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Introduction

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What are ionic liquids?

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Ionic liquids (ILs) are low-melting salts that melt at or below 100°C[1]. They are a combination between organic and inorganic anions, and asymmetric organic cations. An ion is an atomic particle that has electrical charges and neutral charges if combined between cations and anions. ILs have been vastly used throughout chemical science such as catalysis, organic synthesis, separation and analysis, electrochemistry, material chemistry, pretreatment of biomass, energy technology, and many other engineering fields. Ionic liquids have distinguished properties that have contributed to the green revolution in chemical science such as low vapor pressure, high thermal stability, ionic conductivity, structural designability, and the ability to dissolve a wide range of chemical species. There are many types of ionic liquids such as room temperature ionic liquids (RTILs)[2].

Energetic ionic liquids (EILs)

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EILs are bulky compounds that have the ability to store and release high energy such as explosives, propellants, and pyrotechnics. Moreover, with the tunability of cations and anions, researchers have found routes to synthesize ILs to EILs to target high energy matters[3]. Energetic ionic liquids have been known as the environmental-friendly explosives or propellants fuels for energetic applications and this is where new-generation materials chemistry occurs.

Why EILs?

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In contrast with the traditional energetic material – TNT or RDX, energetic ionic salts are considered to be a better option with higher environmental footprint as it has less impact on environment and human receptors[4]. The advantages EILs give such as low viscosity, high thermal stability, high energy density and low toxicity. Researchers focuses on developing the so-called “new-generation energetic materials” or “green energetic materials.” The word green is defined by the low vapour pressure – negligible, therefore, it reduces the “polymorphism” issue that arises with the traditional energetic materials that could potentially risk human and environmental health[1].

History

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Ionic liquids were first discovered in the late 19th century, as azide salts and hydrazinium azide (mp 75°C)[1]. However, ethylammonium nitrate was recognized as the first IL in 1914 with the melting point of 12°C and it was also an example of a pro ionic liquid (PIL)[5]. ILs were historically used as electrolytes or solvents in the late 1900s or early 2000s[3]. An encounter between Dr. Drake and Edwards Air Force Base in 2002 pushed the researchers within the field of chemical materials to discover and explore more about energetic ionic liquids to test the generality of the ionic liquids concept[3]. The growth of EILs over the years has tremendously increased.

Physicochemical properties of EILs

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Physicochemical properties determine EILs performance as novel energetic materials. Nitrogen-rich cations and oxygen-rich anions are typically used in synthesis to create an adaptable material depending on its application[6]. In order to evaluate a good energetic ionic liquid, one must examine its characteristics comprehensively by the usage of experimental instruments such as NMR.

Thermal properties

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As mentioned above, ILs are mainly having melting points below 100°C. The temperature of EILs can be altered by the number of anions and cations involved[6]. For example, EILs will result in a higher melting point if there is an increased amount of hydrogen-bonding as the molecules are highly packed with lower entropy. However, in order to distinguish different types of energetic ionic liquids, one could apply differential scanning calorimeter (DSC) and thermogravimetric analysis (TGA) to determine its melting range[1]. Intermolecular interactions are essentially determining EILs melting point as it is important for the development of the novel energetic materials.

Density and viscosity

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As mentioned, ILs have high-energy-density in general, but EILs need to be more focused as they are still in development. Research estimates and calculates the density of EILs based on quantum mechanical theory and volume parameter method[7]. An Anton Paar Stabinger Viscometer (SVM3000) can be used to calibrate the viscosity range of ILs and provide least uncertainty ± 3%[8].

Oxygen balance

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Energetic ionic liquids are still a novel discovery. Oxygen balance (OB) is one of the parameters to determine EIL's performance and applications. The environmental health depends on the OB released or consumed for putting EILs in use. Most known EILs have low oxidizing anions in order to massively produce high-energy reactions, it requires a large amount of oxidizers and this is where OB comes in and assists[9].

Applications in explosives

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EILs are a class of candidates in the replacement of 2,4,6-trinitrotoluene (TNT) in melt-cast explosives. TNT is hazardous: its production poses significant environmental risk[10] and prolonged exposure can lead to anemia and liver damage[11]. Additionally, the melt-casting process is an efficient and scalable production method[12]. As such, any replacement candidate should have similar properties to TNT to make use of existing melt-cast infrastructure, but be made by sustainable means. Other requisite properties of such a substitute are included below.

