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Journal ofMaterials Chemistry A

FEATURE ARTICLE

Recent developm

EBscuMthii

aDepartment of Chemistry, University of Ot

Canada, K1N 6N5. E-mail: m.murugesu@

+1-613-562-5800 extn 2733bDefense R&D Canada-Suffield, P.O. Box 400

8K6, Canada

Cite this: J. Mater. Chem. A, 2014, 2,8153

Received 13th January 2014Accepted 11th February 2014

DOI: 10.1039/c4ta00204k

www.rsc.org/MaterialsA

This journal is © The Royal Society of C

ents in the field of energetic ionicliquids

Elena Sebastiao,a Cyril Cook,a Anguang Hub and Muralee Murugesu*a

Energetic ionic liquids (EILs) are a subset of the rapidly growing field of ionic liquid research. These liquid and

stable high-energy materials (HEMs) have many benefits, ranging from ease of manufacture and

transportation to enhanced safety, as well as many new applications. This review focuses on the

developments in this field from 2005 to 2012, with emphasis on propulsion applications of these new

materials. Both bipropellant systems (hypergolic EILs) and monopropellant systems (oxygen balanced

EILs) are discussed.

Introduction

Ionic liquids (ILs), or molten salts, are dened as ionic pairswith melting points below 100 �C.1 Operating under the bannerof green chemistry, the eld rst gained interest in the early1990's: ILs were developed with the successful aim of producingnon-volatile and reusable solvents for synthetic applications, aswell as developing new electrolytes.2 Since then, publications inthis eld have exponentially increased in volume (Fig. 1) due tothe development of new and creative applications for ILs: fromsensors,3,4 solid state photocells,5 and batteries,6 to thermaluids,7 lubricants,8,9 hydraulic uids,10 and ionogels;11 amongmany others. With the evolution of energetic materials towards

lena Sebastiao completed herSc in Chemistry at the Univer-ity of Ottawa in 2012. She isurrently pursuing her MSc.nder the direction of Prof.uralee Murugesu, focusing on

he design and engineering ofigh energetic materials. Elenas also a licensed pyrotechniciann Canada.

tawa, 10 Marie Curie, Ottawa, Ontario,

uottawa.ca; Fax: +1-613-562-5170; Tel:

0, Stn. Main, Medicine Hat, Alberta, T1A

hemistry 2014

the development of new ionic salts,12,13 the elds of energeticmaterials and ILs merged to further diversify military andcommercial applications: energetic ionic liquids (EILs)designed for the production of new propellants.

The application of high-energy materials (HEMs) depends onthe energy releasing process which follows one of the two majorpathways: deagration or detonation (Table 1).14 Propellants,which decompose through a thermal process (deagration),differ from explosives, capable of detonation in which theydecompose through a shockwave at supersonic speeds.15

Propellants, also referred to as deagrating explosives, releasetheir stored energy in the form of hot gas evolving over a giventime frame, ranging from milliseconds to seconds.12,15,16

Propellants are employed in a wide spread of military applica-tions, including the launching of projectiles, such as: bullets,rockets and missiles, as well as ejecting pilots. More generalclasses of propellant applications include fuel-like purposes ofdriving turbines, moving pistons, and starting aircra engines,as well as pumping uids and shearing bolts and wires.16

Dr Cyril Cook obtained his MScin chemistry from University ofRennes, France, in 2006 underthe supervision of Dr C. Crevisy.In 2009, he completed his PhDin organic chemistry at Univer-sity Paris-Sud, France, under thesupervision of Dr E. Roulland.During his post-doc position inProf. M. Murugesu group atUniversity of Ottawa, Canada,from 2010 to 2013, he worked onthe synthesis of new energetic

materials and energetic ionic liquids. He now works as a researchscientist at CanAm Bioresearch in Winnipeg, Canada.

J. Mater. Chem. A, 2014, 2, 8153–8173 | 8153

Fig. 1 Exponential curve illustrating the ionic liquid field growth. As ofAugust 26th 2013, a search on SciFinder using the key words “ionicliquid” generates a list of 49 426 publications.

Table 1 Reaction types of an energetic material with a Qex ¼ 4187 kJkg�3

Reaction type Combustion Deagration Detonation

Reaction speed (m s�1) 10�3–10�2 102 104

Mass ow (m3 s�1) 10�3–10�2 102 104

Gaseous products (m3 s�1) 1–10 105 10�2

Reaction time (s m�3) 102–103 10�2 10�4

a Qex ¼ heat of explosion.14

Table 2 Typical sensitivity and performance data for primary andsecondary explosivesa

Primary explosives Secondary explosives

IS (J) #4 $4FS (N) #10 $50ESD (J) 0.002–0.020 $0.1Qdet (kJ kg

�1) 1000–200 5000–6000Pdet (GPa) — 21–39ndet (m s�1) 3500–5500 6500–9000

a IS ¼ impact sensitivity, FR ¼ friction sensitivity, ESD ¼ electrostaticdischarge sensitivity, Qdet ¼ heat of detonation, Pdet ¼ detonationpressure, ndet ¼ detonation velocity.

Journal of Materials Chemistry A Feature Article

Detonation explosives refer to the commonly used term“explosives”. These are widely used in industrial and engi-neering applications, such as: blasting, mining, cratering, andthe manipulation of metals by either cutting, welding, formingor fragmentation.12 Military applications make use of shapedcharges and the detonation properties of these explosives.Detonation explosives may be divided into two classes: primaryand secondary explosives (Table 2).14 Primary explosives are verysensitive to various initiation stimuli and will detonate regard-less of their connement conditions.15 Secondary explosives areless sensitive to stimuli but more powerful than primaryexplosives. The applications listed above utilise secondaryexplosives for the explosive power, and primary explosives asinitiators. Physical properties of some currently employed andwell known explosives are reported in Table 3. The boundarybetween detonating explosives and propellants is however notabsolute: under the right conditions, many propellants canbehave as detonation explosives.

Dr Anguang Hu is a seniordefence scientist in AdvancedEnergetics Group, MilitaryEngineering Section, at DefenceResearch and DevelopmentCanada – Suffield ResearchCentre (DRDC Suffield). Hereceived his Ph. D in condensedmatter Physics in 1992 fromInstitute of Materials Science,Jilin University, the P. R. China.He specializes in quantumchemistry, high-performance

computing, high-pressure physics and chemistry, and high energydensity materials. In 2013, he received an Alan Berman ResearchPublication Award at the US Naval Research Laboratory.

8154 | J. Mater. Chem. A, 2014, 2, 8153–8173

The intrinsic properties of ILs, such as: low vapour pressures,high thermal stabilities, and low melting points, make them idealcandidates for minimizing or even eliminating hazardous condi-tions associated with handling, processing and transportingexplosive materials.2 In order to achieve low melting points, ILsmake use of bulky, asymmetric, poorly packing ions to lower theirlattice energy.17 Favourite choices are quaternary ammonium andN-heteroaromatic cations. The N-heteroaromatic ions have beenfound to be energetic when combined with appropriate counterions.12 This arises from the numerous energetic N–N and C–Nbonds which lead to high heats of formation. N-rich ILs have the

Prof. Muralee Murugesureceived his PhD from Universityof Karlsruhe in 2002 under theguidance of Prof. A. K. Powell.He undertook postdoctoral staysat the University of Florida(2003–2005) with Prof. G.Christou, and jointly at theUniversity of California, Berke-ley and the University of Cal-ifornia, San Francisco under thesupervision of Prof. J. R. Longand the Nobel Laureate Prof. S.

Pruissner (2005–2006). In 2006, he joined the University of Ottawaas an assistant professor and became an associate professor in2011. Prof. Murugesu's research focuses on the design and devel-opment of lanthanide Single-Molecule Magnets and High-Energymaterials.

