Towards Safer Rocket Fuels: Hypergolic Imidazolylidene‐Borane Compounds as Replacements for Hydrazine Derivatives

Currently, toxic and volatile hydrazine derivatives are still the main fuel choices for liquid bipropellants, especially in some traditional rocket propulsion systems. Therefore, the search for safer hypergolic fuels as replacements for hydrazine derivatives has been one of the most challenging tasks. In this study, six imidazolylidene‐borane compounds with zwitterionic structure have been synthesized and characterized, and their hypergolic reactivity has been studied. As expected, these compounds exhibited fast spontaneous combustion upon contact with white fuming nitric acid (WFNA). Among them, compound 5 showed excellent integrated properties including wide liquid operating range (?70–160 °C), superior loading density (0.99 g cm^?3), ultrafast ignition delay times with WFNA (15 ms), and high specific impulse (303.5 s), suggesting promising application potential as safer hypergolic fuels in liquid bipropellant formulations.     

[Bis(imidazolyl)–BH2]+[Bis(triazolyl)–BH2]− Ionic Liquids with High Density and Energy Capacity

[Bis(imidazolyl)–BH₂]⁺[bis(triazolyl)–BH₂]^? and [bis(imidazolyl)–BH₂]⁺[tris(triazolyl)–BH]^? were synthesized, the cations and anions of which were functionalized with B?H groups and azoles. As B?H groups contribute to the hypergolic activity and azole groups improve the energy output, the resulting ionic liquids exhibited ignition delay times as low as 20 ms and energy outputs as high as 461.1 kJ mol^?1. In addition, densities (1.07–1.22 g cm^?3) and density‐specific impulse (≈360 s g cm^?3) values reached a relatively high level. These ionic liquids show great promise as sustainable rocket fuels.     

Cyanoborohydride‐Based Ionic Liquids as Green Aerospace Bipropellant Fuels

In propellant systems, the most common bipropellants are composed of two chemicals, a fuel (or reducer) and an oxidizer. Currently, the choices for propellant fuels rely mainly on hydrazine and its methylated derivatives, even though they are extremely toxic, highly volatile, sensitive to adiabatic compression (risk of detonation), and, therefore, difficult to handle. With this background, the search for alternative green propellant fuels has been an urgent goal of space science. In this study, a new family of cyanoborohydride‐based ionic liquids (ILs) with properties and performances comparable to hydrazine derivatives were designed and synthesized. These new ILs as bipropellant fuels, have some unique advantages including negligible vapor pressure, ultra‐short ignition delay (ID) time, and reduced synthetic and storage costs, thereby showing great application potential as environmentally friendly fuels in bipropellant formulations.     

Ionic Liquid Propellants: Future Fuels for Space Propulsion

Use of green propellants is a trend for future space propulsion. Hypergolic ionic liquid propellants, which are environmentally‐benign while exhibiting energetic performances comparable to hydrazine, have shown great potential to meet the requirements of developing nontoxic high‐performance propellant formulations for space propulsion applications. This Concept article presents a review of recent advances in the field of ionic liquid propellants.     

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.     

Boronium‐Cation‐Based Ionic Liquids as Hypergolic Fluids

Two series of boronium‐cation‐based ionic liquids were prepared and fully characterized by ¹H, ¹³C, and ¹¹B NMR and infrared spectroscopy, differential scanning calorimetry (DSC), and elemental analysis. The structure of bis(1‐methyl‐1H‐imidazole‐3‐yl)dihydroboronium dicyanoborohydride ( 5 a ) was determined by single‐crystal X‐ray diffraction. The densities of these ionic liquids range from 1.05 to 1.28 g cm^?3, and the heats of formation, predicted on the basis of Gaussian 03 calculations, fall between ?164.6 and 430.5 kJ mol^?1. Compound 5 b , bis(1‐allyl‐1H‐imidazole‐3‐yl)dihydroboronium dicyanoborohydride, exhibits the lowest viscosity (35 mPa s) and shortest ignition‐delay time (14 ms) in combination with 100 % HNO₃.     

