (d, 1H; CH), 9.62 ppm (s, 1H; CH); 13C
UniBloc Moisture Analyzer (Shimadzu, MOC63u) in which, a
halogen heater was used during the measurements.
NMR (126 MHz, [D6]DMSO, 25 °C): δ = 15.66, 44.74, 51.10,
General Procedure for Synthesis of Halide Precursor (1-7)
122.78, 136.27 ppm; FT-IR (BRUKER ALPHA, Zn-Se ATR): ῡ =
3129, 3057, 2975, 2864, 1643, 1560, 1446, 1424, 1343, 1160,
996, 943, 830, 754, 662, 620, 565 cm-1.
1 mol of 1-R-imidazole was dissolved in 20 mL of dry
acetonitrile under N2 atmosphere. The alkyl halide (1.1 mol)
was slowly added into the solution, the reaction mixture was
refluxed for 24 to 64 h at 40-80 °C. All the salts were washed
by using dry ethyl acetate and recrystallized by using methanol
and stored under N2 atmosphere (Yield: 85-90%).
1-allyl-3-propyl-imidazolium bromide (5)
Allyl imidazole (0.10 mol, 11.0 g) loaded with propyl bromide
(0.11 mol, 13.80 g) at 70 °C for 48 h. A yellow coloured product
1
was obtained with Yield 92 %; H NMR (500 MHz, [D6]DMSO,
1-methyl-3-pentyl-imidazolium bromide (1)
25 °C): δ = 0.74 (t, 3H; N-CH2-CH2-CH3), 1.77 (m, 2H; N-CH2-CH2-
Methyl imidazole (0.10 mol, 11.05 g) charged with pentyl
bromide (0.11 mol, 13.80 g) 70 °C for 48 h. Yield. 92 %. 1H NMR
(500 MHz, [D6]DMSO, 25 °C): δ = 0.83 (t, 3H; N-CH2-CH2-CH2-
CH2-CH3), 1.15-1.23 (m, 2H; N-CH2-CH2-CH2-CH2-CH3), 1.23-1.33
(m, 2H; N-CH2-CH2-CH2-CH2-CH3) 1.78 (dt, 2H; N-CH2-CH2-CH2-
CH2-CH3), 3.89 (s, 3H; N-CH3), 4.21 (t, 2H; CH), 7.83 (t, 1H; CH),
7.90-7.93 (m, 1H; CH), 9.44 ppm (s, 1H; CH); 13C NMR (126
CH3), 4.23 (t, 2H; N-CH2-CH2-CH3), 4.99 (d, 2H; N-CH2-CH=CH2),
5.24-5.25 (m, 2H; N-CH2-CH=CH2), 5.26-5.29 (m, 2H; N-CH2-
CH=CH2), 5.99-6.07 (m, 1H; N-CH2-CH=CH2 ), 7.96 (d, 1H; CH),
8.08 (d, 1H; CH), 9.69 ppm (d, 1H; CH);13C NMR (126 MHz,
[D6]DMSO, 25 °C): δ = 10.83, 23.36, 50.71, 51.14, 120.63,
122.90, 123.07, 132.35, 136.58 ppm; FT-IR (BRUKER ALPHA,
Zn-Se ATR): ῡ = 3128, 3055, 2964, 2876, 1644, 1560, 1448,
MHz, [D6]DMSO, 25 °C):
δ = 14.20, 21.97, 28.05, 29.59, 36.27,
6 | J. Name., 2012, 00, 1-3
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1424, 1344, 1298, 1220, 1161, 1090, 995, 941, 862, 834, 753, N-CH2-CH2-CH2-CH2-CH3), 1.82 (quin, 2H; N-CH2-CH2-CH2-CH2-
623, 568 cm-1.
CH3), 3.89 (s, N-CH3), 4.19 (t, 2H; N-CH2-CH2-CH2-CH2-CH3), 7.70
(s, 1H; CH), 7.78 (s, 1H; CH), 9.12 ppm (s, 1H, CH); 13C NMR
(126 MHz, [D6]DMSO, 25 °C): δ = 14.11, 21.94, 28.06, 29.52,
36.23, 49.33, 122.66, 123.98, 136.89 ppm; FT-IR (BRUKER
ALPHA, Zn-Se ATR): ῡ = 3150, 3106, 2948, 2865, 2325, 2224,
2170, 1999, 1666, 1570, 1459, 1377, 1343, 1164, 1126, 1027,
838, 746, 652, 619, 529cm-1; ESI-HRMS: m/z calculated for
cation C9H17N2 [M]+: 153.14; found:153.1390; m/z calculated
for anion BH3CN [M]-: 40.04; found: 40.0371.