Additionally, salts are modular compounds, affording greater convenience in molecular design over non-ionic analogues[13]. They are also typically more dense with lower vapour pressures due to Coulombic attraction[14]. Several azole- and ammonium-based EIL’s have been synthesized and characterized.

Technical requirements of EIL replacements for TNT

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  1. A melting point of 80-100°C.[15][16]
  2. High energy density.[15][16]
  3. Lower viscosity than TNT at high temperature.[15][17]
  4. Low vapour pressure.[15][16]
  5. Greater energetic performance than TNT.[15]
  6. Resistance to detonation stimuli.[15]
  7. Compatible with other materials found in explosives (other ingredients and highly energetic materials).[15]
  8. A large difference in melting point and decomposition temperatures.[15]

Imidazolium-based EILs

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Reactions using imidazolium produce EILs.[15][16] Most reactions occur via metathesis reactions using halide or nitrate salts of an imidazolium cation alongside energetic anions in an aqueous form.[15][16][18] Metal salts can be dangerous, high in cost, and not efficient for synthesis of EILs.[15][16] Newer synthesis protocols are being developed to produce greener and safer means of EIL production.[15][17]

Examples of Imidazolium-based EILs

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The 1,3-dialkylimidazolium cation is used as a precursor for EIL synthesis.[15] This is due it's ability to alter the molecular framework and it's functional composition.[15]

The anionic form of 3,5-Dinitro-1,2,4-triazole can be used to synthesize new EILs.[15] A reaction of this anion with a 1-butyl-3-methylimidazolium cation ([BMIm]+) produces a thermally stable EIL containing a low melting point of 35-36°C.[15]

Imidazolium-based azide EIL example
Azolate EIL example

The anionic form of 5,5'-azotetrazolate can be used to synthesize an EIL with desirable qualities.[15] A metathesis reaction of barium 5,5'-azotetrazolate in water with the salt 1-butyl-3-methylimidazolium sulphate produces a room temperature EIL (RT-EIL).[15] These EIL's contain low melting points (3°C), a density of 1.26 g/cm3, and a ΔHf of -1006 kJ mol-1.[15] These properties enable such EILs to be used for synthesis of nanomaterials.[15]

Azide salts can be used to synthesize azide-based EILs.[15][16] The resulting EILs can form supercooled liquids.[15] The determined melting range for such EILs was 19-66°C.[15] The decomposition temperatures ranged through 107-222°C.[15] Note azide salts are soluable in acetronitrile, methanol, and water (polar solvents) and insoluable in diethyl ether, hexane, DCM, and ethyl acetate.[15][16] These azide EILs react vigorously with N2O4 and red-fuming nitric acid.[15]

The cation form of 1-butyl-3-methylimidazolium can participate in reactions with energetic azolate anions to synthesize EILs.[15] Imidazolium-based azolate ionic liquids have been observed to act as a stabilizer to suspensions of nanoparticles such as Ti(0).[15]

Experimentally added energetic nitro or nitrile groups to the imidazolium cation did not produce EILs.[15] The resulting imidazolium azolate ionic liquids are not highly energetic.[15] Reactions through protonation of dinitro-imidazoles or dicyano-substituted imidazoles alongside picric acid or nitric acid do not produce EILs.[15] However, synthesis reactions of mononitro- or monocyano-substituted protonated imidazolium salts are successful.[15]

Triazolyl-functionalized imidazolium EILs can act as good TNT replacements.[15] They have a melting range of 89-95°C and good detonation abilities.[15] The addition of triazole to the imidazolium cations enhances the usefulness for energetic materials.[15]

Dicyanamide and dinitromethanide salts can be used to synthesize EILs.[15][16] Anionic dinitromethanide is particularly useful for such synthesis.[15] Resulting typical EILs have densities ranging through 1.18-1.45 g/cm3, viscosities ranging between 75-1441 cP, detonation velocities ranging through 5715 to 6873 m/s1 and decomposition temperatures above 150°C.[15] These EILs act as insensitive energetic materials.[15]