Hypergolic Ionic Liquids with the 2,2‐Dialkyltriazanium Cation

No flame, no gain : A hypergolic mixture is composed of stable species that readily react/ignite on molecular contact. Both the anion and the cation in an ionic liquid play prominent roles in determining hypergolic properties as well as ignition delay times. With the 2,2‐dialkyltriazanium cation, salts with nitrate, chloride, nitrocyanamide, and dicyanamide anions are hypergolic.

     

Nitrocyanamide‐Based Ionic Liquids and Their Potential Applications as Hypergolic Fuels

Nitrocyanamide ionic liquids with substituted imidazolium, guanidinium, and tetrazolium cations have been synthesized and fully characterized. Aminoguanidinium nitrocyanamide ( 7 ) crystallizes in the triclinic system P$\bar 1$ . The results obtained from theoretical calculations based on 7 are consistent with the single‐crystal structure data. These ionic liquids exhibit desirable physicochemical properties, such as low melting points and good thermal stabilities. Furthermore, they are all impact insensitive materials. Their energetic performances, including heats of formation, detonation pressures, and detonation velocities, were studied by a combination of theoretical and empirical calculations. The ionic liquids 1 – 4 have large liquid ranges and low viscosities. They were shown to be promising candidates as hypergolic ionic liquids through the combustion tests with 100 % HNO₃.     

Synthesis and Improved Properties of Hypergolic Boronium‐Based Ionic Liquids

A series of asymmetric monoimidazolium dihydroboronium‐based ionic liquids (ILs) were synthesized from amine‐boranes. All the resulting ILs were fully characterized by ¹H and ¹³C NMR, IR spectroscopy, elemental analysis or high resolution mass spectrum. Compared with the symmetric bisimidazolium dihydroboronium‐based ILs, these new ILs exhibited improved properties with shorter ignition delay times (IDs), higher densities, and lower phase transition temperature showing the promising application potential as green propellants.     

Transformation of Atmospheric CO2 Catalyzed by Protic Ionic Liquids: Efficient Synthesis of 2‐Oxazolidinones

Protic ionic liquids (PILs), such as 1,8‐diazabicyclo[5.4.0]‐7‐undecenium 2‐methylimidazolide [DBUH][MIm], can catalyze the reaction of atmospheric CO₂ with a broad range of propargylic amines to form the corresponding 2‐oxazolidinones. The products are formed in high yields under mild, metal‐free conditions. The cheaper and greener PILs can be easily recycled and reused at least five times without a decrease in the catalytic activity and selectivity. A reaction mechanism was proposed on the basis of a detailed DFT study which indicates that both the cation and anion of the PIL play key synergistic roles in accelerating the reaction.     

Borohydride Ionic Liquids as Hypergolic Fuels: A Quest for Improved Stability

Hydrazine and its derivatives are used as fuels in rocket propellant systems; however, due to high vapor pressure, toxicity, and carcinogenicity, handling of such compounds is extremely hazardous. Hypergolic ionic liquids have shown great promise to become viable replacements for hydrazines as fuels. Borohydride‐containing ionic liquids have now been synthesized using a more efficient synthetic pathway that does not require liquid ammonia and halide precursors. Among the eight new compounds, 1‐allyl‐3‐n‐butyl‐imidazolium borohydride ( 1 ) and 1, 3‐diallylimidazolium borohydride ( 5 ) exhibit very short ignition‐delay times (ID) of 8 and 3 ms, respectively. The hydrolytic stability of borohydride compounds has been greatly improved by attaching long‐chain alkyl substituents to the imidazole ring. 1,3‐Di‐(n‐octyl)‐imidazolium borohydride ( 3 ) is a water stable borohydride‐containing ionic liquid. 1,3‐Di‐(n‐butyl)‐imidazolium borohydride ( 2 ) is a unique example of a borohydride liquid crystal. These ionic liquids have some unusual advantages, including negligible vapor pressures, good ignition delay (ID) times, and reduced synthetic and storage costs, thereby showing good application potential as environmentally friendly fuels in bipropellant formulations. In addition, they also have potential applications in the form of reducing agents and hydrogen storage materials.     

