Energetic Ionic Liquids as Explosives and Propellant Fuels: A New

Review pubs.acs.org/CR

Energetic Ionic Liquids as Explosives and Propellant Fuels: A New Journey of Ionic Liquid Chemistry Qinghua Zhang*,† and Jean’ne M. Shreeve*,‡ †

Institute of Chemical Materials, China Academy of Engineering Physics (CAEP), Mianyang 621900, China Department of Chemistry, University of Idaho, Moscow, Idaho 83844-2343, United States

‡

Acknowledgments References

1. INTRODUCTION In the past two to three decades, ionic liquids (ILs) have been one of the most exciting scientific discoveries in chemical science. Nowadays, the most accepted definition of an IL is a low-melting salt that melts at or below 100 °C, which is normally composed of a large asymmetric organic cation and an inorganic or organic anion.1,2 Because of some unique properties such as low vapor pressure, liquidity over a wide temperature range, high thermal stability, ionic conductivity, structural designability, and the ability to dissolve a wide range of chemical species, ILs have brought a green revolution to chemistry and chemical engineering in the past dozen years. Research and development that involve ILs are prevalent in nearly every branch of chemistry and material science, including catalysis,3−6 organic synthesis,7−9 separation and analysis,10,11 electrochemistry,12−15 material chemistry,16−19 pretreatment of biomass,20,21 energy technology,22,23 as well as many others. At the initial stages of IL R&D, more research efforts had been devoted to those fields concerning applications of ILs as green media or high-performance electrolytes, and the nonsolvent applications of ILs had been ignored for a long time. However, in recent years it was widely recognized that the use of ILs as green solvents or electrolytes is just one part of the IL story. Indeed, applications of ILs themselves as advanced functional materials are receiving more and more attention. In these material applications, ILs are always discussed as versatile and high-potential “building blocks” for new-generation functional materials in a broad range of fields.24−30 The new IL-based materials can not only overcome the shortcomings of traditional molecular materials, but also offer some additional advantages such as (1) these charge-rich salts dominated by electrostatic forces can act as new environmentally friendly materials with negligible vapor pressure; (2) the properties of IL-based materials that are retained under extreme conditions such as high vacuum, ultrahigh, or ultralow temperature make them promising candidates for a variety of applications; and (3) the designability of the ILs including the choice of cations/ anions, functional group, or even the length of the alkyl chain can readily tailor the properties of the target materials with respect to polarity, solubility, hydrophobicity, hydrophilicity, conductivity, viscosity, density, melting point, and stability, among others. No doubt, the discipline crossing and integration

CONTENTS 1. 2. 3. 4.

Introduction A Glance at the History of EILs New-Generation Energetic Materials: Why EILs? Physicochemical Properties of EILs 4.1. Thermal Properties 4.2. Density 4.3. Viscosity 4.4. Heat of Formation 4.5. Ignition Delay Time 4.6. Oxygen Balance 4.7. How Green Are EILs? 5. Applications of Energetic ILs (EILs) in Explosive Formulations 5.1. Imidazolium-Based EILs 5.2. Triazolium-Based EILs 5.3. Tetrazolium-Based EILs 5.4. Quaternary Ammonium-Based EILs 6. Applications of EILs as Hypergolic Fuels 6.1. Hypergolic ILs Based on Dicyanamide and Nitrocyanamide Anions 6.2. Hypergolic ILs Based on B−H Bonds-Rich Anions 6.3. Hypergolic ILs Based on Complex Aluminum Anions 6.4. Boronium-Based Hypergolic ILs 6.5. Hypergolic ILs Based on Hypophosphite (HP) Anion 6.6. Hydrazinium-Based Hypergolic ILs 6.7. Ignition Mechanism of HILs with WFNA 7. Concluding Remarks Author Information Corresponding Authors Notes Biographies

© 2014 American Chemical Society

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Received: July 10, 2014 Published: September 10, 2014 10527

dx.doi.org/10.1021/cr500364t | Chem. Rev. 2014, 114, 10527−10574

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concerning this topic have appeared very recently.32,33,49 Therefore, those high-melting energetic salts are beyond this Review, and only energetic salts that fit the definition of ILs (herein also named EILs) are included.