Example EIL synthesis using HBTA- anion

The 5-aminotetrazolate anion can be used to synthesize EILs.[15] The reaction between N,N-bis(1H-tetrazol-5-yl)amine anion (HBTA- ) and 1,3-dialkylimidazolium produces energetic salts.[15] Such EIL products had melting points below the desired 100°C.[15] The HBTA-based EIL products are useful due to their short ignition time.[15] There is potential to use HBTA-based EILs as green propellants.[15]

Triazolium-based EILs

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Triazolium cations have been used for the design and synthesis of EILs.[15][16] The high nitrogen nature of triazolium enables great energy capacity.[15][16] Triazolium-based EILs can be produced in good yield via acid-base reactions or direct quaternization using 1,2,4-triazole as starting material.[15] The low melting point of such EILs can be designed through using bulky nitrogen heterocycles.[15] The low melting points are achieved through poor crystal lattice packing due to the low symmetry status of the bulky heterocycle.[15]

Examples of Triazolium-based EILs

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Examples of triazolium-based EILs

An EIL containing a substituted 1,2,4-triazolium cation alongside 4,5-dinitro-imidazolate or 5-nitrotetrazolate produces a high melting point over the desired 100°C.[15] This is due to the weak basicity of nitrogen in the 4th heterocycle position which enables quaternization with a strong acid in methanol.[15] The presence of nitro- or azido- groups in the cation result in EILs with high densities, and good performance as explosives.[15] Note when strongly acidic energetic azoles are used the produced salt contains a lower melting point below 100°C.[15] These EILs contain substantial detonation velocities above 8000 m/s-1, and large positive heats of formation above 830 kJ mol-1.[15]

An EIL produced from an azido derivative of a triazolium cation, nitrate, perchlorate, 5-nitrotetrazolate or 4,5-dinitroimidazolate anions is desirable. Such EILs contain thermal stability, and low melting points typically under 25°C.[15] The densities and heats of formation of these EILs make for successful applications in low-signature propellants.[15]

The use of N-aminoazole and the following quaternization reaction produces energetic salts.[15] These resulting energetic salts contain low melting points below the desired 100°C, thermal stability, and high densities over 1.55 g/cm3.[15]

Low energetic performance of dicyanamide salts results in poor abilities for explosive applications.[15][16]

Energetic salts containing nitrogen-rich anions and a high density are used alongside the cation of 1-amino-1,2,3-triazole (ATZ) or 3-methyl-1-amino-1,2,3-triazole (MAT) resulting in EILs.[15] Melting points were below desired 100°C, hence EIL classification.[15] Some contained good detonation velocities and pressures.[15] All EILs were sensitive to impact.[15]

Azide anion-based EILs can be used as energetic materials.[15][16] The resulting EILs have low volatility, high thermal stability, low vapour toxicity and low sensitivity.[15][16]

EILs can be developed from nitrogen-rich borate anions.[15][16] The densities of these borate EILs rely on the anion and substituents on the boron.[15] These particular EILs show use in the electrochemical field due to the good chemical and electrochemical capacity of borate anions.[15][16]

Example of synthesis pathway for triazolium EIL using borate anions

Preparing an EIL with lanthanide nitrates as the anion alongside substituted 1,2,4-triazolium cations produces successful EILs.[15] The resulting EILs are thermally stable, moisture-stable, and environmentally friendly.[15][19] The use of such EILs can potentially be propellants.[15]

Triazolium-based lanthanide nitrate EIL complex

Tetrazolium-based EILs

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Nitrogen content is directly related to detonation performance. Higher nitrogen content increases density, which is proportional to both the detonation pressure and energy output[20]. Heterocycles with more nitrogen trend to higher enthalpies of formation, and therefore greater energetic content. The heats of formation for imidazole, 1,2,4-triazole and tetrazole are, respectively, ΔH°f(cryst) = 58.5 kJ/mol, ΔH°f(cryst) = 109 kJ/mol, and ΔH°f(cryst) = 237.2 kJ/mol[21]. Greater nitrogen content also has bearing on the environment. Nitrogen-rich heterocycles yield greater fractions of dinitrogen in the decomposition products, making for “greener” detonations[14].