1-allyl-3-butyl-imidazolium chloride (6)
Allyl imidazole (0.10 mol, 11.21 g) charged with butyl chloride
(0.11 mol, 10.52 g) at 80 °C for 63 h. A viscous yellow coloured
1
product was obtained with Yield: 86 %; H NMR (500 MHz,
[D6]DMSO, 25 °C): δ = 0.82 (t, 3H; -CH2-CH2-CH2-CH3), 1.19 (dq,
2H; -CH2-CH2-CH2-CH3), 1.70-1.80 (m, 2H; -CH2-CH2-CH2-CH3),
4.27 (t, 2H; -CH2-CH2-CH2-CH3), 4.98 (d, 2H; -CH2-CH=CH2), 5.26
(dq, 1H; -CH2-CH=CH2), 5.27-5.30 (m, 2H; -CH2-CH=CH2), 6.05
(ddt, 1H; -CH2-CH=CH2), 7.99 (t, 1H; CH), 8.11 (t, 1H; CH), 9.93
1-methyl-3-octyl-imidazolium cyanoborohydride (9)
ppm (s, 1H; CH); 13C NMR (126 MHz, [D6]DMSO, 25 °C): δ = Yield. 88 %: H NMR (500 MHz, [D6]DMSO, 25 °C):
δ = -0.18-
1
13.71,19.20, 31.89, 48.91, 51.05, 120.42, 122.93, 123.10, 0.35 (m, 3H; BH3CN), 0.71 (t, 3H; N-CH2-CH2-C5H10-CH3), 1.10-
132.53, 136.85 ppm; FT-IR (BRUKER ALPHA, Zn-Se ATR): ῡ = 1.11 (m, 10H; N-CH2-CH2-C5H10-CH3) 1.64 (m, 2H; N-CH2-CH2-
3128, 3050, 2959, 2870, 1644, 1560, 1461, 1378, 1331, 1209, C5H10-CH3), 3.72 (s, 3H; N-CH3), 4.02 (t, 2H; N-CH2-CH2-C5H10-
1162, 995, 942, 882, 761, 627, 571, 516 cm-1.
CH3), 7.54 (s, 1H; CH), 7.61 (s, 1H; CH), 8.94 ppm (s, 1H, CH);
13C NMR (126 MHz, [D6]DMSO, 25 °C): δ = 14.34, 22.51, 25.96,
28.79, 28.92, 29.85, 36.23, 49.33, 122.72, 124.05, 136.94 ppm:
FT-IR (BRUKER ALPHA, Zn-Se ATR): ῡ = 3148, 3109, 2925, 2856,
2324, 2221, 2165, 1571, 1462, 1378, 1338, 1165, 1124, 1022,
867, 839, 748, 697, 649, 620 cm-1; ESI-HRMS: m/z calculated
for cation C12H23N2 [M]+: 195.19; found:195.1855; m/z
calculated for anion BH3CN [M]-: 40.04; found: 40.0380.
1-allyl-3-octyl-imidazolium chloride (7)
Allyl imidazole (0.10 mol, 11.13 g) charged with octyl chloride
(0.11 mol, 16.80 g) at 80 °C for 64 h. A yellow coloured viscous
1
product was obtained with Yield 75 %; H NMR (500 MHz,
[D6]DMSO, 25 °C):
δ = 0.83 (m, 3H; -CH2-CH2-C5H10-CH3), 1.13-
1.34 (m, 10H; -CH2-CH2-C5H10-CH3), 1.79 (quin, 2H; -CH2-CH2-
C5H10-CH3), 4.24 (t, 2H; -CH2-CH2-C5H10-CH3), 4.90-4.99 (m, 2 H;
-CH2-CH=CH2), 5.28 (dq, 1H; -CH2-CH=CH2), 5.32 (dq, 1H; -CH2-
1,3-diallyl-imidazolium cyanoborohydride (10)
1
CH=CH2), 6.06 (ddt, 1H; -CH2-CH=CH2), 7.89 (t, 1H; CH), 8.00 (t, Yellow coloured product was obtained, Yield 83 %; H NMR
1H, CH), 9.74 ppm (d, 1H; CH); 13C NMR (126 MHz, [D6]DMSO, (500 MHz, [D6]DMSO, 25 °C):
δ = -0.03-0.51 (m, 3H; BH3CN),
25 °C): δ = 14.37, 22.51, 25.97, 28.80, 28.96, 29.88, 31.62, 4.87 (d, 4H; N-CH2-CH=CH2), 5.29-5.34 (m, 2H; N-CH2-CH=CH2),
49.25, 51.15, 120.40, 122.97, 123.07, 132.48, 136.84 ppm; FT- 5.36-5.39 (m, 2H; N-CH2-CH=CH2), 6.02-6.10 (m, 2H; N-CH2-
IR (BRUKER ALPHA, Zn-Se ATR): ῡ = 3127, 3045, 2925, 2855, CH=CH2), 7.74 (d, 2H; CH), 9.15 ppm (d, 1H; CH); 13C NMR (126
1644, 1560, 1459, 1374, 1333, 1163, 994, 939, 762, 628, 569 MHz, [D6]DMSO, 25 °C): δ = 51.47, 121.03, 123.05, 131.89,
cm-1.
