Rapid ignition of "green" bipropellants enlisting hypergolic copper (II) promoter-in-fuel

Abstract

Ionic liquid-with-promoter fuels and H2O2 oxidizer require optimization and further exploration of inexpensive initiators that can be understood at the molecular level; it is advantageous when exacting structural characterization comparisons can also be made. For the first time, hypergolic energetic copper(II) complexes viz. [CuII(1-H-imidazole)4(BH3CN)][BH3CN] (Cu-P1) and [CuII(1-methyl imidazole)4(BH3CN)2] (Cu-P2) were developed as new ‘promoters’ for “green” space vehicle propulsion. Cu-P1 and Cu-P2 exhibited ignition delay times (IDTs) of 3.75 and 8.5 ms, respectively, with 95% H2O2. The superior solubility of Cu-P1 in ionic and non-ionic fuels helped to promote the hypergolic ignition of green liquid fuels. The ionic liquid, 1-ethyl-3-methyl imidazolium cyanoborohydride ([EMIM][BH3CN]), non-ionic tetraglyme, and mixtures thereof (1:1, wt/wt%) were employed. A 13 wt% of Cu-P1 in [EMIM][BH3CN] improved the IDT from >1000 ms to 9.50 ms with 95% H2O2 and from >5000 ms to 21.25 ms with 70% H2O2. Hence, the development of a suitable energetic promoter, soluble in ionic liquid, gained significant attention for “green” bipropellant development. Interestingly, tetraglyme is non-hypergolic with H2O2; a 13 wt% Cu-P1 induced the hypergolic ignition with an IDT of 9.0 ms (95% H2O2). Moreover, the fuel mixture of [EMIM][BH3CN] and tetraglyme (1:1, wt/wt%) with 13 wt% Cu-P1 also revealed the shortest IDT of 7.75 ms (95% H2O2) and 16.5 ms (70% H2O2). The superior ignition performance, reasonable viscosity and density, long-term stability of promoter in the fuel, and comparable theoretical performance of hypergolic propellants are advantages of our approach.