Given their high nitrogen content, salts with a tetrazolium cation have good potential in explosives applications, but there are drawbacks. Quaternization of tetrazoles is more difficult than for imidazoles and triazoles, and is required for cation formation, and thus of the salt[15]. Furthermore, in contrasting EIL’s with the same hydrotris(1,2,4-triazolyl)borate [HB(tz)3] anion, decomposition temperatures of 81.7°C, 158°C, and 253°C were observed for 5-amino-1,4-dimethyltetrazolium [HB(tz)3], 1-amino-3-methyltriazolium [HB(tz)3], and 1-n-butyl-2,3-dimethylimidazolium [HB(tz)3], respectively, demonstrating decreasing thermal stability with the increasing number of nitrogen atoms in the heterocycle[15].

Examples of Tetrazolium-Based EILs

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Relatively few syntheses for tetrazolium-based EIL's are found in the literature, due to the difficulty in quaternization. Nevertheless, several synthetic routes have been reported. For example, the formation of 1-amino-4,5-dimethyltetrazolium nitrate and 1-amino-4,5-dimethyltetrazolium perchlorate can be achieved by alkylating 1-amino-5-methyltetrazole with iodomethane, in an SN2-type reaction (iodomethane is attacked by the nucleophilic nitrogen, iodide leaves and stabilizes the cationic product), followed by a metathesis reaction involving either silver nitrate or silver perchlorate.

The 1-amino-4,5-dimethyltetrazolium salts above have similar thermal degradation temperatures, but drastically different melting points depending on the counter-ion. The salt is liquid at room temperature when paired with NO3−  (Tm = − 59°C), whereas the ClO4− salt is not (Tm = 51°C)[21]. 1,5-diamino-4-methyltetrazolium dinitramide can be synthesized in a similar fashion, by reflux with methyl iodide and metathesis with silver dinitramide.

The final product of this synthesis has a low melting point of Tm = 85°C, and is stable beyond 150°C, but is sensitive to impact and friction. High nitrogen content in both the cation and anion suggests it is highly energetic, however its detonation characteristics have not yet been investigated[15].

Oxygen balance is a measure of the amount of oxygen required for combustion in the absence of external sources, and is an important consideration with respect to both detonation performance and the environment. A negative oxygen balance indicates the compound has insufficient oxygen to undergo complete combustion, leading to poorer performance and toxic by-products[22]. Several dinitramide salts with favourable oxygen balance have been synthesized by reaction of tetrazolium perchlorate salts with potassium dinitramide, three examples of which are shown below.

Some tetrazolium-dinitramide salts

These dense dinitramide salts all exhibit low melting points (Tm < 100 °C) and thermal stabilities of Td < 160°C. From left to right, these compounds have calculated detonation velocities of 9429 m/s, 9215 m/s, and 8548 m/s[15]. The leftmost two compounds therefore show better detonation performance than both TNT and RDX. Similar 1,5-diaminotetrazolium 5-diaminotetrazolium salts with lanthanide complex anions [La(NO3)6]3− and [Ce(NO3)6]3− show high density (> 2 g/cm3) and low melting points. Salts containing the [La(NO3)6]3− anion showed thermal stability in the range of 180 – 235°C[19].

Quaternary ammonium-based EILs

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Salts with nitrogen-rich cations make for attractive prospects in explosives applications. They are typically dense and stable due to high lattice energies[22], and typically have environmentally friendly decomposition products. Quaternized ammonium-based salts are a class of such prospects. These ammonium salts present an advantage over, for example, tetrazolium analogues, in that their syntheses are relatively easy and examples have been known for quite some time (hydrazinium azide was discovered in 1890)[15]. As such there are many examples of EILs involving quaternized ammoniums.

Examples of Ammonium-Based EILs

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Hydrazinium derivatives are attractive candidates for explosives applications given their high nitrogen content and green decomposition products (water and dinitrogen). Several hydrazinium-azide-type salts can be synthesized by reaction of the the hydrazine derivative with hydrazoic acid. N,N’-diethylhydrazinium azide can be made in this manner, and is liquid at room temperature. However, it shows poor sensitivity to detonation stimuli, requiring rapid heating to explode[23].

Some hydrazinium-azide-type salts


The 1-cyanomethyl-1,1-dimethylhydrazinium perchlorate salt can be synthesized via alkylation of 1,1- dimethylhydrazine with bromoacetonitrile, and subsequent metathesis with perchlorate. This compound melts at 47°C, is thermally stable beyond 160°C, as has been calculated to have a detonation velocity of 7940 m/s.