This journal is © The Royal Society of Chemistry 2014

Table 3 Common explosives and their propertiesa

TNT14,18–21

2-methyl-1,3,5-trinitrobenzene

Tetryl15,21

N-methyl-N,2,4,6-tetranitroaniline

RDX14,18,19,21

1,3,5-trinitro-1,3,5-triazacyclohexane

PETN14,15,21

[3-nitrooxy-2,2-bis-(nitro oxymethyl)-propyl] nitrate

CL2021–25

2,4,6,8,10,12-hexanitro-2,4,6, 8,10,12-hexaazaisowurtzitane Lead azide14,15,21

Tm (�C) 80.4 129.5 204.1 143 167 —Td (�C) 295 162 213 202 213 190Tign (�C) 300 187 210 202 — 327–360Tdet (�C) 3471 — 4081 4076 — —r (g cm�3) 1.65 1.73 1.80 1.76 2.044 4.8OB �73.96 �47.36 �21.61 �10.12 �10.95 �5.49N 18.5 24.4 37.8 17.7 38.4 28.9DHf (kJ mol�1) �62.07 33.89 70.29 �530.53 372.00 468.61DHf (kJ kg

�1) �273.28 118.02 316.45 �1678.16 848.95 1609.02DHdet (kJ mol�1) �929.34 �1246.35 �1127.36 �1838.37 �2609.59 �469.00Qexp (kJ kg�1) 4091.63 4340.50 5075.50 5815.08 5955.43 1610.36Pdet (GPa)

d 21.0c, 20.2b — 37.5c, 34.5b 31.5c, 31.1b 48.23 34.3Vdet (m s�1) 6950c, 7150b 7080 8750c, 8920b 8270c, 8660b 9620c 5500IS (J) 15 3 7.4 3 2–4 2.4–4FS (N) >353 >353 120 60 48 <1ESD (J) 0.46–0.57 — 0.15–0.20 0.19 — 0.005

a Tm¼melting temperature, Td¼ decomposition temperature, Tign¼ thermal ignition temperature, Tdet¼ detonation temperature, r¼ density, OB¼ oxygen balance (for CaHbNcOd to be converted to CO2 and H2O (without crystal water) OB (%)¼ 1600[(d� 2a� b/2)/Mw] whereMw is themolecularweight),N¼%nitrogen content,DHf¼ enthalpy of formation,DHdet¼ enthalpy of detonation, Qexp¼ heat of explosion, Pdet¼ detonation pressure,Vdet ¼ detonation velocity, IS ¼ impact sensitivity, FS ¼ friction sensitivity, ESD ¼ electrostatic discharge sensitivity. b Calculated value.c Experimental value. d Original value has been converted to units employed in this review for clearer comparison.

Feature Article Journal of Materials Chemistry A

added benet of higher densities and better oxygen balance thantheir carbocyclic analogues, and tend to be more environmentallyfriendly since their main decomposition product is dinitrogen gas.The composition of energetic ionic liquids (EILs) reported in theliterature follows a distinct trend in which ions are employed. Thecations generally feature substituted N-heteroaromatic rings orammonium derivatives: a majority of cations includes imidazo-lium,26–38 triazolium,17,18,26,28,31,32,39–45 tetrazolium,32,36,39,40,46,47

Fig. 2 A selection of choice ions toward energetic ionic liquids.


ammonium,26,30,48 iminium,39,44 triazanium,49 or hydrazinium.50–52

Anions commonly consist of azolates,17,27,29,30,38,40,43,44 dicyana-mides,28,31,34,37,48–50,52,53 dinitramides,18,31,45,47,48,54 nitro-cyanamides,36,48,49,52,53 nitrocyanomethanides,32,49 methane-sulfonates,33 bis(triuromethylsulfonyl)imide,33 picrates,35

nitrates,17,18,31,32,34,35,42,45,49,50,53,54 perchlorates,17,31,45,47 azides,41,48,51

borohydrides,55 cyanoborates,26,53 or metallic nitro complexes39,46

(Fig. 2).

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Table 4 Performance Requirements for EILs2,a

Physical properties

Tm <�40 �CY <100 dyne cm�1

r >1.4 g cm�3

h As low as possibleWS Hydrolytically

stable

Hazard sensitivity

IS >5 JFS >120 NEDS >5000 V at 0.25 J

Thermal stability

Td >150 �CIsothermal (75 �C) <1% loss over 24 h

Thermodynamic properties

DHf As positive aspossible

DHcom >25 kJ g�1

Toxicity

LD50 >0.5 g kg�1

AMES Negative

a Tm ¼ melting temperature, Y ¼ surface tension, r ¼ density, h ¼viscosity, WS ¼ water sensitivity, IS ¼ impact sensitivity, FS ¼ frictionsensitivity, EDS ¼ electro-static discharge sensitivity, Td ¼decomposition temperature, DHf ¼ heat of formation, DHcom ¼ heatof combustion, LD50 ¼ lethal dose 50%, AMES ¼ mutagenic potentialtest.

Table 5 UN classification for the transport of dangerous goods14,59

IS (J) FS (N)

Insensitive >40 >360Less sensitive 35–40 ca. 360Sensitive 4–35 80–360Very sensitive <4 10–80Extremely sensitive — <10

Fig. 3 Reaction scheme for symmetrical dimethyl hydrazine basedsalts.50 Anions in grey correspond to salts which fail to meet the ILdesignation.


The main focus of EILs has been towards the production ofnew propellants.56 For EILs to be used as propellants, they mustexhibit several key characteristics (Table 4). Most importantly,they need to possess high energy density, oen found inmaterials with large positive heat of formation (DHf).57 Thisleads to a high combustion chamber temperature, and in turn ahigh specic impulse (Isp) which is a measure of the fuel'sefficiency.53,57 A propellant's physical properties should includelow vapour pressure, low viscosity (h) for facile fuel-oxidizermixing (in the case of bi-propellant systems), and high bulkdensity (r); all allowing for simpler, more efficient devicedesign.26,57 Furthermore, they must exhibit a wide liquid range:low melting point (Tm) and high thermal stability (Td –

decomposition temperature). Evidently, the handling andstorage characteristics will play a crucial role in determining thepractical use of a given propellant. Important factors toconsider are: high resistance to thermal, mechanical (IS –

impact sensitivity, FN – friction sensitivity, Table 5) and elec-trical (EDS – electrical discharge sensitivity) shock, low

8156 | J. Mater. Chem. A, 2014, 2, 8153–8173

corrosivity, low toxicity, low re hazard, no deterioration withinstorage, inertness with the atmosphere and hydrolytic stabili-ties.57 Other indicators used to determine and characterise thepotency of a propellant are detonation pressure (Pdet), anddetonation velocity (ndet).

Despite the intensive use of ILs, EILs have been relativelyunexplored.2,12,58 We herein present a review of recent literature(2005 to 2012) which has focused on propellant applications.For clarity purposes we have divided the aforementioneddiscussion into bi- and monopropellant sections. Each sectionis subdivided by the targeted functional properties of thecompounds' structure. Due to inconsistencies in the reportedproperties of new energetic materials (such as simple safetytests, hypergolicity tests, and computational calculations),many were sorted based on their structures.

Bipropellant energetic ionic liquids

Within rocket fuel applications, hydrazine and its derivativesare commonly used in combination with powerful oxidantssuch as dinitrogen tetroxide (N2O4) and white fuming nitric acid(WFNA). Such mixtures are referred to as hypergolic bipropel-lants, indicating that they ignite upon contact due to anexothermic redox reaction.49 With its dangerous (carcinogenicand toxic) properties and high volatility, hydrazine and itsderivatives pose many risks in all stages of manipulation,resulting in costly safety procedures.26,49 Therefore, muchattention has been directed towards producing an alternative tohydrazine, and EILs have taken a lead due to their intrinsic ILsproperties (very low volatility), signicantly decreasing the toxicrisk. Various EILs have been found to exhibit hypergolicbehaviour. Originally, this behaviour was mainly observed forEILs with dicyanamide and nitrocyanamide anions, leading tothe postulation that the anion was responsible for hypergolicbehaviour.26 For hypergolic EILs to successfully replace


Fig. 4 Reaction scheme for 2,2-dialkyltriazanium based salts.49

Fig. 5 Reaction scheme for N,N-dimethylhydrazinium based EILs.52

Anions in grey correspond to salts which fail to meet the ILdesignation.

Fig. 6 Reaction scheme for 1,5-diamino-4H-tetrazolium basedsalts.47 Anions in grey correspond to salts which fail to meet the ILdesignation.

Fig. 7 Reaction scheme for 1,5-diamino-4-methyl-tetrazolium basedsalts.47 Anions in grey correspond to salts which fail to meet the ILdesignation.


hydrazine they must exhibit the requirements previouslymentioned. In addition, they must possess shorter ignitiondelay (ID) times than the conditions leading to their owndetonation.

Finding a replacement for hydrazine is one of the main goalsof researchers in this eld. Investigating the conversion ofhydrazine, and its derivatives, into EILs was one path exploredfor nding such an alternative. Sabate and co-workers usedN,N0-dimethyl hydrazine dihydrochloride to synthesize a familyof four new hydrazinium salts (Fig. 3).50 These were all reportedto be stable during electrostatic discharge sensitivity tests, andfailed to detonate when exposed to a Bunsen burner ame.However, only the nitrate (1a) and dicyanamide (1c) analoguesare classied as ILs, and are potential hypergols. A trendemerged within the series, showing that salts based on feweroxidizing anions were more resistant to impact and friction,showing no signs of decomposition at the maximum loading forBAM drop hammer and friction tests (maximum loadings IS >40 J and FS > 360 N).59 Such was the case for 1c, which possessedthe largest liquid range of all salts produced at 140 �C; r 1.420 gcm�3, Tm 37 �C, Td 180 �C, DHf 2842 kJ mol�1, oxygen balance(OB) �140. Analogues consisting of more oxidizing anionsdecomposed explosively at lower loadings. Despite failing todetonate when exposed to a ame, 1a showed a substantialenergetic reaction accompanied by a gentle transition fromburning to deagration; r 1.582 g cm�3, Tm 98 �C, Td 220 �C,DHf

1029 kJ mol�1, OB �26.50 Both have a negative oxygen balance(�26 and �140 respectively).