of ILs with other materials has provided a large number of new opportunities for developing a variety of environmentally friendly functional materials. Energetic materials are one of the most important functional materials in the field of material science.31−34 In general, energetic materials are defined as compounds with large amounts of stored chemical energy that can be released under specific conditions (e.g., heat, shock, friction, and electrostatic discharge). Typical energetic materials include explosives, pyrotechnics, and propellants, which are widely used for a variety of military purposes and civilian applications. Along with growing concerns about the environment and safety issues, considerable efforts have been devoted to pursuing environmentally friendly and insensitive energetic materials.35,36 In the development of new high-performance energetic materials, nitrogen-rich heterocycles (e.g., imidazole, pyrazole, triazole, tetrazole, and 1,2,4,5-tetrazine) represent a unique class of energetic molecular frameworks, which have recently attracted significant interest in the design of energetic materials due to their high heats of formation, density, and thermal stability as compared to those of their carbocyclic analogues.37−41 Especially, in recent years, a new class of energetic ionic salts has emerged and received significant attention.42−45 These ionic energetic materials are most often composed of highnitrogen organic cations (e.g., guanidinium, imizadolium, triazolium, and tetrazolium) and bulky anions with one or more energetic groups such as −NO2, −N3, and −CN. From the viewpoint of molecular structure, there are a number of similarities between ILs and energetic ionic materials, such as (1) both are ionic, (2) nitrogen-containing heterocycles are most often used for the formation of cations, and (3) the structures of both cation and anion components can be independently designed and modified depending on the anticipated application. No doubt that the inherent designability of ILs provides great opportunities for developing newgeneration ionic energetic materials, which makes both the properties and the performance of the target energetic material reasonably predictable. In this context, the so-called “energetic ionic liquids (EILs)”, which are relatively environmentally friendly, low-melting, and thermally stable, have emerged as a new class of energetic materials in recent years.46 As the name implies, here EILs are defined as the low-melting ionic materials (i.e., the melting point is below 100 °C) that have potential energetic applications as explosives, pyrotechnics, or propellants. Over the past decade, considerable efforts have been devoted to the R&D of EILs. Numerous EILs with different structures and energetic properties have been explored as environmentally friendly explosives or propellant fuels for a variety of energetic applications.47−50 In a sense, the concept of designing EILs as energetic materials has provided a unique architectural platform for developing new-generation liquid energetic materials. In this Review, we present an overview of EILs, including history, syntheses, properties, and applications in the fields of energetic materials. The main aim of this work is not only to present the latest advances of EILs as new energetic materials, but also to emphasize the new possibilities and the future challenges in this field. It should be pointed out that, although a large number of heterocyclic-based nitrogen-rich energetic salts have been synthesized and applied as high-performance energetic materials in the past decade, most of them have fallen outside the scope of EILs (i.e., the energetic salts with melting points of 100 °C). Technically, the low-temperature phase behaviors of EILs (i.e., Tm and Tg) and the hightemperature phase behavior (i.e., Td) can be accurately measured by the method of differential scanning calorimeter (DSC) and thermogravimetric analysis (TGA), respectively. It should be pointed out that, in this Review, all of the thermal property data of EILs in Tables 1−5 are collected from the melting peak unless they are expressly stated. The melting point of an EIL can be tailored by appropriately selecting the anion and the cation. Because a broad range of energetic cations and anions have been employed, the existing EILs have a broad range of melting points, from below room temperature (RT-EILs) to about 100 °C (solid EILs), in which the number of EILs that are liquid at room temperature is much smaller than solid EILs. As compared to those high-melting energetic salts, EILs display low melting points when large asymmetric ions with a shielded or delocalized charge are used, thereby decreasing the low lattice energies of EILs. When compared to traditional ILs, due to the presence of some energetic groups such as −NO2, −N3, −CN, and the azoles, the strong hydrogen-bonding interactions in EILs nearly always cause closer crystal lattice packing, thereby resulting in higher melting points. To obtain RT-EILs, the rational design and combination of suitable cations and anions is very important. In general, the nitrogen-rich heterocycles with low symmetry (e.g., imidazolium, triazolium, and tetrazolium,), which can cause poor crystal lattice packing, are always chosen as the cation of EILs. For example, with an identical anion (e.g., CF3COO−), the EIL with an aminoguanidinium cation may have a lower

4. PHYSICOCHEMICAL PROPERTIES OF EILs As a new class of energetic materials, the physicochemical properties of EILs determine their performance in practical applications. Similar to traditional ILs, the properties of EILs can also be tuned by proper selection of a suitable cation/anion 10529