Salts of 1,1,1-trimethylhydrazine have been seen to have great thermal stability (up to 322°C), and typically have detonation velocities and pressures comparable to or greater than that of TNT, with generally greater resistance to detonation stimuli.

Some quaternized guanidines in perchlorate, nitrate, and dinitroamide salts exhibit desirable properties for melt-cast explosives. These can be synthesized by treating a given guanidine derivative with phosphoryl chloride and a primary amine with the desired substituent to form an imine, followed by alkylation using iodomethane and metathesis with the desired silver-(anion) compound.

Some generalized guanidimium-based salts

The imine can also be treated with an acid HX such that X is the desired final anion. Typical X and R groups include nitrate and perchlorate, and saturated alkanes like n-butane. These salts have decomposition temperatures ranging from 127°C - 325°C, are often liquid at room temperature, but their heats of formation vary wildly in magnitude and sign. Some examples of these types of salts are given in the figure to the left.

Many EILs formed by metathesis of 2-tetrazenium-halides also show thermal stability up to 200°C and melting points under 100°C — a significant difference. Most of those reported are also resistant to detonation stimuli.

Industrial applications

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EILs can be used for vast purposes. The use of these ionic liquids is observed in the food industry, pharmaceuticals, batteries, fuel cells, solar cells, nuclear fuel reprocessing, cellulose processing, gas processing and super-captors. [17][16]