In hopes of achieving larger positive heats of formation,Shreeve and co-workers introduced an extra N–N bond in thecation partner, leading to a family of seven EIL based on a 2,2-dialkyltriazanium cation (Fig. 4).49,60 Only four of these EILs(the 2a, 2b, 2c, and 2d analogues) were found to be hypergolicwith either N2O4 or WFNA, with 2b giving the best results; Tm99.0 �C, Td 145.6 �C, IDN2O4

10 ms, IDWFNA 4 ms, Pdet 22.2 GPa,ndet 8034 m s�1, Isp 228 s, IS > 60 J. With the second bestperformance results in this series, the RTEIL 2d would make abetter choice for practical applications; Tm �0.19 �C, Td145.7 �C, IDN2O4

8 ms, IDWFNA 16 ms, Pdet 16.0 GPa, ndet 7169 ms�1, Isp 226 s, IS > 60 J. The cation was then proved to play avital role since the chloride precursor (2a) was also found to behypergolic. Therefore, the anion is not solely responsible forthe hypergolic properties as previously expected.49,61 Thisgroup later developed a series of N,N-dimethylhydraziniumEILs (Fig. 5) which paired cations of varying side chain lengthswith dicyanamide and nitrocyanamide anions in attempts toinvestigate which components were most relevant to hyper-golicity.52 The whole series was found to be hypergolic withWFNA with IDs ranging from 22 ms to 1642 ms. The authorsobserved that the nitrocyanamide analogues (4a–4g)possessed greater thermal stabilities as well as higher densi-ties, viscosities, and ignition delay times than their dicyana-mide analogues. They held the alkyl substituents accountablefor trends in the densities and viscosities of both series, withthe –CH2CN (g) analogues having the highest values for bothproperties. Hydrazinium derivatives 3b, 3d, and 3e are the besthypergols with ID times of 22 ms, 24 ms, and 30 ms respec-tively. 3d proved to have the overall most desirable properties;Tg <�60 �C, Td 199.2 �C, IDWFNA 24 ms, Pdet 9.09 GPa, ndet 6057m s�1, Isp 204.0 s.

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Fig. 8 5-Aminotetrazolate based EILs.44 7a 4-amino-1-ethyl-1,2,4-triazolium 5-aminotetrazolate, 7b 4-amino-1-methyl-1,2,4-triazolium5-aminotetrazolate, 7c aminoguanidinium 5-aminotetrazolate.

Fig. 10 Reaction scheme for 1-amino-1,2,3-triazolium based salts.18



Also aiming for high heats of formation, Klapotke and co-workers sought to explore N-rich heteroaromatic compounds. Afamily of ve salts based on aminotetrazoles was synthesized,granting some of the highest nitrogen containing organicsubstances, yet exhibiting surprisingly high thermal stabilities(Fig. 6 and 7).47,62–66 Only two compounds fall within the ILscharacterisation (5a, 6c) with 6c being the most promising; r1.719 g cm�3, Tm 85 �C, Td 184 �C, DHcom-cal �2171 kJ mol�1,DHcom-exp �1976 kJ mol�1, DHdet �1091 kJ mol�1, DUcom DV¼0

�2000 kJ mol�1, OB �25.3, IS 7 J, FS 24 N. With a calculateddetonation pressure of 33.6 GPa and a detonation velocity of8827 m s�1, 6c is comparable to the high explosive RDX (Pdet ¼34.7 GPa, ndet ¼ 8750 m s�1).14 In addition to having similarperformance results, 6c is also comparable in terms of itsimpact sensitivity to RDX (IS ¼ 7.4 J), and less sensitive whencompared to other common explosives including Tetryl (IS ¼ 3J) and PETN (IS ¼ 3 J).14,47 Unfortunately this EIL exhibited thehighest friction sensitivity (FS 24 N) of all the salts produced,making it more sensitive than PETN (FS 60 N). It is, however,more stable than the primary explosive lead azide (FS 1 N).14 Nohypergolic tests were reported for these compounds.

Increasing the nitrogen content of stable room temperatureEILs (RTEILs) has proven to be a challenge as N-rich heterocy-clic rings generally possess higher melting points for theirrespective salts.44 Shreeve and co-workers also sought to employ5-aminotetrazole based salts, but rather than utilising it as aweak base, as commonly employed,40,67–70 they used it as a weakacid to provide the anion.44 The eight salts published were all

Fig. 9 Reaction scheme for 1-amino-1,2,3-triazolium based salts.18


8158 | J. Mater. Chem. A, 2014, 2, 8153–8173

found to be stable at room temperature for several months.More impressively, they were all stable in water, even whenheated to 100 �C. Only three of these salts are ILs (Fig. 8): 7a (Tg�38 �C, Td 171 �C, Pdet 16.4 GPa, ndet 7397 m s�1), 7b (Tg�24 �C,Td 174 �C, Pdet 18.9 GPa, ndet 7334 m s�1), and 7c (Tm 96 �C, Td211 �C, IS > 60 J, Pdet 20.1 GPa, ndet 8149 m s�1).44 This resultedin the rst 5-aminotetrazolate based family of ILs. Furthermore,7a and 7b fall within the designation of RTEILs; the low meltingpoints being attributed to the asymmetry of the 4-amino-1-alkyl-1,2,4-triazolium cation. As of 2008, 7b has been the RTEIL withthe highest nitrogen content percentage (68% weight). Hyper-golic tests are also required for these compounds.

A recent example of tetrazolate based EILs has been reportedby Pang and co-workers18 who employed a couple of 1-amino-1,2,3-triazolium based counter-cations (Fig. 9 and 10). Six of thenine compounds synthesized are ILs and potential hypergols,though with quite high melting points (>80 �C). The IL with thelowest melting point, 9c, has the best oxygen balance and thesecond best calculated performance values of the series; Tm 82.2�C, Td 176.5 �C, OB �10, Pdet 29.7 GPa, ndet 8320 m s�1. Theimpact sensitivity of 9c could not be accurately determined dueto its deliquescent nature. In comparing the series, it was foundthat the methyl group on 3-methyl-1-amino-1,2,3-triazole basedsystems helped to lower the melting point and reduce the

Fig. 11 Triazolium based EILs azides.41 10a 1-methyl-4-amino-1,2,4-triazolium azide, 10b 1-allyl-4-amino-1,2,4-triazolium azide, 10c 1-(2-azidoethyl)-4-amino-1,2,4-triazolium azide, 10d 1-(2-hydroxyethyl)-4-amino-1,2,4-triazolium azide, 11a 1-amino-3-methyl-1,2,3-tri-azolium azide, 11b 1-amino-3-allyl-1,2,3-triazolium azide.


Fig. 13 Reaction scheme for azide anion based ILs; 15 triethy-lammonium azide and 16 N,N-dimethylisopropylammonium azide.48


impact sensitivity. Furthermore, comparisons with 1,2,3-tri-azole based systems suggest that incorporating amino groupsfurther assists in lowering the melting point.45 Overall, thisseries was thermally stable, with liquid ranges reaching as far as100 �C.

Azido groups embody another alternative to utilise N–Nbonds for energy storage: they have been calculated to addapproximately 280 kJ mol�1 per azido group to the energycontent of a molecule.48 Further calculations suggest that theheat of formation of the azide anion in the gas phase (197.2 kJmol�1) is signicantly greater than that of the dicyanamide andnitrocyanamide anions (113.4 kJ mol�1 and �27.1 kJ mol�1

respectively), which are commonly employed in the design ofhypergolic ILs.48,49,71,72 Klapotke and co-workers reported severalsystems outlining the properties of hydrazinium azide basedEILs and energetic salts.41,51,73–79 Unfortunately, thesecompounds were volatile and/or hygroscopic materials whichalso liberated the undesirable and unstable HN3 elimination by-product. In 2008, Schneider and co-workers introduced tri-azolium azide based EILs (Fig. 11).41 Instead of employing AgN3

to replace the intermediates' halide counter-ion as is commonpractice, a polymeric quaternary ammonium azide exchangeresin was used. This alternative preparative route was deemedsafer and easier to perform; however, a 2% ammoniumcontamination could not be avoided. These new azide EILs, andpotential hypergols, were remarkably less sensitive compared tonotoriously sensitive covalent azides, and even to RDX (�50 kgcm or 7.4 J).41 The 2-azidoethyl-1,2,4-triazolium analogue (10c)was found to be stable enough for safe handling. The thermalstability, negligible volatility and low vapour toxicity for azide-based EILs were attributed to the quaternary nitrogen. Therelatively low melting points were attributed to the allyl-,2-hydroxethyl-, and 2-azidoethyl functionalities. Unfortunately,the more energetic allyl- and the 2-azidoethyl-side chain func-tionalities lowered the decomposition temperatures.