136.49 ppm; FT-IR (BRUKER ALPHA, Zn-Se ATR): ῡ = 3140,
3105, 2986, 2853, 2324, 2222, 1645, 1561, 1446, 1423, 1331,
1293, 1155, 1126, 992, 943, 839, 745, 628, 598, 558cm-1; ESI-
HRMS: m/z calculated for cation C9H13N2 [M]+:149.11;
found:149.1070; m/z calculated for anion BH3CN [M]-: 40.04;
found: 40.0346.
Synthesis of Silver Salt Precursor (Silver Dicyanamide, AgDCA)
It was prepared by drop-wise addition of aqueous solution of
AgNO3 (0.12 mol, 20.78 g,) into a saturated solution of
equimolar sodium dicyanamide (0.12 mol, 10.89 g,). The
resulting white solid was filtered and washed with water. The
light sensitive product was dried in an oven over night at 90 °C
(Yield: 95%).
1-allyl-3-ethyl-imidazolium cyanoborohydride (11)
1
Colourless liquid, Yield 85 %; H NMR (500 MHz, [D6]DMSO,
25 °C): δ = -0.01 - 0.53 (m, 3 H, BH3CN), 1.43 (t, 3H, N-CH2-CH3),
Synthesis of ILs 8-14
4.23 (m, 2H, N-CH2-CH3), 4.84 (t, 2H; N-CH2-CH=CH2), 5.33 (m,
2H; N-CH2-CH=CH2), 5.99-6.07 (m, 1H; N-CH2-CH=CH2), 7.66 (d,
1H; CH), 7.76 (d, 1H; CH), 9.14 ppm (d, 1H; CH); 13C NMR (126
MHz, [D6]DMSO, 25 °C): δ = 15.38, 43.15, 51.39, 121.07,
122.61, 122.75, 136.07 ppm; FT-IR (BRUKER ALPHA, Zn-Se
ATR): ῡ = 3143, 3106, 2985, 2324, 2222, 2168, 1645, 1563,
1447, 1423, 1388, 1295, 1158, 1125, 1027, 993, 946, 867, 838,
749, 621, 564 cm-1; ESI-HRMS: m/z calculated for cation
C8H13N2 [M]+: 137.11 ; found:137.1065; m/z calculated for
A solution of sodium cyanoborohydride (1.1 mol) in DMF was
drop-wise added in to the salt solution in DMF of 1-7 (10 mL)
and stirred at room temperature for 4 h. The precipitate of
sodium chloride was decanted by filtration. The filtrate was
evaporated under reduced pressure. The isolated product was
purified using dichloromethane and dried under high vacuum
to form a colourless or yellow liquid.
anion BH3CN [M]-: 40.04; found: 40.0401
.