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Table 1. Important Physicochemical Properties of Imidazolium-Based EILsj

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Table 1. continued

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Table 1. continued

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Table 1. continued

a

Melting point. bGlass transition temperature. cThermal decomposition temperature. dDensity. eHeat of formation. fCalculated detonation velocity. Calculated detonation pressure. hImpact sensitivity. iFriction sensitivity. jDefinitions: detonation velocity (D) can be defined as the rate, speed, or velocity of propagation of detonation waves in an explosive; detonation pressure (P) is the peak dynamic pressure in the shock front, which is a measure of the explosive’s shock wave energy; impact sensitivity (IS) represents the ease with which an explosive can be set off by a blow impact and is expressed in terms of the distance through which a standard weight is allowed to drop to cause an explosive to explode; friction sensitivity (FS) represents the ease with which an explosive can be set off by a blow friction and is expressed in terms of what occurs when a pendulum of known weight scrapes across an explosive (ignites or explodes or snaps or crackles).96 g

([N(NO2)2]−) (Im-42, Table 1) exhibited a glass transition temperature of −79 °C in a heating and cooling cycle without melting, whereas the 1,3-dimethylimidazolium dinitromethanide had a melting point of 60 °C (Im-46, Table 1). In addition to the structural changes of the energetic component ions, the degree of anion−cation contact with respect to the type and strength of interactions is also a major factor determining the melting points of EILs. Thus, a thorough understanding of the

melting point than an EIL with a guanidinium cation.63 For the selection of energetic anions, it seems that those EILs based on nitrate (NO3−), azide (N3−), dicyanamide ([N(CN)2]−), and nitrocyanamide ([N(CN)(NO2)]−) anions showed lower melting points. In general, the introduction of fuel-rich functional groups such as the allyl and vinyl groups into the cations can decrease the melting points of the resultant EILs. For example, the 1,3-diallyl-imidazolium dinitromethanide 10533


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pycnometer. In general, the densities of EIL are closely related to the structures of component ions, which depend on how closely the component ions can pack together and, hence, on the size and shape of the ions and the ion−ion interactions. For most EILs designed for explosive applications, the densities are greater than 1.5 g cm−3, while for most hypergolic ILs designed as propellant fuels, their densities lie most often between 0.9 and 1.3 g cm−3. This notable difference in densities might be attributed to the different design of the molecular structure, thereby resulting in different molecular organization or packing of the EILs. For instance, to obtain higher detonation performance for explosive applications, dense and energetic groups such as −NO2, −N3, and −CN that can increase strong H-bond interactions in the molecules are nearly always introduced into the backbone of EILs, thereby resulting in higher densities of the EILs. In contrast, for designing hypergolic ILs as propellant fuels, unsaturated substituents, such as allyl, propargyl, and dicyanamide, are nearly always introduced into the cation or anion to enhance the fuel-rich nature of ILs, thereby resulting in relatively lower densities of most hypergolic ILs as compared to those of EILs designed for explosive applications. Moreover, particular attention has also been paid to develop new theoretical methods that are capable of estimating or calculating the densities of EILs with a maximum of accuracy. Different calculation methods such as group-contribution and quantum mechanical theory have been successfully employed to predict the densities of EILs. Among them, a convenient volume parameter method has been developed for the rapid and accurate estimation of densities of RTILs and energetic salts.67 In this method, the density of the salt can be calculated according to eq 1:

intermolecular interactions in known EILs is of significant importance for the design of new EILs. Supercooling and glass formation often render an accurate determination of the melting point difficult. Many EILs that are liquids at room temperature (e.g., many hypergolic ILs for propellant application) have no melting points and only show a glass transition in a heating and cooling cycle. In fact, glass transition is indicative of the cohesive energy within the salt, which is decreased by repulsive Pauli forces from the overlap of closed electron shells and increased through the attractive Coulomb and van der Waals interactions.64 When glass transition occurs during cooling, the EIL changes from a supercooled liquid to an amorphous solid. The glass transition properties of EILs are governed by complex van der Waals forces and electrostatic interactions, which are primarily influenced by the size, symmetry, H-bonding interactions, and charge delocalization of EILs. In particular, the presence of strong hydrogen bonds in energetic cations and anions significantly influences the low temperature phase behavior of EILs (i.e., Tm, Tg). In general, Tg can be decreased through reducing the packing and cohesive energy of the EILs, for example, by decreasing the cation size, or increasing the asymmetry of the cation. Low vapor pressure and high thermal stabilities are thought of as two of the most important physical properties of EILs. Similar to most traditional ILs, all of the EILs have no boiling point and only undergo an exothermic decomposition (explode) upon heating at high temperature. The decomposition temperature is a measure of thermal stability of EILs, which is generally defined as where there is 10% mass loss using thermogravimetric analysis (TGA). There is no doubt that high Td values (e.g., >300 °C) for ideal EILs are highly desirable. Unfortunately, the number of reported EILs with a Td of >300 °C is still very limited. For instance, some EILs (Am-8, Am-35, Table 4) consisting of quaternary guanidinium cations and the perchlorate anion exhibited the best thermal stabilities (Td > 320 °C). For most other EILs involved in this Review, the decomposition temperatures lie mostly between 120 and 300 °C. In fact, the decomposition process of EILs is immensely complex, generally involving thermal decomposition kinetics and many factors that can influence the thermal decomposition behavior including the structures of component ions, the strength of anion−cation interactions, hydrogen bonds, and different energetic functional groups.65,66 With an identical anion, the introduction of highly energetic groups such as nitro, azido, nitroamino, nitroester groups into the cations will significantly decrease the thermal stabilities of EILs. For an identical cation, the anions also have an important influence on the decomposition behavior. For instance, it seems that the most thermally stable EILs reported were those with a bis(trifluoromethanesulfonyl)imide (NTf2) or ClO4 anion, although they are not highly desirable for explosive applications due to the low nitrogen content of NTf2 and the chlorine content of ClO4−, which is unfriendly to the environment.

ρ = W /(0.6022V )

(1)

where ρ is the density, W is the molar weight of the ionic salt, and V is the molecular volume of the ionic salt. According to eq 1, if the molecular volume of an EIL can be accurately predicted or calculated, the high-precision estimation of EIL density will be achieved. By means of the Cambridge Structure Database (CSD, ConQuest Version 1.8, 2006) and the group additivity method (i.e., summing the volume occupied by each atom or group or molecular fragment), it is convenient to derive the molecular volume of a wide range of EILs and energetic ionic salts with different structures. Moreover, to increase the prediction accuracy, other factors such as strong hydrogen bonding have been considered in this volume parameter method. The calculation results of more than 200 salts including RTILs and energetic ionic materials have shown the high efficiency and universality of this volume parameter method, with a mean relative absolute error of 1.5%.68 4.3. Viscosity

As is the case for traditional ILs, the viscosity of EILs is an important issue that must be addressed. For explosive applications (e.g., as potential replacement of TNT in meltpour explosives), most reported EILs are solids at room temperature, and therefore their viscosity studies, in particular the melt viscosities, have long been ignored. For propellant applications (e.g., as hypergolic fuels in liquid bipropellants), the hypergolic ILs with low viscosities are highly desirable because low viscosity can facilitate the rapid mixing of

4.2. Density

It is well-known that density is one of the major indices to reflect the energy performance of energetic materials, because the higher density usually means the higher mole number of an EIL that can be packed into a limited volume, indicating higher energy contribution to the explosive composition. In the literature, the density of EILs is usually calculated from X-ray or empirical formula or determined experimentally by use of a gas 10534


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Scheme 1. Born−Haber Cycle for the Formation of EILsa