Continuing in the pursuit of higher heats of formations,Shreeve and co-workers set out to synthesize EILs based on 2-

Fig. 12 Reaction scheme for 2-azido-N,N,N-trimethylethylammo-nium based ILs, as well as the bis(2-azidoethyl)ammonium and bis(2-azidoethyl)dimethylammonium based ILs.48 The IL 12b was alsoreported by Schneider and co-workers.81


azido-N,N-dimethylethylamine (DMAZ), an alternative(reduced-hazard) liquid fuel developed by the U.S. Army, aswell as azide counter-anion based ILs (Fig. 12 and 13).48 Theseazide based EILs were obtained either through metathesisreactions with silver azide or through direct neutralizationwith hydrazoic acid. Of the eleven salts presented, all but onefall within the IL range, with 12b, 12c, 12d, 13a, and 13bclassifying as RTEILs. Drop tests with N2O4 and WFNA resul-ted in 12c, 12b, 13a, 13b, 15, and 16 exhibiting hypergolicity.Among the RTEILs, 12c and 13a possess the shortest ignitiondelay times for WFNA (8 ms and 16 ms, respectively). With anignition delay of 8 ms with WFNA, 12c exhibits the samebehaviour as monomethyl-hydrazine with inhibited redfuming nitric acid (IDIRFNA 8 ms),80 displaying promise as ahypergolic propellant. Despite not being RTEILs, 15 and 16(Tm 80 �C, IS > 40 J, IDWFNA hypergolic, IDN2O4

explosion, Isp 245s; and Tm 75 �C, IS > 40 J, IDWFNA hypergolic, Isp 249 s,respectively) were reported as the rst azide-based ILs exhib-iting hypergolicity. Both were hypergolic with N2O4, and 15was found to explode when exposed to WFNA. Both of thesecompounds are, however, extremely hygroscopic.

Fig. 14 Dicyanoborate-based EILs.26

J. Mater. Chem. A, 2014, 2, 8153–8173 | 8159

Fig. 15 Aluminum borohydride based EIL.55Fig. 17 Ionic liquid solubilized boranes.85


Hypergolic redox reactions observed between alkali metalborohydride salts andWFNA in preliminary experiments turnedthe attention of Shreeve and co-workers toward borohydridebased anions: borohydride, cyanoborate and dicyanoborate.Using these anions, several ILs were synthesized but theborohydride and cyanoborate analogues were found to be toowater-sensitive for practical applications.26 However, the dicya-noborate based EILs, could themselves be synthesized in water,generating ten new EILs found to be hypergolic with WFNA(Fig. 14). The 17f analogue possessed the shortest ID times (Tm<�80 �C, Td 189 �C, ID 4 ms) and the 17j analogue had thelowest viscosity (Tm <�80 �C, Td 266 �C, h 12.4 mPa s, ID 8ms).26

Regardless of the cation, the dicyanoborate analogues exhibitedlower melting temperatures, lower densities, lower viscositiesand lower ignition delays than their corresponding dicyana-mide and nitrocyanamide analogues. The drastic reduction inignition delay times (81 / 28 ms, 130 / 4 ms, 46 / 8 ms)suggests that the B–H bond is responsible for the hyper-golicity.26 These promising results initiated a shi towardsinvestigating hydride-based anions.

In parallel, searching for the ultimate green rocket fuelalternative, Schneider and co-workers focused not only on theproperties of EILs but also on the properties of the oxidizingpartner. Hydrogen peroxide was deemed attractive due to itsgood oxidizing properties, low corrosivity and low vapourtoxicity in comparison to WFNA, in addition to environmentallyfriendly decomposition products.55 However, hydrogenperoxide and WFNA are comparable in terms of storage andhandling risks.82,83 In order to achieve better fuel performance,they utilised an aluminum borohydride anionic complex, takingadvantage of the large combustion energies of these lightnontoxic metals, while simultaneously providing a large anddense hydride content. This anion was coupled with a trihex-yltetradecylphosphonium cation, which would not be reducedby the borohydride components of the anion, unlike thecommonly used heterocyclic cations, to give a RTEIL (Fig. 15). It

Fig. 16 EILs doped with boron nanoparticles.28

8160 | J. Mater. Chem. A, 2014, 2, 8153–8173

is noteworthy that this EIL is completely stable in air (Tg�70 �C,Td > 150 �C). Drop tests of this EIL into oxidizers resulted inhypergolicity with 90% H2O2 (ID < 30 ms), 98% H2O2

(ID < 30 ms), N2O4 (EIL ignited with the vapours of N2O4 prior tothe liquids mixing), and WFNA (resulted in explosion ratherthan combustion).

With boron gaining a lot of attention as a benecial compo-nent of EILs, Rogers and co-workers developed an originalapproach to include boron nanoparticles suspended in an EIL,using its solvent properties (Fig. 16).28 This technique producedair stable nanoparticles, which were free of the oxide layer(requiring ignition temperatures of 1500 �C) that normally coatsboron additives. Milling boron nanoparticles with 1-methyl-4-amino-1,2,4-triazolium dicyanamide (20), and dispersing thesewithin the EIL by sonication, led to no signicant changes in theignition delay with respect to the clean hypergolic EIL withWFNA (ID 45 � 14 ms vs. 37 � 6 ms). It did, however, result in ashorter, much more intense burn (ame duration 43 � 4 ms vs.77� 18ms). A similar experiment was carried out with 1-butyl-3-methylimidazolium dicyanamide (19), but led to little enhance-ment in hypergolicity and a complex burn pattern.Investigations into the surface binding of these ILs to the oxide-free boron nanoparticles found that both the cation and anion ofboth ILs interacted with the surface.84 The ILs differed in thedegree of interaction of each species; for 20 the cation's inter-actions dominated, while for 19 the anion's interactions domi-nated; the differences being attributed to the amino group of the1-methyl-4-amino-1,2,4-triazolium cation, which also appearedto induce a thicker capping layer. These results suggest that thedicyanamide anion is in this case responsible for the hyper-golicity, and the strong anionic interactions with the nano-particles will negatively affect performance. For these additivesto be benecial, the cation's coordinative capabilities need todominate and secure a thicker capping layer.

Another, more recent example of utilising the solvent prop-erties of ILs comes from Shreeve et al. (Fig. 17), whom employedILs to solubilise various boranes. In preliminary tests, thesolids: ammonia borane (a), hydrazine borane (b), and hydra-zine bis-borane (c) were all found to exhibit hypergolic behaviorwith WFNA (80, 4, and 12 ms respectively).85 Dissolving a and bin the non-hypergolic IL 21, resulted in hypergolic solutions (88and 390 ms respectively). Solutions with known hypergolic EILs19 (47 ms with WFNA) and 22 (44 ms with WFNA) with all threeboranes (a–c) proved to all be hypergolic as well. The dissolved


Fig. 18 Bis(1-substituded-imidazole-3-yl)dihydroboronium based EILs.53 Fig. 19 Tetranitratoaluminate based EIL.46


boranes reduced the IDs of the EILs signicantly, with valuesranging from 3 to 34 ms. Assumingly, due to varying solubilitiesof the boranes in each IL, the authors did not assess thecontribution of each borane to the hypergolicity of the resultingliquid. The best performing solutions with IDs of 3 ms were the19a and 19b with mole ratios of 2.70 : 1 and 2.4 : 1 respectively.

Highly reactive B–H bonds led to the incorporation of boroninto the cationic partner of EILs, combining this reactivity withthe stability and high heats of formation of imidazole rings.53 Ofthe various EILs achieved (Fig. 18) the 24d analogue proved tobe the most promising; r 1.05 g cm�3, Tm �80 �C, Td 266 �C, h35 mPa s, IDWFNA 14 ms, Isp 162.4 s. Generally, the allylanalogues possess lower melting temperatures, and shorterignition delay times.

With the advances over the last decade, hypergolic EILs areshowing great promise as replacements for common propel-lants; such as hydrazine. They have opened new doors towardsfuel optimization beyond simple fuel mixing, where solidadditives may now be easily incorporated. Targeting N-richsystems, EILs could provide much cleaner sources of fuel,decomposing mainly into N2. Unfortunately, despite all of theadvancements in bipropellant systems, a problem which yetremains is the corrosive nature of the employed oxidants. Thus,in order to nd greener fuels, future research must focus onalleviating this problem, likely aiming for milder oxidizers withenvironmentally friendly decomposition products.

Monopropellant energetic ionic liquids

In addition to hydrazine's use as a bi-propellant, its purposealso includes monopropellant applications. The concept ofusing EILs for monopropellant applications was rst suggestedby Karl Christe in 1998.86 Since then, a few attempts atdesigning alternative EILs that could act as monopropellantswere investigated. The targeted monopropellants, or self-oxidizing EILs, are a benecial combination of a cationic fuelpartner with an oxygen-rich anionic oxidizer, making itunnecessary to resort to external strong oxidants. This reducesthe cost related to the use of toxic and corrosive oxidizers, andmakes monopropellants suitable for straightforward greenapplications.