1-methyl-3-pentyl-imidazolium cyanoborohydride (8)
1
Yield 92 %; H NMR (500 MHz, [D6]DMSO, 25 °C):
δ = -0.00-
0.53 (m, 3H; BH3CN), 0.88 (t, 3H; N-CH2-CH2-CH2-CH2-CH3),
1-allyl-3-propyl-imidazolium cyanoborohydride (12)
1.22-1.28 (m, 2H; N-CH2-CH2-CH2-CH2-CH3), 1.29-1.36 (m, 2H;
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1
Yellow coloured product was obtained, Yield: 78 %; H NMR 1,3-diallyl-imidazolium dicyanamide (15)
(500 MHz [D6]DMSO, 25 °C: δ = -0.00-0.53 (m, 3H; BH3CN),
Reddish brown coloured product was obtained with Yield:
83%
1H NMR (500 MHz [D6]DMSO, 25 °C): δ = 4.87 (d, 4H; N-
0.84 (t, 3H; N-CH2-CH2-CH3), 1.82 (m, 2H; N-CH2-CH2-CH3), 4.16
(t, 2H; N-CH2-CH2-CH3), 4.86 (d, 2H; N-CH2-CH=CH2), 5.29-5.33
(m, 1H; N-CH2-CH=CH2), 5.34-5.37 (m, 1H; N-CH2-CH=CH2),
6.01-6.09 (m, 1H; N-CH2-CH=CH2), 7.69 (d, 1H; CH ), 7.77 (d,
1H; CH), 9.12 ppm (d, 1H; CH); 13C NMR (126 MHz, [D6]DMSO,
25 °C): δ = 10.83, 23.28, 50.96, 51.39, 120.86, 122.94, 123.01,
131.99, 136.46 ppm; FT-IR (BRUKER ALPHA, Zn-Se ATR): ῡ =
3141, 3105, 2969, 2969, 2969, 2324, 2222, 2168, 1645, 1562,
1449, 1424, 1387, 1344, 1296, 1159, 1125, 993, 945, 839, 750,
625, 565cm-1; ESI-HRMS: m/z calculated for cation C9H15N2
[M]+: 151.12 ; found:151.1234; m/z calculated for anion BH3CN
;
CH2-CH=CH2), 5.27-5.35 (m, 2H, N-CH2-CH=CH2), 5.38 (dd, 2H,
N-CH2-CH=CH2), 6.06 (ddt, 2H, N-CH2-CH=CH2), 7.75 (d, 2H;
CH), 9.19 ppm (s, 1H, CH); 13C NMR (126 MHz, [D6]DMSO, 25
°C): δ = 51.46, 119.57, 120.84, 123.15, 132.08, 136.67 ppm; FT-
IR (BRUKER ALPHA, Zn-Se ATR): ῡ = 3137, 3092, 3015, 2225,
2190, 2123, 1645, 1561, 1445, 1423, 1304, 1157, 1105, 991,
942, 903, 842, 745, 663, 624, 599, 557, 520 cm-1; ESI-HRMS:
m/z
calculated
for
cation
C9H13N2
[M]+:149.11;
found:149.1073; m/z calculated for anion C2N3 [M]-: 66.01;
found: 66.0104.
[M]-: 40.04; found: 40.0401
.
1-allyl-3-butyl-imidazolium dicyanamide45 (16)
1-allyl-3-butyl-imidazolium cyanoborohydride (13)
Brown coloured product was obtained with Yield 78 %;
1H
1
Yellow coloured product was obtained, Yield: 80 %; H NMR
(500 MHz [D6]DMSO, 25 °C): δ = -0.02–0.51 (m, 3H; BH3CN),
0.90 (t, 3H; N-CH2-CH2-CH2-CH3), 1.27 (dq, 2H; N-CH2-CH2-CH2-
CH3), 1.79 (m, 2H; N-CH2-CH2-CH2-CH3), 4.20 (t, 2H; N-CH2-CH2-
CH2-CH3), 4.85 (dt, 2H, N-CH2-CH=CH2), 5.30 (dq, 1H; N-CH2-
CH=CH2), 5.37 (dq, 1H; N-CH2-CH=CH2), 6.04 - 6.10 (m, 1H; N-
CH2-CH=CH2), 7.72 (t, 1H; CH) 7.81 (t, 1H; CH), 9.17 ppm (t, 1H;
CH); 13C NMR (126 MHz, [D6]DMSO, 25 °C): δ = 13.71, 19.28,
31.77, 49.20, 51.40, 120.78, 123.00, 123.07, 132.10 ppm; FT-
IR (BRUKER ALPHA, Zn-Se ATR): ῡ =3140, 3104, 2962, 2935,
2872, 2323, 2221, 2165, 1645, 1562, 1461, 1424, 1334, 1158,
1125, 993, 945, 867, 841, 750, 625, 571 cm-1; ESI-HRMS: m/z
