Thermal Decomposition and Hypergolic Reaction of a Dicyanoborohydride Ionic Liquid

In this study, in situ infrared spectroscopy techniques and thermogravimetric analysis coupled with mass spectrometry (TGA-MS) are employed to characterize the reactivity of the ionic liquid, 1-butyl-3-methylimidazolium dicyanoborohydride (BMIM⁺DCBH^-), in comparison to the well-characterized 1-butyl-3-methylimidazolium dicyanamide (BMIM⁺DCA^-) ionic liquid. TGA measurements determined the enthalpy of vaporization (ΔH_vap) to be 112.7 ± 12.3 kJ/mol at 298 K. A rapid scan Fourier transform infrared spectrometer was used to obtain vibrational information useful in tracking the appearance and disappearance of species in the hypergolic reactions of BMIM⁺DCBH^- and BMIM⁺DCA^- with white fuming nitric acid (WFNA) and in the thermal decomposition of these energetic ionic liquids. Attenuated total reflectance measurements recorded the infrared spectra of the reactant sample (BMIM⁺DCBH^-) and the liquid reaction products after reacting with WFNA. Computational chemistry efforts, aided by the experimental results, were used to propose key reaction pathways leading to the hypergolic ignition of BMIM⁺DCBH^- + WFNA. Experimental results indicate that the hypergolic reaction of BMIM⁺DCBH^- with WFNA generates both common and unique intermediates as compared to previous BMIM⁺DCA^- + WFNA investigations: nitrous oxide was generated during both hypergolic reactions indicating that it may play a crucial role in the hypergolic ignition process, NO₂ was generated in significantly higher concentrations for BMIM⁺DCBH^- than for BMIM⁺DCA^-, CO₂ was only generated for BMIM⁺DCA^-, and HCN was only generated during thermal decomposition and hypergolic ignition of BMIM⁺DCBH^-.