Most known EILs suffer from relatively low oxygen content,resulting in poorer performance than traditional energeticmaterials.46 Even those EILs containing small anions, such asnitrates, perchlorates, and dinitramides, have insufficient


oxygen content to completely oxidize their correspondingcations;39 hence EILs must be coupled with powerful oxidizers.Since CO2 essentially dissociates into CO and O2 at elevatedtemperatures (Boudouard equilibrium),87 the best performancemay be achieved, in addition to low molecular weight exhaustgas products, by oxidizing the carbon content to CO rather thanCO2.46 EILs with such an oxygen balance (OBCO ¼ 1600[(d � a �b/2)/Mw] whereMw is the molecular weight for a compound withformula CaHbNcOd vs. OBCO2

¼ 1600[(d� 2a� b/2)/Mw) could bepotential monopropellants. In addition to removing the needfor corrosive oxidants such as WFNA, this class of compoundsbenets from enhanced stability. In combining fuel in cationicform and an oxidizer in anionic form, a more oxidizer-resistantcation and an anion protected from premature reduction aregenerated. More specically, the formal positive charge of thecation increases its ionization potential, while the formalnegative charge of the anion decreases its electron affinity.46

In 2006, Christe and co-workers produced an oxygen-balanced (OBCO) monopropellant EIL using thetetranitratoaluminate anionic complex and the 1-ethyl-4,5-dimethyltetrazolium cation (DHf free gaseous cation 836 kJmol�1, anion �1486 kJ mol�1) (Fig. 19).46 The anion wasselected for its high oxygen content (10.5 of its 12 oxygen atomsare available for reduction), while the cation was chosen due toits high heat of formation and its ability to form ILs. Theisomeric mixture (impurities were not removed since theyresulted in a lower melting temperature without affecting theenergetic properties) has a glass transition temperature of�46 �C and a strongly exothermic decomposition with onset at183 �C and maximum at 217 �C. Keeping the EIL at 75 �C for 4hours results in a 10.4% weight loss, attributed to the loss ofNO2 and O2, along with the formation of Al–O–Al bridges, ascommonly reported for polynitratoaluminates.88 A spectacularignition was achieved using a hot 40-gauge Ni/Cr wire wrappedaround a glass capillary lled with the EIL within a few secondsof applying current, or by heating it to about 200 �C.46 Thepredicted idealized combustion reaction, showing the produc-tion of alumina observed upon combustion is:

2[N4C5H11]+[Al(NO3)4]

� / Al2O3 + 8N2 + 11H2O + 10CO (1)

Unfortunately, EIL 25 was subject to hydrolysis when exposed towater. This problem was overcome by an anion substitutionfrom aluminum nitrate to lanthanide nitrate complexes, givingmoisture stable EILs; the improved stability being attributed tothe strong coordination affinity between lanthanide ions and

J. Mater. Chem. A, 2014, 2, 8153–8173 | 8161

Fig. 20 Hexanitratolanthanate based EILs.39 Anions in grey correspond to salts which fail to meet the IL designation.


oxygenated ligands.39 Of the ten ILs synthesized (26a–f and 27a–f, Fig. 20), the b, c, and d analogues (all RTEILs) had a negativeoxygen balance (OBCO), while the a analogues had a positiveoxygen balance. The remaining set, the f analogues, wereperfectly balanced at zero (OBCO). Despite not being RTEILs, thea and f analogues easily adopt supercooled phases that onlysolidify aer being stored for days at room temperature. Thewhole series was found to be hydrophilic and moisture-stable.Heating 1 mg of 26f to 200 �C led to rapid gas evolution, leavingwhite lanthanum oxide powder residue, following the reaction:

2[N6C2H7]3+[La(NO3)6]

3�/La2O3 + 24N2 + 21H2O+ 12CO (2)

A main drawback to these two balanced analogues is theirimpact sensitivity (IS 27 J). Repeating this test six times for 26fand 27f led to three and two explosions, respectively.

An original approach to obtaining improved oxygen balancewas brought forth by Drake and co-workers who investigatedpolyoxyamine systems.89 Methylene bisoxyamine (H2N–O–CH2–

O–NH2) based systems showed interesting results, but were fartoo unstable (unexpected and untimely deagrations shown byseveral of their salts). Having postulated that the geminal oxy-amines may be unstable under acidic conditions, attention wasturned to 1,2-bis(oxyamine)ethane based salts in the hope ofgenerating more robust systems (Fig. 21). Half of the synthe-sized compounds proved to be ILs, with 29c qualifying as aRTEIL; Tm < 0 �C, Td 115–120 �C, IS # 20 kg cm, FS < 0.45 kg.However, this RTEIL proved to be very sensitive, having a highlyexplosive response, completely destroying the scratcher fromthe Julius Peters style friction tester. Themost stable IL was 28b;

Fig. 21 1,2-Bis(oxyamine)ethane based EILs.54 Anions in grey corre-spond to salts which fail to meet the IL designation.

8162 | J. Mater. Chem. A, 2014, 2, 8153–8173

Tm 73–76 �C, Td 80 �C, IS 94 kg cm, FS 22.8 kg; showing lessimpact sensitivity (OlinMatheson style drop weight tester with 2kg mass) than the HMX standard (IS 34 kg cm). Its melting anddecomposition temperatures, however, fall short of the desir-able values. On the whole, ethylene bis(oxyamine) salts wereextremely sensitive to impact and friction, possessed fairly shortliquid ranges which were followed by rapid and highlyexothermic decompositions, and were unstable (exhibitingvisible changes) even when stored in inert atmospheres. Theinstability of bis(oxyamine) salts is in contrast to mono-oxy-amine (such as hydroxylamine and picryloxyamine). Indeed, forthe latter, the anhydrous unprotonated species are quiteunstable, but the corresponding protonated salts can be storedindenitely at ambient conditions.54,90–92

In comparison to bipropellant systems, monopropellantshave been underdeveloped. Though, in concept, they show greatpromise as the ultimately green rocket fuel alternative, none ofthe present systems meet all the required criteria: they are toosensitive to either stimuli or moisture. Further investigationswould surely lead to better systems.

A comprehensive comparison of all EILs discussed in thisreview can be observed in Tables 6 and 7 as a visual aid, dis-playing the physical and energetic data available. The moststriking observation is the signicant lack of uniformity inreporting pertinent physical properties (energetic sensitivityand reactivity) making an objective well-founded comparison ofthe potential of each EIL challenging.

Conclusions

Accompanying the extensive development of the eld of ionicliquids (ILs) over the past twenty years, interest has beengrowing specically within the area of energetic ionic liquids(EILs). Mainly focused toward the design of viable, powerfulalternatives to currently available rocket fuels, ideal EILs needto surpass the basic requirements of ILs (ionic pairs that meltbelow �40 �C rather than 100 �C), in addition to have inter-esting energetic properties. The latter requirement involves agood compromise between stability in ambient conditions (air,moisture), relative sensitivity toward certain stimuli and highenergy release. In this review we have outlined recent advancesin the design and preparation of EILs, along with the energeticdata available for these compounds. Inconsistencies in


Tab

le6

Comparativetable

ofim

portan

tphysical

propertiesofEILspresentedin

thisreview

q

Com

poun

dRef.

r (gcm

�3)

hb (mPa

s)Tmb

(�C)

Tdb

(�C)

Tdeta

(�C)

DHf-cata

(kJmol

�1)

DHf-an

a

(kJmol

�1)

DHf-salta

(kJmol

�1)

DHcoma

(kJmol

�1)

DHdeta

(kJmol

�1)

DUcomb

(kJmol

�1)

Thermal

shock

Isothermal

(75

� Cfor48

h)