calculated for cation C10H17N2 [M]+: 165.14; found:165.1395;
NMR (500 MHz [D6]DMSO, 25 °C): δ = 0.90 (t, 3H, N-CH2-CH2-
CH2-CH3), 1.23-1.31 (m, 2H, N-CH2-CH2-CH2-CH3), 1.79 (m, 2H;
N-CH2-CH2-CH2-CH3), 4.20 (t, 2H; N-CH2-CH2-CH2-CH3), 4.84-
4.86 (m, 2H, N-CH2-CH=CH2), 5.30 (dq, 1H; N-CH2-CH=CH2),
5.37 (dq, 1H; N-CH2-CH= CH2), 6.02-6.10 (m, 1H; N-CH2-
CH=CH2), 7.73 (d, 1H, CH), 7.82 (t, 1H, CH), 9.21 ppm (s, 1H;
CH);
13C NMR (126 MHz, [D6]DMSO, 25 °C): δ = 13.70, 19.28,
31.78, 49.18, 51.39, 119.57, 120.73, 123.02, 132.11, 136.53
ppm; FT-IR (BRUKER ALPHA, Zn-Se ATR): ῡ = 3137, 3094, 2961,
2935, 2874, 2225, 2190, 2123, 1645, 1562, 1461, 1446, 1424,
1303, 1160, 992, 944, 901, 844, 751, 662, 636, 623, 520 cm-1;
ESI-HRMS: m/z calculated for cation C10H17N2 [M]+:165.14;
found:165.1388; m/z calculated for anion C2N3 [M]-: 66.01;
m/z calculated for anion BH3CN [M]-: 40.04; found: 40.0389
.
found: 66.0111
.
1-allyl-3-octyl-imidazolium dicyanamide45 (17)
1-allyl-3-octyl-imidazolium cyanoborohydride (14)
1
1
Yellow coloured product was obtained, Yield 83 %; H NMR
(500 MHz [D6]DMSO, 25 °C): δ = -0.03-0.50 (m, 3H; BH3CN),
0.86 (t, 3H; -CH2-CH2-C5H10-CH3), 1.23-1.29 (m, 10H; N-CH2-
CH2-C5H10-CH3), 1.80 (quin, 2H; N-CH2-CH2-C5H10-CH3), 4.18 (t,
2H; N-CH2-CH2-C5H10-CH3), 4.85 (dt, 2H; N-CH2CH=CH2), 5.29
(dq, 1H; N-CH2CH=CH2), 5.37 (dq, 1H; N-CH2CH=CH2), 6.02-6.10
(m, 1H; N-CH2-CH=CH2), 7.73 (t, 1H; CH), 7.82 (t, 1H; CH), 9.17
Brown coloured product was obtained with Yield 80 %; H
NMR (500 MHz [D6]DMSO, 25 °C): δ = 0.85 (m, 3H; N-CH2-CH2-
C5H10-CH3), 1.21-1.29 (m, 10H; N-CH2-CH2-C5H10-CH3), 1.80
(quin, 2H; N-CH2-CH2-C5H10-CH3), 4.18 (t, 2H; N-CH2-CH2-C5H10-
CH3), 4.85 (d, 2H; N-CH2-CH=CH2), 5.29 (dq, 1H, N-CH2-
CH=CH2), 5.37 (dq, 4H, N-CH2-CH=CH2) 6.06 (ddt, 1H; N-CH2-
CH=CH2), 7.73 (t, 1H, CH) 7.82 (t, 4H; CH), 9.18 ppm (s, 1H; CH);
ppm (t, 1H; CH), 13C NMR (126 MHz, DMSO-d6)
δ = 14.39,
13C NMR (126 MHz, [D6]DMSO, 25 °C)
: δ = 14.38, 22.53, 25.97,
22.53, 25.97, 28.77, 28.94, 29.78, 31.62, 49.45, 51.39, 120.65,
123.02 , 123.06, 132.18, 136.51 ppm; FT-IR (BRUKER ALPHA,
Zn-Se ATR): ῡ = 3140, 3102, 2926, 2857, 2323, 2221, 2165,
1642, 1562, 1458, 1373, 1337, 1158, 1127, 1025, 992, 944,
866, 752, 629, 568 cm-1; ESI-HRMS: m/z calculated for cation
C14H25N2 [M]+: 221.20; found:221.2009; m/z calculated for
28.77, 28.93, 29.78, 31.62, 49.44, 51.39, 119.56, 120.65,
123.05, 132.15, 136.53 ppm; FT-IR (BRUKER ALPHA, Zn-Se
ATR): ῡ = 3138, 3093, 2926, 2857, 2227, 2191, 2125, 2563,
1458, 1305, 1160, 992, 943, 901, 754, 629 cm-1; ESI-HRMS: m/z
calculated for cation C14H25N2 [M]+:221.20; found:221.2012;
m/z calculated for anion C2N3 [M]-: 66.01; found: 66.0108
.
anion BH3CN [M]-: 40.04; found: 40.0344
.
