Atmospheric Ignition Chemistry of Green Hypergolic Bipropellant 1-Ethyl-3-Methylimidazolium Cyanoborohydride – Hydrogen Peroxide in an Acoustic Levitator: Exploring a Potent Universal Propellant

Abstract

Hypergolic ionic liquids (HILs) represent a critical pool of reactive ionic liquids which ignite spontaneously in absence of oxygen when mixed with an oxidizer such as white fuming nitric acid (WFNA, HNO3) or hydrogen peroxide (H2O2). These HILs have emerged as greener alternatives to the toxic hydrazine family of fuels for operations in space under anaerobic conditions. Here, we report on the unusual atmospheric ignition chemistry of the 1-ethyl-3-methylimidazolium cyanoborohydride ([EMIM][CBH])-H2O2 bipropellant while comparing with the parent hypergolic reaction by exploiting a chirped-pulse triggered droplet merging technique in an ultrasonic levitation apparatus under controlled environment. Significant enhancements of the ignition performance and noticeable differences of product distribution in the simulated atmosphere containing molecular oxygen are revealed. The critical mechanistic role of the surrounding oxygen plausibly involves the reaction with a suitable intermediate (iminomethyl)boronic acid, [(HO)2BCH=NH] formed in the initial reactions of the IL and H2O2 which eventually generates catalytic hydroperoxyl (⋅HO2) and hydroxyl (⋅OH) radicals - further promoting the ignition reaction. This systematic case study conceptually demonstrates that under atmospheric conditions, well-defined HIL - oxidizer combinations provide excellent ignition performance and effectively be used as universal fuel both in space and in the terrestrial atmosphere simplifying the multistage propulsion system.