hypergolic ILs with the oxidizer and therefore encourage rapid initiation and combustion processes. Nowadays, the search for new hypergolic ILs with low viscosities has also been a hot topic in the fields of EILs and green propellants. In general, most EILs exhibit relatively high viscosities (>20 cP) at room temperature. The high viscosity may be caused by the presence of an extensive H-bonded network within ionic salts, which results in lower mobility of free ionic species. Moreover, the viscosities of EILs are also highly dependent on strong interactive forces such as electrostatic or van der Waals interactions, and hydrogen bonding; greater interactions lead to higher viscosities. In the cases of imidazolium-based hypergolic ILs, it has been shown that employing the proper alkyl chains or introducing specified functional groups (e.g., soft ether group) can decrease the viscosity through reduced van der Waals interactions, while delocalization of the charge on the anion, such as through fluorination, also favors the decrease of the viscosity by weakening hydrogen bonding. From previous results, it seems that the anion structure of EILs has a more obvious effect on the viscosity than the cation. However, it is still unclear why the viscosity is more dependent on the anion structure rather than cation structure. Among the reported hypergolic ILs, the use of electron-rich anions such as N(CN)2− and BH2(CN)2− in combination with imidazolium-based cations can favor the formation of slightly viscous EILs. For example, 1-allyl-3-methylimidazolium dicyanoborate (HIL-47, Table 5) has a low viscosity of 12.4 cP at 25 °C, which is lower than that of any known hypergolic IL. However, this viscosity is still much larger than the known hydrazine-based fuels (e.g., 0.51 cP for unsymmetric dimethylhydrazine at 25 °C). Therefore, the search for slightly viscous hypergolic ILs is also the goal of developing new ILs-based propellants with practical application potential.

a

The number of moles of the respective products are given by a, b, c, and d.

ΔHf °(EIL, 298 K) = ΣΔHf °(cation, 298 K) + ΣΔHf ° (anion, 298 K) − ΔHL

(2)

where ΔHL is the lattice energy of the salt. For 1:1 salts and considering the nonlinear nature of the cations and anion used, ΔHL (kJ mol−1) can be predicted by eq 3 suggested by Jenkins et al.:70 ΔHL = UPOT + [p(n M/2 − 2) + q(n X/2 − 2)]RT

(3)

where nM and nX depend on the nature of the ions Mp+ and Xq−, respectively, and are equal to 3 for monatomic ions, 5 for linear polyatomic ions, and 6 for nonlinear polyatomic ions. The equation for lattice potential energy UPOT has the form shown in eq 4: UPOT(kJ mol−1) = γ(ρm /M m)1/3 + δ

(4)

where ρm is density (g cm−3) and Mm (g) is the chemical formula mass of the energetic salt, and values for g and the coefficients γ (kJ mol−1) and δ (kJ mol−1) are taken from the literature.71 On the basis of the above approach, the heats of formation of numerous EILs including imidazolium, triazolium, and tetrazolium-based cations have been calculated by using an isodesmic approach. For example, the heats of formation of 119 energetic salts have been efficiently calculated, indicating that it is a straightforward and convenient calculation approach and thus can be widely used to calculate the heats of formation of EILs.72 Moreover, by virtue of a similar calculation method, the effects of different energetic substituents (e.g., −NO2, −NF2, −CN, −N3, and −NH2) on the heats of formation for some tetrazole salts were studied, and results showed that in most cases the presence of energetic groups can increase the ΔHf values.73

4.4. Heat of Formation

Heat of formation (ΔHf) is also a very important property parameter of energetic materials, which is closely related to the energy and the prediction of detonation performance. For example, to estimate the detonation velocity and detonation pressure of an EIL through theoretical calculation methods, its ΔHf data must be first obtained. Nowadays, different approaches including a bond energy additive method, a group additive method, molecular orbital theory, as well as high-level ab initio calculations have been used to calculate the ΔHf values of energetic materials. Among them, the ab initio calculation method has been widely used due to its high accuracy. In this approach, the geometric optimization of the EIL structure and frequency analyses is first carried out, and single energy points are calculated at high levels. In general, DFT (B3LYP) and MP2 calculations are used in conjunction with an empirical approach based on molecular volumes to calculate the lattice enthalpies and entropies of energetic ionic salts. All of the optimized structures are characterized to be true local-energy minima on the potential-energy surface without imaginary frequencies. Condensed phase heats of formation of EILs can be determined using the gas-phase heat of formation and heat of phase transition (lattice energy) according to Hess’s law of constant summation (Born−Haber energy cycle) (Scheme 1). On the basis of Scheme 1, calculation of ΔHf (heats of formation) of the energetic ionic salts can be simplified by eq 2:69