Synthesis and Characterization of 5‐Cyanotetrazolide‐Based Ionic Liquids

New salts based on imidazolium, pyrrolidinium, phosphonium, guanidinium, and ammonium cations together with the 5‐cyanotetrazolide anion [C₂N₅]^? are reported. Depending on the nature of cation–anion interactions, characterized by XRD, the ionic liquids (ILs) have a low viscosity and are liquid at room temperature or have higher melting temperatures. Thermogravimetric analysis, cyclic voltammetry, viscosimetry, and impedance spectroscopy display a thermal stability up to 230 °C, an electrochemical window of 4.5 V, a viscosity of 25 mPa s at 20 °C, and an ionic conductivity of 5.4 mS cm^?1 at 20 °C for the IL 1‐butyl‐1‐methylpyrrolidinium 5‐cyanotetrazolide [BMPyr][C₂N₅]. On the basis of these results, the synthesized compounds are promising electrolytes for lithium‐ion batteries.     

Temperature Dependence of the Viscosity and Conductivity of Mildly Functionalized and Non‐Functionalized [Tf2N]− Ionic Liquids

A series of bis(trifluoromethylsulfonyl)imide ionic liquids (ILs) with classical as well as mildly functionalized cations was prepared and their viscosities and conductivities were determined as a function of the temperature. Both were analyzed with respect to Arrhenius, Litovitz and Vogel–Fulcher–Tammann (VFT) behaviors, as well as in the context of their molecular volume (V_m). Their viscosity and conductivity are highly correlated with V_m/T or related expressions (R²≥0.94). With the knowledge of V_m of new cations, these correlations allow the temperature‐dependent prediction of the viscosity and conductivity of hitherto unknown, non‐ or mildly functionalized ILs with low error bars (0.05 and 0.04 log units, respectively). The influence of the cation structure and mild functionalization on the physical properties was studied with systematically altered cations, in which V_m remained similar. The T_o parameter obtained from the VFT fits was compared to the experimental glass temperature (T_g) and the T_g/T_o ratio for each IL was calculated using both experimental values and Angell’s relationship. With Walden plots we investigated the IL ionicity and interpreted it in relation to the cation effects on the physical IL properties. We checked the validity of these V_m/T relations by also including the recently published variable temperature viscosity and conductivity data of the [Al(OR^F)₄]^? ILs with R^F=C(H)(CF₃)₂ (error bars for the prediction: 0.09 and 0.10 log units, respectively).     

Computer‐Aided Design of Ionic Liquids as CO2 Absorbents

Ionic liquids (ILs), vary strongly in their interaction with CO₂. We suggest simple theoretical approach to predict the CO₂ absorption behavior of ILs. Strong interaction of the CO₂ with the IL anions corresponds to chemical absorption whereas weak interaction indicates physical absorption. A predictive estimate with a clear distinction between physical and chemical absorption can be simply obtained according to geometries optimized in the presence of a solvation model instead of optimizing it only in gas phase as has been done to date. The resulting Gibbs free energies compare very well with experimental values and the energies were correlated with experimental capacities. Promising anions, for ionic liquids with reversible CO₂ absorption properties can be defined by a reaction Gibbs free energy of absorption in the range of ?30 to 16 kJ mol^?1.     

Borohydride Ionic Liquids and Borane/Ionic‐Liquid Solutions as Hypergolic Fuels with Superior Low Ignition‐Delay Times

In propellant systems, fuels of choice continue to be hydrazine and its derivatives, even though they comprise a class of acutely carcinogenic and toxic substances which exhibit rather high vapor pressures and require expensive handling procedures and costly safety precautions. Hypergolic ionic liquids tend to have low volatility and high thermal and chemical stability, and often exhibit wide liquid ranges, which could allow the use of these substances as bipropellant fuels under a variety of conditions. A new family of borohydride ionic liquids and borane–ionic‐liquid solutions is described which meets nearly all of the desired important criteria for well‐performing fuels. They exhibit ignition‐delay times that are superior to that of any known hypergolic ionic liquid and may thus be legitimate replacements for hydrazine and its derivatives.