Session: Physical Chemistry Poster:

Hypergolic ionic liquids (ILs) ignite spontaneously upon contact with an oxidizer (e.g. HNO₃) and present low vapor toxicities and environmental hazards. They have become of interest within the aerospace community as green propellant alternatives to hydrazine for rocket propulsion applications. The designability of hypergolic ILs includes the choice of cation and anion within a single ion pair and the introduction of functional group into the constituents, each of which may be modified independently. This presentation reports investigations into thermal decomposition and oxidation of various ILs which consist of 1-methyl-4-amino-1,2,4-triazolium (MAT⁺) and dialkyl imidazolium (dialkyl-IM⁺) cations and dicyanamide (DCA⁻) and dicyanoborohydride (DCBH⁻) anions. The work was to reveal preignition kinetics and dynamics of individual constituent ions and their combinations. The direct dynamics trajectory simulations were employed to identify possible pathways in the IL reactions, followed by potential energy surface mapping and RRKM kinetics modeling to pinpoint the most probable reaction mechanisms and products. Theoretical results were validated by comparing product structures and spectroscopy with available experimental data.
Anions play a decisive role in IL hypergolicity, and DCA⁻ and DCBH⁻ are the two commonly used in propellant formulations. The preignition reaction of DCA⁻ with HNO₃ is dominated by the formation of O₂N-NHC(O)NCN⁻ and O₂N-NHC(O)NC(O)NH-NO₂⁻ (i.e. DNB⁻). Compared to DCA⁻, the DCBH⁻-containing ILs present remarkably shorter ignition delay time. The enhanced performance of DCBH⁻ can be rationalized by boron-hydride-specific preignition reactions, of which the most important is the elimination of H₂ via the combination of a hydride in DCBH⁻ and a proton in HNO₃. Cations not only tune IL physicochemical properties, but also participate in preignition via intra-ion-pair reactions. The cation's capability of proton transfer and/or alkyl abstraction with counter anion may significantly affect the oxidation activities of DCA⁻-containing ILs, as exemplified by the reactive trend: 1-butyl-3-methyl-imidazolium(BMIM⁺)-DCA⁻ > 1-allyl-3-methyl-imidazolium(AMIM⁺)-DCA⁻ >> MAT⁺DCA⁻.

Wenjing Zhou

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