Synthesis and Properties of Azide-Functionalized Ionic Liquids as Attractive Hypergolic Fuels

Hypergolic ionic liquids (ILs) have shown a great promise as viable replacements for toxic and volatile hydrazine derivatives used as propellant fuels, and hence, have attracted increasing interest over the last decade. To take advantage of the reactivity and high energy density of the azido group, a family of low-cost and easily prepared azide-functionalized cation-based ILs, including fuel-rich anions, such as nitrate, dicyanamide, and nitrocyanamide anions, were synthesized and characterized. All the dicyanamide- and nitrocyanamide-based ILs exhibited spontaneous combustion upon contact with 100 % HNO₃ . The densities of these hypergolic ILs varied in the range 1.11-1.29 g cm^-3 , and the density-specific impulse, predicted based on Gaussian 09 calculations, was between 289.9 and 344.9 s g cm^-3 . The values of these two key physical properties are much higher than those of unsymmetrical dimethylhydrazine (UDMH). Among the studied compounds, compound IL-3b, that is, 1-(2-azidoethyl)-1-methylpyrrolidin-1-ium dicyanamide, shows excellent integrated properties including the lowest viscosity (30.9 M Pa s), wide liquid operating range (-70 to 205 °C), shortest ignition-delay time (7 ms) with 100 % HNO₃ , and superior density specific impulse (302.5 s g cm^-3 ), suggesting promising applications with potential as bipropellant formulations.

Synthesis and hypergolic properties of flammable ionic liquids based on the cyano (1H-1,2,3-triazole-1-yl) dihydroborate anion

A family of ionic liquids based on the cyano (1H-1,2,3-triazole-1-yl) dihydroborate anion was developed, which showed hypergolic reactivity with white fuming nitric acid. Furthermore, these ionic liquids can be ignited upon contact with 90 wt% H₂O₂ in the presence of iodine.

Rational Design and Facile Synthesis of Boranophosphate Ionic Liquids as Hypergolic Rocket Fuels

The design and synthesis of new hypergolic ionic liquids (HILs) as replacements for toxic hydrazine derivatives have been the focus of current academic research in the field of liquid bipropellant fuels. In most cases, however, the requirements of excellent ignition performances, good hydrolytic stabilities, and low synthetic costs are often contradictory, which makes the development of high-performance HILs an enormous challenge. Here, we show how a fuel-rich boranophosphate ion was rationally designed and used to synthesize a series of high-performance HILs with excellent comprehensive properties. In the design strategy, we introduced the {BH₃ } moiety into the boranophosphate ion for improving the self-ignition property, whereas the complexation of boron and phosphite was used to improve the hydrolytic activity of the borohydride species. As a result, these boranophosphate HILs exhibited wide liquid operating ranges (>220 °C), high densities (1.00-1.10 g cm^-3 ), good hydrolytic stabilities, and short ignition delay times (2.3-9.7 milliseconds) with white fuming nitric acid (WFNA) as the oxidizer. More importantly, these boranophosphate HILs could be readily prepared in high yields from commercial phosphite esters, avoiding complex and time-consuming synthetic routes. This work offers an effective strategy of designing boranophosphate HILs towards safer and greener hypergolic fuels for liquid bipropellant applications.

Synthesis and Properties of Triaminocyclopropenium Cation Based Ionic Liquids as Hypergolic Fluids

A novel family of hydrophobic triaminocyclopropenium cation based ionic liquids have been synthesized, and their structures and physicochemical properties characterized by NMR and IR spectroscopy, elemental analysis, differential scanning calorimetry, and hypergolic tests. The experimental results showed that all of these ionic liquids exhibited the expected hypergolic reactivity with the oxidizer white fuming nitric acid. Among them, the hypergolic ionic liquid based on the cyanoimidazolylborohydride anion showed excellent integrated properties, including high decomposition temperature (194 °C), high density (0.95 g cm^-3 ), moderate viscosity (44 MPa s), ultrafast ignition delay time (6 ms), and high specific impulse (301.9 s); this demonstrates its potential as an environmentally friendly alternative to toxic hydrazine derivatives.

Prospective Symbiosis of Green Chemistry and Energetic Materials

A global increase in environmental pollution demands the development of new "cleaner" chemical processes. Among urgent improvements, the replacement of traditional hydrocarbon-derived toxic organic solvents with neoteric solvents less harmful for the environment is one of the most vital issues. As a result of the favorable combination of their unique properties, ionic liquids (ILs), dense gases, and supercritical fluids (SCFs) have gained considerable attention as suitable green chemistry media for the preparation and modification of important chemical compounds and materials. In particular, they have a significant potential in a specific and very important area of research associated with the manufacture and processing of high-energy materials (HEMs). These large-scale manufacturing processes, in which hazardous chemicals and extreme conditions are used, produce a huge amount of hard-to-dispose-of waste. Furthermore, they are risky to staff, and any improvements that would reduce the fire and explosion risks of the corresponding processes are highly desirable. In this Review, useful applications of almost nonflammable ILs, dense gases, and SCFs (first of all, CO₂ ) for nitration and other reactions used for manufacturing HEMs are considered. Recent advances in the field of energetic (oxygen-balanced and hypergolic) ILs are summarized. Significant attention is paid to the SCF-based micronization techniques, which improve the energetic performance of HEMs through an efficient control of the morphology and particle size distribution of the HEM fine particles, and to useful applications of SCFs in HEM processing that makes them less hazardous.

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

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

Ultrafast igniting, imidazolium based hypergolic ionic liquids with enhanced hydrophobicity

Exploring ultrafast igniting and hydrolytically stable ionic liquids (ILs) has a wide scope in hypergolic rocket fuels.

