Rational Design and Facile Synthesis of Boranophosphate Ionic Liquids as Hypergolic Rocket Fuels

Abstract

The design and synthesis of new hypergolic ionic liquids (HILs) as replacements for toxic hydrazine derivatives have been the focus of current academic research in the field of liquid bipropellant fuels. In most cases, however, the requirements of excellent ignition performances, good hydrolytic stabilities, and low synthetic costs are often contradictory, which makes the development of high-performance HILs an enormous challenge. Here, we show how a fuel-rich boranophosphate ion was rationally designed and used to synthesize a series of high-performance HILs with excellent comprehensive properties. In the design strategy, we introduced the {BH } moiety into the boranophosphate ion for improving the self-ignition property, whereas the complexation of boron and phosphite was used to improve the hydrolytic activity of the borohydride species. As a result, these boranophosphate HILs exhibited wide liquid operating ranges (>220 °C), high densities (1.00-1.10 g cm ), good hydrolytic stabilities, and short ignition delay times (2.3-9.7 milliseconds) with white fuming nitric acid (WFNA) as the oxidizer. More importantly, these boranophosphate HILs could be readily prepared in high yields from commercial phosphite esters, avoiding complex and time-consuming synthetic routes. This work offers an effective strategy of designing boranophosphate HILs towards safer and greener hypergolic fuels for liquid bipropellant applications.

Theoretical Study on Hydrolytic Stability of Borohydride-Rich Hypergolic Ionic Liquids

“…In general, the hypergolic process of a bipropellant system consists of a complex interplay between heat and mass transfer as well as the kinetics for reactive species production and species identity. − Essentially, the hypergolicity of HILs with an oxidizer is a tremendous redox reaction in which HILs (fuel) are the reducer. , Therefore, there is a widespread sentiment in the design of HILs that the HIL’s cation significantly influences the physicochemical properties, while the anion is strongly related to the hypergolic activity because the reducibility of HILs mainly comes from anions. − Among the developed HILs’ anions, several classes of borohydride-rich anions (e.g., [BH 4 ] − , [BH 3 CN] − , [BH 2 (CN) 2 ] − , [BH 3 CNBH 2 CN] − , [PO­(OCH 3 ) 2 BH 3 ] − , and so on) have demonstrated their superior ignition reactivity (faster self-ignition and combustion) with oxidizers such as white fuming nitric acid (WFNA) and dinitrogen tetroxide (N 2 O 4 ). − These results are not surprising because these borohydride-rich anions exhibit strong reducibility. Unfortunately, in many cases, the reducibility of anions is incompatible with the hydrolytic stability, thereby making the storage of HILs very difficult.…”

Synthesis and Properties of Azide‐Functionalized Ionic Liquids as Attractive Hypergolic Fuels

“…The hypergolicity of some dicyanamide‐based ionic liquids was first discovered by Schneider and co‐workers in 2008 . Since then, several families of hypergolic ILs with different structures have been reported, including azide (N 3 − ), dicyanamide ([N(CN) 2 ] − ), nitrocyanamide ([N(NO 2 )CN] − ), borohydride, etc.…”

The Electrolysis of Ammonium Dinitramide in Dimethyl Sulfoxide

“…The discovery of IL hypergols , which ignite instantly upon contact with an appropriate oxidizer, has also opened up a new research avenue related to bipropellant applications. Thus, the exploration of high‐performance hypergolic EILPs has become of interest in the field of IL chemistry, and there have been numerous studies related to new hypergolic EILPs and their hypergolic mechanisms .…”