4.5. Ignition Delay Time

As a new class of environmental friendly propellant fuels, hypergolicity is one of the most important properties of hypergolic ILs, which refers to the phenomenon of spontaneous ignition when the EIL-based fuel is contacted with an oxidizer such as 100% HNO3 or N2O4. Theoretically, the faster is the ignition reaction rate, the better. To evaluate the ignition rate of this redox reaction, the ignition delay (ID) time is defined as the time interval between the initial fuel/ oxidizer contact and the start of combustion. In principle, less than 50 ms is the acceptable time for ignition delay in real-life applications. Recently, the goal of developing green propellant fuels is to design and synthesize new hypergolic ILs with ID times of 150 °C. The densities and viscosities of these EILs range between 1.18 and 1.45 g cm−3 and between 75 and 1441 cP, respectively. The detonation velocities lie between 5715 and 6873 m s−1, while the detonation pressures are much lower (830 kJ mol−1) and good detonation velocities of >8000 m s−1, which suggested their potential application in the field of energetic materials. In the same year, several EILs composed of substituted 1,2,4triazolium-based cations and nitrodicyanomethanide and dinitrocyanomethanide anions were reported (Scheme 16, Tri-43−Tri-46).108 The important physicochemical properties such as melting point, thermal stability, and density were measured. All of these EILs exhibited high decomposition temperatures of >200 °C. The melting points and densities of dinitrocyanomethanide salts are higher than those of their nitrodicyanomethanide analogues. Several functionalized 1,2,4-triazolium salts with cyanomethyl, vinyl, and propargyl substituents coupled with energetic

the product; and (iii) the possible loss of a portion of the products during multiple extraction steps. Recently, a new greener synthetic protocol leading to new EILs (Scheme 10, Im-61−Im-68) was developed, that is, through the reactions with the zwitterionic 1,3-dimethyl-imidazolium-2-carboxylate or new [HCO3]−-based IL precursors and neutral energetic azoles with strong or weak acidity (Scheme 10).94 In this strategy, only the byproducts CO2 and H2O were produced, thereby overcoming the previously experienced limitations related to ion exchange processes and product purification. More importantly, this new synthetic protocol can be expanded to the design and synthesis of a large library of azolium azolate salts, further highlighting its application potential for the synthesis of new EILs. 5.2. Triazolium-Based EILs

In comparison with the imidazolium cation, the triazolium cation has higher nitrogen content in the heterocyclic ring, thus theoretically possessing more energy. Over the past decades, the triazolium cation has been widely used for the design of EILs or other high-melting energetic salts.97−101 As early as 2004, the possibility of synthesizing triazolium-based EILs was explored. The syntheses of N-aminoazole precursors and their subsequent quaternization were demonstrated to lead to new energetic salts (Scheme 11, Tri-1−Tri-6).61,102 Most of the Scheme 11. Structures of 1,2,4-Triazolium-Based EILs (Tri1−Tri-6)a

a

Adapted from ref 102.

new energetic salts exhibited low melting points (1.55 g cm−3), and good thermal stabilities. Shortly thereafter, a new class of triazolium-based energetic salts were synthesized, most of which exhibited low melting points of 1.50 g cm−3), good thermal stabilities, high combustion energies, and molar enthalpies of formation. To learn the effect of substitution at the nitrogen atoms of the triazolium ring on the heat of formation and charge delocalization, studies of some triazolium-based dinitramide EILs by ab initio quantum chemistry calculations, in particular about the possible decomposition pathways of these EILs, were reported.98 The results showed that 1,4-substituted triazolium rings have lower energy than the 1,2-substituted analogues. Moreover, it was proved that the presence of only small energy barriers, or often 10541


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Scheme 12. Design and Synthesis of Azido-Containing Triazolium-Based EILs (Tri-7−Tri-15)a

a


8300 m s−1 and detonation pressure of >30 GPa. However, most of them were shown to be sensitive to impact. Because of its high nitrogen content, the azide anion (N3−) is a highly energetic anion. In 2008, six azide anion-based EILs derived from azidoethyl-, alkyl-, and alkenyl-substituted derivatives of 1,2,4- and 1,2,3-amino-triazoles were reported (Scheme 19, Tri-71−Tri-76).112 Interestingly, some azide salts are liquid even at room temperature. More importantly, these EILs are not simple protonated salts like the hydrazinium azides reported previously, which exhibited high thermal stability and negligible vapor toxicity. Moreover, the presence of the azide anion did not make the EILs too sensitive to be handled safely, indicating that these new, less volatile, less sensitive, liquid azides may hold great potential as energetic materials. Nitrogen-rich borate anions (e.g., dihydrobis(1,2,4-triazolyl)borate and hydrotris(1,2,4-triazolyl)borate) were also used for the development of EILs. The synthesis of some azolium poly(1,2,4-triazaolyl)borate salts, which were prepared by a simple and highly effective method, that is, the metathesis reactions between barium or silver salts of dihydrobis(1,2,4triazolyl)borate ([H2B(tz)2]) and hydrotris(1,2,4-triazolyl)borate [HB(tz)3] and 1-amino-4-methyl-1,2,4-triazolium iodide ([NH2mtz]I) precursor, was reported (Scheme 20).113 Two new EILs (Tri-77 and Tri-78) were obtained, and surprisingly