Exploiting hydrophobic borohydride-rich ionic liquids as faster-igniting rocket fuels

A family of hydrophobic borohydride-rich ionic liquids was developed, which exhibited the shortest ignition delay times of 1.7 milliseconds and the lowest viscosity (10 mPa s) of hypergolic ionic fluids, demonstrating their great potential as faster-igniting rocket fuels to replace toxic hydrazine derivatives in liquid bipropellant formulations.

Oxygen-rich anion based energetic salts with high detonation performances

Novel energetic salts based on 2,2,2-trinitroethyl nitrocarbamate anion were synthesized, which exhibited promising detonation performances (detonation pressure: 29.4 to 33.1 GPa; detonation velocity: 8213 to 8596 m s⁻¹).

Energetic salts based on 3-hydrazino-4-amino-1,2,4-triazole (HATr): synthesis and properties

The synthesized energetic salts of 3-hydrazino-4-amino-1,2,4-triazole were fully characterized with respect to their prospective use in energetic applications.

Exploring Sustainable Rocket Fuels: [Imidazolyl-Amine-BH2](+)-Cation-Based Ionic Liquids as Replacements for Toxic Hydrazine Derivatives

The application of hypergolic ionic liquids as propellant fuels is a newly emerging area in the fields of chemistry and propulsion science. Herein, a new class of [imidazolyl-amine-BH2](+)-cation-based ionic liquids, which included fuel-rich anions, such as dicyanamide (N(CN)2(-)) and cyanoborohydride (BH3CN(-)) anions, were synthesized and characterized. As expected, all of the ionic liquids exhibited spontaneous combustion upon contact with the oxidizer 100 % HNO3. The densities of these ionic liquids varied from 0.99-1.12 g cm(-3), and the heats of formation, predicted based on Gaussian 09 calculations, were between -707.7 and 241.8 kJ mol(-1). Among them, the salt of compound 5, that is, (1-allyl-1H-imidazole-3-yl)-(trimethylamine)-dihydroboronium dicyanamide, exhibited the lowest viscosity (168 MPa s), good thermal properties (Tg <-70 °C, Td >130 °C), and the shortest ignition-delay time (18 ms) with 100 % HNO3. These ionic fuels, as "green" replacements for toxic hydrazine-derivatives, may have potential applications as bipropellant formulations.

Solid-state Reaction of Azolium Hydrohalogen Salts with Silver Dicyanamide--Unexpected Formation of Cyanoguanidine-azoles, Reaction Mechanism and Their Hypergolic Properties

Cyanoguanidines as well as azoles are important bioactive groups, which play an important role in the medical application; meanwhile, the high nitrogen content makes them excellent backbones for energetic materials. A Novel and simple method that combined these two fragments into one molecular compound was developed through the transformation of dicyanamide ionic salts. In return, compounds 4–11 were synthesized, and fully characterized by IR, MS, NMR and elemental analysis. Meanwhile, the structures of compounds 4, 8 and 11 were confirmed by X-ray crystal diffraction. Detailed reaction mechanisms were studied through accurate calculations on the reaction energy profiles of the azolium cations and DCA anion, which revealed the essence of the transformation proceeding. Meanwhile, compound 8 exhibits excellent hypergolic property, which could be potentially novel molecular hypergolic fuel.

3,4-Diamino-1,2,4-triazole based energetic salts: synthesis, characterization, and energetic properties

A series of energetic salts based on 3,4-diamino-1,2,4-triazole with promising detonation performances have been synthesized using a metathesis reaction method or a protonation reaction method.

Alkyl ammonium cation stabilized biocidal polyiodides with adaptable high density and low pressure

The effective application of biocidal species requires building the active moiety into a molecular back bone that can be delivered and decomposed on demand under conditions of low pressure and prolonged high-temperature detonation. The goal is to destroy storage facilities and their contents while utilizing the biocidal products arising from the released energy to destroy any remaining harmful airborne agents. Decomposition of carefully selected iodine-rich compounds can produce large amounts of the very active biocides, hydroiodic acid (HI) and iodine (I2). Polyiodide anions, namely, I3(-), I5(-), which are excellent sources of such biocides, can be stabilized through interactions with large, symmetric cations, such as alkyl ammonium salts. We have designed and synthesized suitable compounds of adaptable high density up to 3.33 g cm(-3) that are low-pressure polyiodides with various alkyl ammonium cations, deliverable iodine contents of which range between 58.0-90.9%.

Energetic salts based on dipicrylamine and its amino derivative

Energetic salts based on dipicrylamine and its amino derivative were synthesized. All salts were fully characterized by multinuclear NMR spectroscopy ((1)H, (13)C), vibrational spectroscopy (IR), and elemental analysis. Ethylenediammonium di-DPA (DPA=dipicrylamine) and 1,3-diaminoguanidinium DPA were further confirmed by single-crystal X-ray diffraction. These salts exhibit reasonable physical properties, such as high densities (1.71-1.81 g cm(-3)), good thermal stabilities (T(d) =155-285 °C), and low solubilities in water. The impact sensitivity of 1-methyl-3,4,5-triamino-1,2,4-triazolium DPA is lower than that of 2,4,6-trinitrotoluene (TNT), and for some other energetic salts their impact sensitivities are comparable to that of TNT. Based on experimental densities and theoretical calculations carried out by using the Gaussian 03 suite of programs, all the salts have calculated detonation pressures (22.5-27.8 GPa) and velocities (7226-7917 m s(-1)) that exceed those of conventional TNT. The toxicities of these salts measured by luminescent bacteria toxicity tests are much lower than that of TNT, and two binary eutectic mixtures with melting points that fall between 70 and 100 °C were identified.