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 MPas), good thermal properties (T-g < -70 degrees C, T-d > 130 degrees 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.