1a50 (201

2)1.58

2a—

98e

220e

4472

i,j

——

192i,l

——

—Burnsfast

—1c

1.42

0b—

37e

180e

2102

i,j

——

552i,l

——

—Burns

slow

ly—

2a49 (200

9)1.47

——

——

756.8k

�230

.3k

�50.2k

——

——

—2b

1.47

—99

.0f

145.6f

—75

6.8k

�307

.9k

�95.6k

——

——

—2c

1.15

—10

.7f

134.2f

—75

6.8k

113.4k

363.7k

——

——

—2d

1.26

—�0

.19f

145.7f

—75

6.8k

�27.1k

228.5k

——

——

—2e

1.20

—74

.4f

153.3f

—75

6.8k

200.3k

465.4k

——

——

—2f

1.35

—47

.8f

142.5f

—75

6.8k

32.2

k29

6.8k

——

——

—2g

1.48

——

134.2f

—75

6.8k

�127

.7k

138.2k

——

——

—

3a52 (201

0)1.10

c—

60.5

f25

3.5f

—64

6.5k

113.4k

258.38

i,k

——

——

—3b

1.06

c67

.530

.9f

267.1f

—61

3.9k

113.4k

243.66

i,k

——

——

—3c

1.01

c11

3.9

20.4

f26

3.3f

—57

8.3k

113.4k

232.73

i,k

——

——

—3d

1.05

c78

.6—

199.2f

—81

4.6k

113.4k

453.14

i,k

——

——

—3e

1.13

c22

8.6

—17

4.3f

—90

9.4k

113.4k

538.55

i,k

——

——

—3f

1.15

c16

1.8

—23

6.0f

—47

8.3k

113.4k

109.57

i,k

——

——

—3g

1.17

c10

57.0

—14

4.8f

—84

4.9k

113.4k

470.29

i,k

——

——

—4a

1.24

c—

35.2

f29

2.4f

—64

6.5k

�27.1k

119.28

i,k

——

——

—4b

1.17

c—

25.4

f29

6.9f

—61

3.9k

�27.1k

105.11

i,k

——

——

—4c

1.11

c11

9.5

9.0f

285.5f

—57

8.3k

�27.1k

93.49i,k

——

——

—4d

1.16

c84

.9—

208.2f

—81

4.6k

�27.1k

314.50

i,k

——

——

—4e

1.21

c26

9.8

—18

9.3f

—90

9.4k

�27.1k

403.69

i,k

——

——

—4f

1.26

c18

5.9

—26

9.1f

—47

8.3k

�27.1k

�28.68

i,k

——

——

—4g

1.28

c13

10.0

—19

3.5f

—84

4.9k

�27.1k

333.24

i,k

——

——

—

5a47 (200

5)1.90

2a—

9719

2—

——

192.0i,m,

169.0i,k

�959

i ,�8

99b,i

�859

i�8

33i

——

6c1.71

9a—

8518

4—

——

385.3i,m,

381.2i,k

�196

9i,

�179

2b,i

�989

i�2

000i

——

7a44 (200

8)1.39

c ,1.44

a—

�38d

171g

—82

8.2k

—52

3.4k

——

——

—7b

1.46

c ,1.49

a—

�24d

174g

—86

6.7k

—54

6.0k

——

——

—7c

1.51

c ,1.49

a—

9621

1g—

646.7k

—30

2.3k

——

——

—

8a18 (201

2)1.64

a,1

.75b

—98

.617

7.7

—97

2.1k

�306

.4k

119.3k

——

——

—8c

1.69

a,1

.69b

—98

.820

0.8

—97

2.1k

112.8k

577.1k

——

——

—9a

1.59

a,1

.59b

—87

.024

1.8

—95

1.6k

�306

.4k

116.4k

——

——

—9c

1.57

a,1

.70b

—82

.217

6.5

—95

1.6k

�247

.7k

234.1k

——

——

—

This journal is © The Royal Society of Chemistry 2014 J. Mater. Chem. A, 2014, 2, 8153–8173 | 816


3

Tab

le6

(Contd.)

Com

poun

dRef.

r (gcm

�3)

hb (mPa

s)Tmb

(�C)

Tdb

(�C)

Tdeta

(�C)

DHf-cata

(kJmol

�1)

DHf-an

a

(kJmol

�1)

DHf-salta

(kJmol

�1)

DHcoma

(kJmol

�1)

DHdeta

(kJmol

�1)

DUcomb

(kJmol

�1)

Thermal

shock

Isothermal

(75

� Cfor48

h)

9d1.59

a,1

.59b

—92

.622

4.3

—95

1.6k

112.8k

,o57

4.0k

——

——

—9e

1.56

a,1

.57b

—89

.619

7—

951.6k

386.1k

1056

.8k

——

——

—

10a

41 (200

8)—

—43

f12

9f

——

—56

9i,m

�275

1.67

i,m

——

——

10b

——

�57d

,f

109f

——

—67

4i,m

�393

0.85

i,m

——

——

10c

——

h10

6f

——

—93

3i,m

�366

0.66

i,m

——

——

10d

——

�50d

,f

129f

——

—38

9i,m

�325

1.25

i,m

——

——

11a

——

50f

156f

——

—62

3i,m

�281

0.72

i,m

——

——

11b

——

�62d

,f

114f

——

—72

4i,m

�397

9.81

i,m

——

——

12a

48 (201

0)1.22

c—

92f

139f

——

—57

8k—

——

——

12b

1.15

c—

9f

235f

——

—51

8k—

——

——

12c

1.24

c—

28f

245f

——

—38

0k—

——

——

12d

1.36

c—

RTf

219f

——

—25

1k—

——

——

13a

1.21

c—

RTf

222f

——

—89

4k—

——

——

13b

1.32

c—

RTf

222f

——

—75

2k—

——

——

141.41

c—

58f

162f

——

—77

7k—

——

——

151.01

c—

80f

Sublim

ates

——

—45

7k—

——

——

160.99

c—

75f

Sublim

ates

——

—43

0k—

——

——

17a

26 (201

1)0.91

c39

.4<�

80f

222f

——

——

——

——

—17

b0.92

c22

.3<�

80f

303f

——

——

——

——

—17

c0.96

c19

.8<�

80f

252f

——

——

——

——

—17

d0.99

c29

.9<�

80f

220f

——

——

——

——

—17

e0.96

c17

.3<�

80f

307f

——

——

——

——

—17

f0.93

c35

.0<�

80f

189f

——

——

——

——

—17

g0.94

c16

.6<�

80f

259f

——

——

——

——

—17

h1.00

c13

.5<�

80f

203f

——

——

——

——

—17

i1.03

c21

.0<�

80f

217f

——

——

——

——

—17

j0.99

c12

.4<�

80f

266f

——

——

——

——

—

1855 (201

1)—

—�7

0d,e,f

>150

e,f

——

——

——

——

Nomass

loss

1920 (200

8)1.06

39.14

�630

0—

——

——

——

——

With

boron

nan

o-pa

rticles

13 (201

1)—

——

——

——

——

——

——

8164 | J. Mater. Chem. A, 2014, 2, 8153–8173 This journal is © The Royal Society of Chemistry 2014


Tab

le6

(Contd.)

Com

poun

dRef.

r (gcm

�3)

hb (mPa

s)Tmb

(�C)

Tdb

(�C)

Tdeta

(�C)

DHf-cata

(kJmol

�1)

DHf-an

a

(kJmol

�1)

DHf-salta

(kJmol

�1)

DHcoma

(kJmol

�1)

DHdeta

(kJmol

�1)

DUcomb

(kJmol

�1)

Thermal

shock

Isothermal

(75

� Cfor48

h)

2020 (200

8)—

92�6

6d14

3—

——

——

——

——

With

boron

nan

o-pa

rticles

13(201

1)—

——

——

——

——

——

——

a85 (201

2),

93 (200

3)

——

111–11

4—

——

—�1

47.2

m,n

——

——

—

b85 (201

2),

94 (196

1)

——

61—

——

—�2

5.9m

,n—

——

——

c85 (201

2)—

——

——

——

�54.3m

,n—

——

——

2195 (200

1)1.17

66.5

�81d

360

——

——

——

——

—

21a1.7:1

85 (201

2)—

——

——

——

——

——

——

21b0.8:1

——

——

——

——

——

——

—21

c0.7:1

——

——

——

——

——

——

—19

a2.70

:1—

——

——

——

——

——

——

19b2.4:1

——

——

——

——

——

——

—19

c1.2:1

——

——

——

——

——

——

—

2296 (200

2)0.95

50�5

5—

——

——

——

——

—

22a2.1:1

85 (201

2)—

——

——

——

——

——

——

22b0.47

:1—

——

——

——

——

——

——

22c0.26

:1—

——

——

——

——

——

——

23a

53 (201

2)1.28

c—

90e

217e

—59

9.0k

�307

.9k

�164

.6k

——

——

—23

b1.12

c43

30e

196e

—59

9.0k

113.4k

134.5k

——

——

—23

c1.20

c60

<�80

e22

7e—

731.8k

�27.1k

1.6k

——

——

—23

d1.12

c—

62e

306e

—73

1.8k

�70.9k

291.5k

——

——

—



Tab

le6

(Contd.)

Com

poun

dRef.

r (gcm

�3)

hb (mPa

s)Tmb

(�C)

Tdb

(�C)

Tdeta

(�C)

DHf-cata

(kJmol

�1)

DHf-an

a

(kJmol

�1)

DHf-salta

(kJmol

�1)

DHcoma

(kJmol

�1)

DHdeta

(kJmol

�1)

DUcomb

(kJmol

�1)

Thermal

shock

Isothermal

(75

� Cfor48

h)