Synthesis of ILs (15-17)
Conclusions
A solution of salts (1 mol) 3, 6 and 7 in methanol slowly added
A new series of imidazolium-based ILs 8-17 have been
designed and synthesized with good yields. The investigation
has helped in resolving the problem of hydrolytic stability of
hypergolic IL fuels. The hydrophobic properties of the [BH3CN]-
based ILs were found to vary with different -allyl or n-alkyl
substituents of the cation. All the ILs are found to be liquid at
into the suspension of silver dicyanamide in methanol The
contents were stirred in dark place for 24 h at 30 °C for 24 h.
After the stirring, the insoluble silver halide was removed by
filtration. The resulting filtrate was evaporated under reduced
pressure and dried under vacuum to obtain the yellow or dark
red coloured liquid.
8 | J. Name., 2012, 00, 1-3
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Journal Name
ARTICLE
19 S. Huang, Q. Xiujuan, W. Zhang, T. Liu and Q. Zhang, Chem.
Asian J., 2015, 10, 2725–2732.
20 Q. Zhang, P. Yin, J. Zhang and J. M. Shreeve, Chem. Eur. J.,
2014, 20, 6909–6914.
room temperature, hypergolic with WFNA and exhibit positive
∆Hf. The ILs
8 & 10-13 exhibit the ultrafast ID < 5 ms and fulfils
the requirement of new fuel. The IL 11 has acceptable viscosity
of 16.62 mPa·s and density of 0.95 g.cm-3 at 25 °C which is
comparable with known hypergolic IL fuel. ILs 15-17 showed
high density > 0.99 g.cm-3 among the developed ILs. Amongst
all the ILs, the IL 10 can be considered as the most suitable
candidates for the bipropellant formulation. Therefore
distinctive properties of ILs viz. superior ID, water stability,
high density, liquideous over wide temperature range,
acceptable viscosity, and high ∆Hf make them interesting
equivalents as fuels for space propulsion. In addition, these
new ILs may find extensive scope in industrial applications
such as energy storage, separation technique, as a solvent for
chemical reactions, catalysis and absorption of CO2.
21 Y. Zhang and J. M. Shreeve, Angew. Chem. Int. Ed., 2011, 50
935–937.
,
22 K. Wang, Y. Zhang, D. Chand, D. A. Parrish and J. M. Shreeve,
Chem. Eur. J., 2012, 18, 16931–16937.
23 H. Gao, Y.-H. Joo, B. Twamley, Z. Zhou and J. M. Shreeve,
Angew. Chem. Int. Ed., 2009, 48, 2792–2795.
24 S. Schneider, T. Hawkins, M. Rosander, J. Mills, G. Vaghjiani
and S. Chambreau, Inorg. Chem., 2008, 47, 6082–6089.
25 Y. Joo, H. Gao, Y. Zhang and J. M. Shreeve, Inorg. Chem.,
2010, 49, 3282–3288.
26 P. D. Mccrary, G. Chatel, S. A. Alaniz, O. A. Cojocaru, P. A.
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Rogers, Energy & Fuels, 2014, 28, 3460–3473.
27 H. Gao, Y.-H. Joo, B. Twamley, Z. Zhou and J. M. Shreeve,
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28 Y. Zhang, H. Gao, Y. Guo, Y.-H. Joo and J. M. Shreeve, Chem.
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29 P. D. Mccrary, P. S. Barber, S. P. Kelley and R. D. Rogers,
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30 S. Huang, X. Qi, W. Zhang, T. Liu and Q. Zhang, Chem. Asian
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31 W. Zhang, X. Qi, S. Huang, J. Li and Q. Zhang, J. Mater. Chem.
Acknowledgements
We gratefully acknowledge the financial support from DRDO,
Ministry of Defence (ERIP/ER/1003883/M/01/908/2012/D, R
&D/1416, dated, 28-3-2012) New Delhi, India. One of the
authors, Vikas Bhosale is thankful to the UGC, New Delhi for
the Ph.D. Fellowship.
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32 R. Fareghi-Alamdari, F. Ghorbani-Zamani and N. Zekri, RSC
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