Effect of Boron Clusters on the Ignition Reaction of HNO₃ and Dicynanamide-Based Ionic Liquids

Abstract

Many ionic liquids containing the dicynamide anion (DCA, formula N(CN)) exhibit hypergolic ignition when exposed to the common oxidizer nitric acid. However, the ignition delay is often about 10 times longer than the desired 5 ms for rocket applications, so that improvements are desired. Experiments in the past decade have suggested both a mechanism for the early reaction steps and also that additives such as decaborane can reduce the ignition delay. The mechanisms for reactions of nitric acid with both DCA and protonated DCAH are considered here, using accurate wave function methods. Complexation of DCA or DCAH with borane clusters BH or BH is found to modify these mechanisms slightly by changing the nature of some of the intermediate saddle points and by small reductions in the reaction barriers.

Theoretical Investigation of the Reactivity of Sodium Dicyanamide with Nitric Acid

“…Because experiments have indicated that the reaction takes place primarily on the anion, most previous theoretical studies of the hypergolic reaction have focused on the reaction between the DCA – anion and HNO 3 . ,− In 2008, Chambreau and co-workers proposed a generalized mechanism for the reaction of DCA – + HNO 3 that was intended to imply fully solvated DCA – . The mechanism begins with the protonation of DCA – at the terminal N atom , to form HDCA, followed by NO 3 – attack of the electrophilic C atom.…”

“…More recent computational studies have begun to look outside of the fundamental DCA – + HNO 3 reaction. Reactions of HNO 3 with the intermediate HDCA have been calculated, , as it is formed during the initial step in the hypergolic reaction sequence, and proton transfer from HNO 3 to DCA – is favored at equilibrium in the condensed phase . Chambreau and co-workers showed that in the condensed phase, the reaction of HDCA + HNO 3 is calculated to have lower free energy barriers, but also substantially less free energy release, than the bare DCA – + HNO 3 reaction .…”

“…In the gas phase, Schmidt and Gordon calculate the effective NO 2 shift barrier to be lower for the reaction of HDCA + HNO 3 than for DCA – + HNO 3 , although the earlier barriers are higher . Most recently, the effect of anionic and neutral boron clusters, which have been shown to reduce ignition delay, − and the relationship between ionicity of the IL ion pair and ignition delay have been examined. However, the explicit calculation of the reaction coordinate of a DCA – -based ion pair plus HNO 3 is lacking.…”

Thermal Decomposition and Hypergolic Reaction of a Dicyanoborohydride Ionic Liquid

“…Characterizing the energetics of complex, rapid reactions takes a combination of experimental and computational efforts. − In the present work, this approach is used to study a dicyanoborohydride-based EIL, selected due to its known hypergolicity and inclusion of boron. Energetic ionic liquids with the dicyanoborohydride anion have low ignition delays and high propellant performance characteristics, making these chemicals a desirable fuel for bipropellant systems. ,− By incorporation of boron into the chemical structure of a fuel, the energy density can be enhanced with an increase in the fuel’s volumetric enthalpy of combustion. ,− …”

Ignition Delay Reduction with Sodium Addition to Imidazolium-Based Dicyanamide Ionic Liquid

“…Diffusion-limited reactivity of these ILs was addressed in work conducted by Chambreau et al in 2016 for gaseous HNO 3 reacting with aerosol droplets of IL, but the molecular dynamics (MD) simulations for liquid–liquid diffusion indicate that mixing of the two liquids, BMIM + DCA – and HNO 3 , is very rapid . Under 10 ms is considered a desirable ignition delay for aerospace applications. , Most of the ionic liquids examined to date have ignition delays that are longer, and considering fuel additives may be an option to reduce these ignition delays into a desirable range for in-space applications. − …”