24a

1.15

c84

4<�

80e

198e

—59

9.0k

�307

.9k

273.7k

——

——

—24

b1.09

c69

<�80

e19

0e—

599.0k

113.4k

88.9

k—

——

——

24c

1.14

c10

3<�

80e

236e

—73

1.8k

�27.1k

430.5k

——

——

—24

d1.05

c35

�80e

266e

—73

1.8k

�70.9k

248.9k

——

——

—

2546 (200

6)—

—�4

6d,f

217f

—83

6l�1

486l

——

——

Spectacu

lar

ignition,

self-

sustained

burning

�10.4%

(NO2,O

2,

Al–O–A

lbridges)p

26a

39 (200

8)1.87

c—

81f

211f

—91

0.7k

�192

4.5k

�142

5.9k

——

——

—26

b1.76

c—

�24d

,f

231f

—86

6.6k

�192

4.5k

�148

7.6k

——

——

—26

c1.67

c—

�32d

,f

232f

—82

8.2k

�192

4.5k

�154

1.2k

——

——

—26

d1.59

c—

�33d

,f

229f

—78

2.6k

�192

4.5k

�159

5.8k

——

——

—26

f2.06

c—

88d,f

185f

—97

4.3k

�192

4.5k

�122

6.0k

——

——

—27

a1.88

c—

76d,f

212f

—91

0.7k

�191

1.6k

�141

5.4k

——

——

—27

b1.77

c—

�28d

,f

227f

—86

6.6k

�191

1.6k

�147

7.2k

——

——

—27

c1.67

c—

�33d

,f

228f

—82

8.2k

�191

1.6k

�152

7.4k

——

——

—27

d1.60

c—

�37d

,f

230f

—78

2.6k

�191

1.6k

�158

5.7k

——

——

—27

f2.08

c—

90d,f

187f

—97

4.3k

�191

1.6k

�121

8.3k

——

——

—

28b

54 (200

6)—

—73

–76

80—

——

——

——

——

28c

——

57–59

75–80

——

——

——

——

—29

c—

—<0

115–12

0—

——

——

——

——

aCalcu

lated

values.bMeasu

red

values.cDen

sity

measu

red

gaspy

cnom

eter

(25

� C).

dTg(glass

tran

sition

tempe

rature)is

repo

rted

.eDSC

/TGA

measu

remen

twith

5� C

min

�1.fDSC

/TGA

measu

remen

twith

10� C

min

�1 .

gNitrogen

ambien

tatmosph

ereformeasu

remen

ts.

hNot

observed

.iOriginal

valuehas

been

converted

toun

itsem

ployed

inthis

review

forclearer

compa

rison.jCom

putedus

ingEXPL

O5.

kCalcu

latedwithG3method

.lCom

putedus

ing(M

P2).

mCalcu

latedwithG2method

.nHeatof

form

ationin

solidph

asewhichwas

estimated

from

heatof

form

ationin

gasph

aseby

using20

kcal

(84kJ

mol

�1)as

theheatof

sublim

ation–calculated

byG2method

.oSeealso

ref.40

.pWeigh

tloss

aer

4hisothermal.qr¼

density,h¼

viscosity,

Tm

¼meltingtempe

rature,Td¼

decompo

sition

tempe

rature,Tdet¼

detonationtempe

rature,DHf-cat¼

heatof

form

ationof

thecation

,DHf-an

¼heatof

form

ationof

thean

ion,

DHf-salt¼

heatof

form

ationof

thesalt,D

Hcom¼

heatof

combu

stion,DHdet¼

heatof

detonation,DUcom¼

constan

tvolumecombu

stionen

ergy,Thermal

shock¼

compo

unds

areexpo

sedto

aBun

sen

burner

am

e,Isothermal

¼up

onbe

ingke

epat

aconstan

ttempe

rature

of75

� Cfor48

hthecompo

und

does

not

deteriorate.

Num

bers

inbo

ldindicate

that

theminim

umrequ

irem

ents,a

sou

tlined

inTab

le4,

aremet.



Tab

le7

Comparativetable

ofim

portan

tphysical

propertiesofEILspresentedin

thisreview

ab

Com

poun

dRef.

Na

OBa

ISb(J)

FSb(N

)EDSb

IDWFNAb

(ms)

IDN

2O

4b

(ms)

IDH

2O2b

(ms)

I spa(s)

P deta

(GPa

)n d

eta

(ms�

1 )V e

xp.ga

sa

(Lkg

�1)

1a50 (201

2)30

c�2

6>3

0e>3

60e

(�)l

——

—22

5aa

24.1

z,aa

8117

aa

893a

a

1c58

c�1

40>4

0e>3

60e

(�)l

——

—19

9aa

15.9

z,aa

6885

aa

671a

a

2a49 (200

9)38

c�1

29c

>60e

——

Nhm

26—

——

——

2b71

c�7

0c>6

0e—

—4

10—

228v

22.2

v80

34v

—2c

59c

�146

c>6

0e—

—22

Shr

—20

1v12

.1v

6516

v—

2d52

c�8

9c>6

0e—

—16

8—

226v

16.0

v71

69v

—2e

51c

�164

c>6

0e—

—Nhm

Nhm

—19

0v11

.1v

6207

v—

2f45

c�1

12c

>60e

——

Nhm

Nhm

—21

1v15

.5v

7009

v—

2g41

c�7

0c>6

0e—

—Nhm

Nhm

—22

7v21

.2v

7644

v—

3a52 (201

0)49

.19

�176

c—

——

58—

—18

9.4w

10.82w

6317

w—

3b44

.36

�191

c—

——

22—

—18

4.6w

9.79

w61

31w

—3c

38.35

�214

c—

——

46—

—18

0.0w

8.58

w59

32w

—3d

41.83

�196

c—

——

24—

—20

4.0w

9.09

w60

57w

—3e

41.46

�189

c—

——

30—

—21

0.0w

10.08w

6191

w—

3f40

.73

�164

c—

——

40—

—18

5.8w

10.88w

6281

w—

3g49

.72

�164

c—

——

1286

——

202.1w

11.03w

6318

w—

4a42

.98

�114

c—

——

126

——

213.4w

14.71w

7029

w—

4b39

.04

�132

c—

——

198

——

208.0w

12.56w

6681

w—

4c34

.10

�161

c—

——

228

——

201.2w

10.89w

6408

w—

4d36

.82

�141

c—

——

130

——

221.5w

12.19w

6596

w—

4e37

.47

�134

c—

——

134

——

227.2w

12.78w

6611

w—

4f36

.29

�113

c—

——

247

——

206.2w

14.09w

6880

w—

4g44

.22

�112

c—

——

1642

——

220.4w

14.52w

6899

w—

5a47 (200

5)41

.5�5

.57

60—

——

——

32.2

x83

83x

—6c

56.4

�25.3

724

——

——

—33

.6x

8827

x—

7a44 (200

8)63

.92,

63.94d

�126

c—

——

——

——

16.4

y73

97y

—

7b68

.82,

68.41d

�109

c—

——

——

——

18.9

y73

34y

—

7c79

.21,

78.80d

�86c

>60e

——

——

——

20.1

y81

49y

—

8a18 (201

2)47

.6�1

6*21

.5f

——

——

——

30.5

x83

66x

—8c

63.3

�28*

3.1f

——

——

——

27.0

x79

48x

—9a

43.5

�35*

28.9

f—

——

——

—23

.7x

7596

x—

9c39

.4�1

0*—

——

——

——

29.7

x83

20x

—9d

59.2

�41*

3.4f

——

——

——

23.1

x75

09x

—9e

60.1

�59*

6.9f

——

——

——

23.0

x75

24x

—



7

Tab

le7

(Contd.)

Com

poun

dRef.

Na

OBa

ISb(J)

FSb(N

)EDSb

IDWFNAb

(ms)

IDN

2O4

b

(ms)

IDH

2O2

b

(ms)

I spa(s)

P deta

(GPa

)n d

eta

(ms�

1)

V exp

.ga

sa

(Lkg

�1)

10a

41 (200

8)70

c�1

08c

>20z

,g>3

60j

——

——

——

——

10b

59c

�149

c13

z,g

——

——

——

——

—10

c71

c�9

8c>1

5,<2

0z,g

——

——

——

——

—10

d57

c�1

08c

17z,g

353j

——

——

——

——

11a

70c

�108

c>2

0z,g

>360

j—

——

——

——

—11

b59

c�1

39c

<6z,g

——

——

——

——

—

12a

48 (201

0)55

.80,

56.35d

�154

c>4

0e—

—Nhm

Nhm

—22

7w—

——

12b

48.01,

48.42d

�168

c>4

0e—

—20

Nhm

—20

2w—

——

12c

43.73,

42.21d

�123

c>4

0e—

—8

Nhm

—21

8w—

——

12d

40.15,

39.09d

�85c

>40e

——

Nhm

Nhm

—23

1w—

——

13a

54.98

53.65d

�147

c>4

0e—

—Nhm

16—

221w

——

—

13b

50.16,

48.94d

�113

c>4

0e—

—Nhm

226

—23

1w—

——

1457

.83,

56.67d

�86c

>40e

——

Nhm

Nhm

—24

5w—

——

1534

.53,

33.98d

�222

c>4

0e—

—Hyp

ern

Exp

o—

245w

——

—

1637

.80,

36.90d

�209

c>4

0e—

—Hyp

ern

Nhm

—24

9w—

——

17a

26 (201

1)30

.77,

29.97d

�237

c—

——

6—

——

——

—

17b

20.29,

19.94d

�267

c—

——

26—

——

——

—

17c

20.90,

20.37d

�251

c—

——

18—

——

——

—

17d

34.15,

34.07d

�215

c—

——

32—

——

——

—

17e

27.45,

26.74d

�235

c—

——

28—

——

——

—

17f

33.74,

33.79d

�222

c—

——

4—

——

——

—

17g

21.99,

21.50d

�255

c—

——

8—

——

——

—

17h

22.71,

22.67d

�238

c—

——

6—

——

——

—



Tab

le7

(Contd.)