Scheme 13. Chemical Structures of Some 1,2,4-Triazolium Azolate EILs (Tri-16−Tri-27)a

a

Adapted from refs 104 and 105.

anions, perchlorate, nitrate, dicyanamide, and dinitramide, were synthesized (Scheme 17a, Tri-47−Tri-59).109 The melting points of most salts are below 100 °C, which puts them in the EIL class. Their densities range between 1.25 and 1.76 g cm−3, of which the dicyanamides are the lowest. Among these EILs, these dicyanamide salts show the least promise for explosive applications because they have the lowest energetic performance. 1,3-Disubstituted 1,2,3-triazolium cations were also used for the design and synthesis of new EILs. In 2005, a new family of energetic salts, that is, 1-amino-3-alkyl-1,2,3-triazolium nitrates, was designed and synthesized (Scheme 17b, Tri-60− Tri-64).110 All of these nitrate salts fall into the EIL class (mp 10542


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Scheme 14. Structures of 12 Triazolium-Based EILs (Tri-28−Tri-39)a

a


of 1,2,4-triazolyl substituents on the boron atom. For example, the density of [NH2mtz][HB(tz)3] is 1.39 g cm−3, which is higher than that of [NH2mtz][H2B(tz)2] (1.23 g cm−3). Moreover, borate anions have good chemical and electrochemical stabilities that can be applied, for example, in lithium ion batteries as nonaqueous electrolytes. Thus, this new class of borate-based EILs has potential application in the field of electrochemistry also, especially considering the fact that both of them exhibit conductivities similar to those of equimolar KCl solutions in acetonitrile/water. Nitrate is an energetic anion and also a familiar stable ligand for lanthanides. Recently, lanthanide nitrate complex anions were also used as energetic components for the building of EILs.114 A new class of energetic salts based on anionic lanthanide nitrate complexes [Cat]+3[Ln(NO3)6]3−, where [Cat]+ is 4-aminotriazolium, 4-amino-1-methyltriazolium, 4amino-1-ethyltriazolium, and 4-amino-1-butyltriazolium, was reported (Tri-79−Tri-86, Scheme 21).115 It was found that the use of lanthanide nitrates as main energetic anionic components to pair with substituted 1,2,4-triazolium cations can form a series of moisture-stable and thermally stable EILs

Scheme 15. Structures of Three EILs Comprised of 1,2,4Triazolium Cation Paired with 3-Nitro-1,2,4-triazolate-5-one and 5-Nitroimino-tetrazolate Anions (Tri-40−Tri-42)a

a


both of them are liquid at room temperature. The glass transition temperatures of 1-amino-4-methyl-triazolium dihydrobis(1,2,4-triazolyl)borate ([NH2mtz][H2B(tz)2]) and 1-amino-4-methyl-triazolium hydrotris(1,2,4-triazolyl)borate ([NH2mtz][HB(tz)3]) are −35.3 and −18.6 °C, respectively. The densities of these new borate EILs were found to be anion dependent and to increase in direct proportion to the number

Scheme 16. Structures of Four EILs Comprised of Substituted 1,2,4-Triazolium Cations and Nitrodicyanomethanide and Dinitrocyanomethanide Anions (Tri-43−Tri-46)a

a

Adapted from ref 108. 10543


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Scheme 17. Structures of Triazolium-Based EILs (Tri-47−Tri-64)a

a


Scheme 18. Synthesis of Energetic Salts Based on 1-Amino1,2,3-triazole (Tri-65−Tri-70)a

a

Scheme 20. Synthesis of 1-Amino-4-methyl-1,2,4-triazolium EILs Based on Borate Anionsa


a

Scheme 19. Triazolium Azide Salts and AzidoFunctionalized Triazolium Salts (Tri-71 −Tri-76)a

Scheme 21. Chemical Structures of 1,2,4-Triazolium-Based EILs with Anionic Lanthanide Nitrate Complexes (Tri-79− Tri-86)a

a

a



decomposition product, some have a neutral or positive oxygen balance. Furthermore, important for ease of synthesis and for environmental reasons, these liquids are prepared by using readily available nitrate salts as precursors. According to theoretical calculations, these EILs are potential propellants.


