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

Graphical Rocket man : New [imidazolyl−amine−BH 2 ]-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. 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−BH 2 ] + -cation-based ionic liquids, which included fuel-rich anions, such as dicyanamide (N(CN) 2 − ) and cyanoborohydride (BH 3 CN − ) anions, were synthesized and characterized. As expected, all of the ionic liquids exhibited spontaneous combustion upon contact with the oxidizer 100 % HNO 3 . 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 % HNO 3 . These ionic fuels, as “green” replacements for toxic hydrazine-derivatives, may have potential applications as bipropellant formulations.