Com

poun

dRef.

Na

OBa

ISb(J)

FSb(N

)EDSb

IDWFNAb

(ms)

IDN

2O4

b

(ms)

IDH

2O2

b

(ms)

I spa(s)

P deta

(GPa

)n d

eta

(ms�

1)

V exp

.ga

sa

(Lkg

�1)

17i

37.05,

37.07d

�199

c—

——

6—

——

——

—

17j

29.80,

28.85d

�221

c—

——

8—

——

——

—

1855 (201

1)0c

�324

c—

——

Exp

oHyp

ern,s

<30t,<

30u

——

——

1920 (200

8)34

c�2

14c

——

—47

——

——

——

With

boron

nan

o-pa

rticles

13 (201

1)—

——

——

44�

3p—

——

——

—

2020 (200

8)59

c�1

31c

——

—31

——

——

——

With

boron

nan

o-pa

rticles

13 (201

1)—

——

——

45�

14q

——

——

——

a85 (201

2)45

c�2

33c

——

—80

——

381.8x

16.1

x89

02x

—b

61c

�174

c—

——

4—

—27

0.2v,2

70.0

x14

.1v ,15

.7x

8151

v ,85

51x

—c

47c

�214

c—

——

12—

—30

8.6x

11.5

x75

06x

—

2195 (200

1)12

c�1

63c

——

—Nhm

——

——

——

21a1.7:1

85 (201

2)16

c�1

71c

——

—88

——

——

——

21b0.8:1

21c

�165

c—

——

390

——

——

——

21c0.7:1

20c

�174

c—

——

Nhm

——

——

——

19a2.70

:136

c�2

17c

——

—3

——

——

——

19b2.4:1

39c

�207

c—

——

3—

——

——

—19

c1.2:1

37c

�214

c—

——

34—

——

——

—

2296 (200

2)27

c�2

46c

——

—44

——

——

——

22a2.1:1

29c

�244

c—

——

4—

——

——

—



Tab

le7

(Contd.)

Com

poun

dRef.

Na

OBa

ISb(J)

FSb(N

)EDSb

IDWFNAb

(ms)

IDN

2O4

b

(ms)

IDH

2O2

b

(ms)

I spa(s)

P deta

(GPa

)n d

eta

(ms�

1)

V exp

.ga

sa

(Lkg

�1)

85 (201

2)22

b0.47

:133

c�2

33c

——

—5

——

——

——

22c0.26

:131

c�2

39c

——

—31

——

——

——

23a

53 (201

2)29

.39,

29.71d

�144

c—

——

Nhm

——

183.1x

——

—

23b

40c

�188

c—

——

26—

—16

0.4x

——

—23

c37

.27,

36.74d

�149

c—

——

34—

—18

1.3x

——

—

23d

35c

�205

c—

——

Hyp

ern

——

164.5x

——

—24

a24

.06,

23.87d

�173

c—

——

64—

—17

8.2x

——

—

24b

33c

�209

c—

——

18—

—15

9.0x

——

—24

c31

.11,

30.95d

�175

c—

——

18—

—17

6.5x

——

—

24d

29c

�223

c—

——

14—

—16

2.4x

——

—

2546 (200

6)28

c0*

——

——

——

——

——

26a

39 (200

8)32

.91,

33.66d

6.3*

——

——

——

——

——

26b

31.19,

31.27d

�5.9

*—

——

——

——

——

—

26c

29.65,

29.68d

�16.9*

——

——

——

——

——

26d

26.98,

27.13d

�36.0*

——

——

——

——

——

26f

39.26,

38.38d

0*27

h,e

——

——

——

——

—

27a

32.85,

32.67d

6.3*

——

——

——

——

——

27b

31.05,

30.90d

�5.9

*—

——

——

——

——

—

27c

29.61,

29.62d

�16.9*

——

——

——

——

——

27d

26.94,

27.23

�35.9*

——

——

——

——

——

27f

39.20,

38.72d

0*27

i,e

——

——

——

——

—



4

Tab

le7

(Contd.)

Com

poun

dRef.

Na

OBa

ISb(J)

FSb(N

)EDSb

IDWFNAb

(ms)

IDN

2O4

b

(ms)

IDH

2O2

b

(ms)

I spa(s)

P deta

(GPa

)n d

eta

(ms�

1)

V exp

.ga

sa

(Lkg

�1)

28b

54 (200

6)27

.09

�36c

9z,g

22.8

j—

——

——

——

—28

c35

.17,

34.21d

�20c

3z,g

1.5j

——

——

——

——

29c

36.60,

34.62d

5c#2z

,g<0

.4k,j

——

——

——

——

aCalcu

latedvalues.b

Measu

redvalues.c

Valuecalculated

byau

thorsof

this

review

.dFo

undN%

byelem

entalan

alysis.e

BAM

method

s.fTestedon

atype

12toolingaccordingto

the“u

pan

ddo

wn”method

.gTestedon

anOlin

Matheson

styledrop

weigh

ttester.h3/6explosions.

i2/6explosions.

jFriction

testingon

aJulius

Peters-style

friction

tester.kHad

ahighly

explosive

resp

onse

destroyingthestrike

ror

scratcher

completely.

lRou

ghsensitivity

toelectrostaticdischarge

usingaTesla

coil

(ca.

20kV

)where(+)¼

sensitive

and

(�)¼

insensitive.

mNot

hyp

ergo

lic.

nHyp

ergo

lic.

oResultedin

explosionrather

than

combu

stion.pFlam

edu

ration

130�

31ms.

qFlam

edu

ration

43�

4ms.

rHyp

ergo

licwhen

aseconddrop

offuel

was

drop

ped

into

N2O4.sIL

ignited

withthevapo

ursof

N2O4priorto

theliqu

idsmixing.

t90

%H

2O2.u98

%H

2O2.vCom

putedus

ingCheetah6.0.

wCom

putedus

ingCheetah5.0.

xCom

putedus

ingthe

empiricaleq

uation

sof

Kam

letan

dJacobs

.yCom

putedus

ingCheetah4.0.

zOriginal

valuehas

been

converted

toun

itsem

ployed

inthis

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reporting standard energetic data and measurements led tosome difficulties in ranking EILs in terms of their energeticpotential. Future efforts should be made to fully characterizenewly synthesized EILs with standardized tests and consistentmeasurements.

Reported EILs thus far can be divided into two categories:potential bipropellants (hypergolic systems), and mono-propellants (self-oxidizing systems). The rst categoryencompasses a wide range of cations, from N-rich ammo-nium and hydrazinium to tetrazolium and other cationic N-heteroaromatic rings; as well as various anions, such as nitroand cyano containing or borohydride based anions. Theenergetic behaviour of such bipropellants requires a reactionwith an external oxidizer; some of which tend to be sensitiveto ambient oxidative conditions. The second categoryconsists of an ionic pair comprised of a combustible N-richcation with an oxidizing anion, making these EILs oxygen-balanced and prone to self-oxidation. Due to this advanta-geous combination, monopropellant EILs are less sensitivetoward ambient conditions and do not require the use of anexternal oxidizer.

Regarding future developments, a major breakthrough forhypergolic systems would be to determine exactly whichfundamental property is responsible for hypergolicity. Azides,dicyanamides, borohydrides and even halides have been foundto exhibit this behaviour, though these anions do not guaranteea hypergolic reaction. Identifying the route to hypergolicitywould lead to a direct and more efficient design of new EILbipropellant systems, ideally requiring milder and less corro-sive oxidizers. The development of new oxidizers may also be analternative to maximizing the potential of current EILs. Anotherroute to investigate could be the pursuit of new oxidizing cata-lysts, which would remove the use of stœchiometric oxidizers.97

Furthermore, such catalysts could be developed to capitalize onenergetic systems which have not exhibited hypergolic behav-iour. Conversely, for monopropellant systems, stable andinsensitive oxygen-balanced EILs remain elusive. They arenevertheless of great promise due to their self-sufficient ener-getic behaviour and controlled release of their high energydensity. If stable under ambient conditions and not ultra-sensitive, such powerful monopropellants could lead the waytoward more efficient green fuel systems of the future. Despitefew researchers reporting the viscosity of their new EILs, acommon attribute which needs to be rectied prior to the viableapplications as propellants would be in lowering the viscosity ofthese liquids. The average of the viscosities reported herein is 40mPa s, while hydrazine and some of its derivatives are compa-rable to water, which is approximately 0.8 mPa s.98

Though there is still room for improvement, this eld hasdeveloped quite well and promises exciting results. Prior tomoving forward toward the effective design of EILs, a betterunderstanding of existing systems is necessary. Therefore,efforts should be focused on a profound analysis of the vast poolof established ionic pair combinations: completing the ener-getic portfolio of each compound, in parallel with computa-tional investigations regarding the dynamics and synergy of thewhole system in its environment.

J. Mater. Chem. A, 2014, 2, 8153–8173 | 8171


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