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 (T_g<-70 C, T_d>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. Rocket man: New [imidazolyl-amine-BH₂]-cation-based hypergolic ionic liquids exhibit hypergolic properties upon contact with an oxidizer. Of the new ionic liquids, seven dicyanamide-based ionic liquids are water-stable and have ID<50 ms. Compared to traditional hypergolic ionic liquids, they exhibited shorter ID times and higher densities, thus suggesting their potential application in liquid bipropellant fuels or formulations.