(Table 2). Among them, some salts are liquid at room temperature. The thermal degradation temperatures of these EILs are higher than 220 °C. The salts are hydrophilic and are soluble in water and lower alcohols. On the basis of CO as a

5.3. Tetrazolium-Based EILs

To design EILs with the most attractive energetic properties, both the cation and the anion should have high nitrogen 10544


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Table 2. Physicochemical Properties of Triazolium-Based EILsh

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Table 2. continued

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Table 2. continued

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Table 2. continued

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Table 2. continued

a

Melting point. bGlass transition temperature. cThermal decomposition temperature. dDensity. eHeat of formation. fCalculated detonation velocities. gCalculated detonation pressure. hNote that the definitions of detonation velocity and pressure are given in the footnote of Table 1. 10549


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Scheme 22. Synthesis of Tetrazolium-Based EILs (Tetra-1−Tetra-3)a

a


content, which in turn enhances the density and concomitantly the detonation characteristics. As compared to the imidazolium and triazolium cations, the tetrazolium cations are more energetic due to their higher nitrogen content. However, the quaternization of the tetrazole ring is more difficult than for imidazole and triazole molecules, thereby making the design and synthesis of tetrazolium-based EILs more challenging; therefore, the number of reported tetrazolium-based EILs is relatively limited. In 2004, the feasibility of designing tetrazolium-based EILs via the quaternization of 1-amino-5-methyltetrazole with nitric or perchloric acid or with iodomethane followed by metathesis reaction with silver nitrate or silver perchlorate was reported (Scheme 22, Tetra-1 and Tetra-2).102,103 Two tetrazoliumbased EILs were obtained, where the 1-amino-4,5-dimethyltetrazolium nitrate salt was liquid (Tg = −59 °C) at room temperature. A year later, the synthesis of a tetrazolium-based EIL, 1,5-diamino-4-methyl-1H-tetrazolium dinitramide (Scheme 22, Tetra-3), was described.116 This EIL shows a melting point of 85 °C and a decomposition temperature of >150 °C. Because of its high nitrogen (57.0 wt %) and oxygen content (28.9 wt %), this EIL is potentially a highly energetic material. However, its sensitivity to impact and friction is relatively high; that is, the impact sensitivity (IS) and friction sensitivity (FS) are 7 J and 24 N, respectively. Similarly, some protonated tetrazolium-based dinitramide salts were also synthesized by the reaction of potassium dinitramide with some tetrazolium perchlorate salts (Scheme 23, Tetra-4−Tetra-6).117 All three EILs based on the dinitramide anion exhibited melting points tetrazolium. Moreover, energetic azolate anions were also used to form new energetic salts, when paired with the 1,5-diaminotetrazolium or 5-aminotetrazolium cation (top, Scheme 25).107 However, the resulting salts exhibited much higher melting points (mp >100 °C) than the triazolium-based analogues and therefore did not fit the definition of EILs (Table 3). When the same cations were paired with lanthanide nitrate complex anions (e.g., [La(NO3)6]3− or [Ce(NO3)6]3−), the resulting energetic salts have low melting points of 2 g 10550


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As mentioned above, great effort was devoted to the synthesis and properties of high-energy EILs comprised of azido-, nitro-, and N-amino-substituted azolium cations with nitrate or perchlorate anions. The studies involving the EILs based on quaternary ammonium cations are relatively limited, mainly due to the high melting salts. The first example of using quaternary guanidinium cations for synthesis of EILs resulted from the quaternization of cyclic and a cyclic guanidines by acids or alkyl iodide, followed by the subsequent metathesis reactions with nitrate, perchlorate, and dinitroamide salts (Scheme 27, AM-5−AM-22).118 Most of the resulting salts exhibited low melting points of 100 °C (Td: 93.3 °C). The borane-IL solutions also showed excellent hypergolic performance. For example, with TEAB composition mass ratios greater than 20%, the mixture systems containing borane additives exhibited very short ID times of

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