See also

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References

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  2. ^ Marcus, Yizhak (23 April 2016). "The Properties of Ions Constituting Ionic Liquids. In: Ionic Liquid Properties". Ionic Liquid Properties: 7–24 – via SpringerLink.
  3. ^ a b c Acc. Chem. Res. (November 3, 2007). "The Second Evolution of Ionic Liquids: From Solvents and Separations to Advanced Materials—Energetic Examples from the Ionic Liquid Cookbook". Accounts of Chemical Research. 40: 1182–1192 – via ACS Publications.
  4. ^ ACS Sustainable Chem. Eng. (February 20, 2013). "Energetic Ionic Materials: How Green Are They? A Comparative Life Cycle Assessment Study". ACS Sustainable Chemistry and Engineering. 1: 448–455 – via ACS Publications.
  5. ^ Welton, Tom (26 April 2018). "Ionic liquids: a brief history". Biophysical Reviews. 10: 691–706 – via SpingerLink.
  6. ^ a b Maiti, Amitesh (3 June 2013). "Ionic Liquids and Energetic Materials". Propellants, Explosives, Pyrotechnics. 38: 319 – via Wiley Online Library.
  7. ^ Zhang, Xiangwen; Pan, Lun; Wang, Li; Zou, Ji-Jun (April 28, 2018). "Review on synthesis and properties of high-energy-density liquid fuels: Hydrocarbons, nanofluids and energetic ionic liquids". Chemical Engineering Science. 180: 95–125 – via ScienceDirect.
  8. ^ Zhang, Qingguo; Cai, Siyi; Zhang, Wenbo; Lan, Yalin; Zhang, Xinyuan (May 2017). "Density, viscosity, conductivity, refractive index and interaction study of binary mixtures of the ionic liquid 1–ethyl–3–methylimidazolium acetate with methyldiethanolamine". Journal of Molecular Liquids. 233: 471–478 – via ScienceDirect.
  9. ^ Jones, C. Bigler; Haiges, Ralf; Schroer, Thorsten; Christe, Karl O. (20 July 2006). "Oxygen‐Balanced Energetic Ionic Liquid". Angewandte Chemie. 45: 4981–4984 – via Wiley Online Library.
  10. ^ Chatterjee, Soumya; Deb, Utsab; Datta, Sibnarayan; Walther, Clemens; Gupta, Dharmendra K. (2017-10-01). "Common explosives (TNT, RDX, HMX) and their fate in the environment: Emphasizing bioremediation". Chemosphere. 184: 438–451. doi:10.1016/j.chemosphere.2017.06.008. ISSN 0045-6535.
  11. ^ LEVINE, B (1990-08-01). "Toxic interactions of the munitions compounds TNT and RDX in F344 rats*1, *2". Fundamental and Applied Toxicology. 15 (2): 373–380. doi:10.1016/0272-0590(90)90062-o. ISSN 0272-0590.
  12. ^ Johnson, Eric C.; Sabatini, Jesse J.; Chavez, David E.; Sausa, Rosario C.; Byrd, Edward F. C.; Wingard, Leah A.; Guzmàn, Pablo E. (2018-06-15). "Bis(1,2,4-oxadiazole)bis(methylene) Dinitrate: A High-Energy Melt-Castable Explosive and Energetic Propellant Plasticizing Ingredient". Organic Process Research & Development. 22 (6): 736–740. doi:10.1021/acs.oprd.8b00076. ISSN 1083-6160.
  13. ^ Zhang, Qinghua; Shreeve, Jean’ne M. (2014-10-22). "Energetic Ionic Liquids as Explosives and Propellant Fuels: A New Journey of Ionic Liquid Chemistry". Chemical Reviews. 114 (20): 10527–10574. doi:10.1021/cr500364t. ISSN 0009-2665.
  14. ^ a b "New Energetic Salts Based on Nitrogen-Containing Heterocycles". dx.doi.org. Retrieved 2020-10-18.
  15. ^ a b c d e f g h i j k l m n o p q r s t u v w x y z aa ab ac ad ae af ag ah ai aj ak al am an ao ap aq ar as at au av aw ax ay az ba bb bc bd be bf bg bh bi bj bk bl bm bn bo bp bq br bs bt bu bv bw bx Zhang, Qinghua; Shreeve, Jean’ne M. (2014-09-10). "Energetic Ionic Liquids as Explosives and Propellant Fuels: A New Journey of Ionic Liquid Chemistry". Chemical Reviews. 114 (20): 10527–10574. doi:10.1021/cr500364t. ISSN 0009-2665.
  16. ^ a b c d e f g h i j k l m n o p q Sebastiao, Elena; Cook, Cyril; Hu, Anguang; Murugesu, Muralee (2014). "Recent developments in the field of energetic ionic liquids". J. Mater. Chem. A. 2 (22): 8153–8173. doi:10.1039/C4TA00204K. ISSN 2050-7488.
  17. ^ a b c Maiti, Amitesh (2013-06-07). "Ionic Liquids and Energetic Materials". Propellants, Explosives, Pyrotechnics. 38 (3): 319–319. doi:10.1002/prep.201380331.
  18. ^ Yang, Haijun; Liu, Yuejia; Ning, Hongli; Lei, Jianlei; Hu, Gang (2017). "Synthesis, structure and properties of imidazolium-based energetic ionic liquids". RSC Advances. 7 (53): 33231–33240. doi:10.1039/C7RA05601J. ISSN 2046-2069.
  19. ^ a b Tao, Guo-Hong; Huang, Yangen; Boatz, Jerry A.; Shreeve, Jean'ne M. (2008-12-08). "Energetic Ionic Liquids based on Lanthanide Nitrate Complex Anions". Chemistry - A European Journal. 14 (35): 11167–11173. doi:10.1002/chem.200800828.
  20. ^ Agrawal, Hemant; Mishra, Arvind (2017-09-30). "A Study on Influence of Density and Viscosity of Emulsion Explosive on Its Detonation Velocity". Modelling, Measurement and Control C. 78 (3): 316–336. doi:10.18280/mmc_c.780305.
  21. ^ a b Xue, Hong; Arritt, Sean W.; Twamley, Brendan; Shreeve, Jean'ne M. (2004-12-06). "Energetic Salts fromN-Aminoazoles". Inorganic Chemistry. 43 (25): 7972–7977. doi:10.1021/ic048890x. ISSN 0020-1669.
  22. ^ a b Klapötke, Thomas M.; Stierstorfer, Jörg (2009). "Azidoformamidinium and 5-aminotetrazolium dinitramide—two highly energetic isomers with a balanced oxygen content". Dalton Trans. (4): 643–653. doi:10.1039/B811767E. ISSN 1477-9226.
  23. ^ Hammerl, Anton; Holl, Gerhard; Kaiser, Manfred; Klapötke, Thomas M.; Kränzle, Rainer; Vogt, Martin (2002-01-02). <322::aid-zaac322>3.0.co;2-s "N,N′-Diorganylsubstituted Hydrazinium Azides". Zeitschrift für anorganische und allgemeine Chemie. 628 (1): 322–325. doi:10.1002/1521-3749(200201)628:1<322::aid-zaac322>3.0.co;2-s. ISSN 0044-2313.
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