Fourier Transform Infrared Studies in Hypergolic Ignition of Ionic Liquids

Hypergolic ionic liquids: to be or not to be?

Hypergolic ionic liquids (HIL) - ionic liquids which ignite spontaneously upon contact with an oxidizer - emerged as green space propellants. Exploiting the previously marked hypergolic [EMIM][CBH] - WFNA (1-ethyl-3-methylimidazolium cyanoborohydride - white fuming nitric acid) system as a benchmark, through the utilization of a novel chirped-pulse droplet-merging technique in an ultrasonic levitation environment and electronic structure calculations, this work deeply questions the hypergolicity of the [EMIM][CBH]-WFNA system. Molecular oxygen is critically required for the [EMIM][CBH]-WFNA system to ignite spontaneously. State-of-the-art electronic structure calculations identified the resonantly stabilized N-boryl-N-oxo-formamide [(H3B-N(O)-CHO)-; BOFA] radical anion as the key intermediate in driving the oxidation chemistry upon reaction with molecular oxygen of the ionic liquid. These findings challenge conventional wisdom of 'well-established' test protocols as indicators of the hypergolicity of ionic liquids thus necessitating truly oxygen-free experimental conditions to define the ignition delay upon mixing of the ionic liquid and the oxidizer and hence designating an ionic liquid as truly hypergolic at the molecular level.

Structural Properties of HEHN- and HAN-Based Ionic Liquid Mixtures: A Polarizable Molecular Dynamics Study

Molecular dynamics simulations of binary mixtures comprising 2-hydroxyethylhydrazinium nitrate (HEHN) and hydroxylammonium nitrate (HAN) were conducted using the polarizable APPLE&P force field to investigate fundamental properties of multimode propulsion (MMP) propellants. Calculated densities as a function of temperature were in good agreement with experiments and similar simulations. The structural properties of neat HEHN and HAN-HEHN provided insights into their inherent, protic nature. Radial distribution functions (RDFs) identified key hydrogen bonding sites located at N-H···O and O-H···O within a first solvation shell of approximately 2 Å. Angular distribution functions further affirmed the relatively strong nature of the hydrogen bonds with nearly linear directionality. The increased hydroxylammonium cation (HA+) mole fraction shows the influence of competitively strong hydrogen bonds on the overall hydrogen bond network. Dominant spatial motifs via three-dimensional distribution functions along with nearly nanosecond-long hydrogen bond lifetimes highlight the local bonding environment that may precede proton transfer reactions.

Unraveling the initial steps of the ignition chemistry of the hypergolic ionic liquid 1-ethyl-3-methylimidazolium cyanoborohydride ([EMIM][CBH]) with nitric acid (HNO3) exploiting chirped pulse triggered droplet merging

The composition of the products and the mechanistic routes for the reaction of the hypergolic ionic liquid (HIL) 1-ethyl-3-methylimidazolium cyanoborohydride ([EMIM][CBH]) and nitric acid (HNO₃) at various concentrations from 10% to 70% were explored using a contactless single droplet merging within an ultrasonic levitation setup in an inert atmosphere of argon to reveal the initial steps that cause hypergolicity. The reactions were initiated through controlled droplet-merging manipulation triggered by a frequency chirp pulse amplitude modulation. Utilizing the high-speed optical and infrared cameras surrounding the levitation process chamber, intriguing visual images were unveiled: (i) extensive gas release and (ii) temperature rises of up to 435 K in the merged droplets. The gas development was validated qualitatively and quantitatively with Fourier Transform Infrared Spectroscopy (FTIR) indicating the major gas-phase products to be hydrogen cyanide (HCN) and nitrous oxide (N₂O). The merged droplet was also probed by pulsed Raman spectroscopy which deciphered features for key functional groups of the reaction products and intermediates (-BH, -BH₂, -BH₃, -NCO); reaction kinetics revealed that the reaction was initiated by the interaction of the [CBH]^- anion of the HIL with the oxidizer (HNO₃) through proton transfer. Computations indicate the formation of a van-der-Waals complex between the [CBH]^- anion and HNO₃ initially, followed by proton transfer from the acid to the anion and subsequent extensive isomerization; these rearrangements were found to be essential for the formation of HCN and N₂O. The exoergicity observed during the merging process provides a molar enthalpy change up to 10 kJ mol^-1 to the system, which could be sufficient for a significant fraction of the reactants of about 11% to overcome the reaction barriers in the individual steps of the computationally determined minimum energy pathways.

Influence of Adding Carbonaceous Fuels to Ionic Liquids on Propellant Properties

To make ionic liquids (ILs) accessible and economical, ethylene glycol was mixed in 1-ethyl-3-methylimidazolium-dicyanamide ([EMIm]DCA) to obtain droplets that could experimentally collide white fuming nitric acid. To investigate the ignition delay (ID) time theoretically in terms of hydrodynamics, alcohol fuels and kerosene were used as combustibles, while the intermiscibility between them and nitric acid (HNO₃) was calculated using the ternary phase-field method alongside finite element analysis. The specific impulses of blend fuels were calculated by a thermodynamic method and compared to ILs. When the droplet was ethylene glycol/[EMIm]DCA with a 2.1 mm diameter and a 1.69 m/s colliding velocity, the ID time was the shortest. Kerosene was not an applicable additive for [EMIm]DCA owing to its lower intermiscibility with ILs and HNO₃ than alcohol fuels; alcohol fuels, however, were appropriate. The concentration of ethylene glycol in the oxidizer pool increased faster than the concentration of propylene glycol, triggering more rapid hypergolic ignition in the first 50 ms. The protocols regarding the hypergolic ignition conditions were verified, i.e., the size of the droplet had to be minute when the colliding velocity was as fast as possible; this was carefully calculated using ethylene glycol. According to thermodynamic calculations, the addition of alcohol fuels can improve the specific impulse of fuels, with ethylene glycol performing the best. The feasibility of adding alcohol fuels to ILs was confirmed via experiments and thermodynamic computations, with the simulation results providing some guidance on selecting the experimental or engineering conditions or both.

Thermal and Catalytic Decomposition of 2-Hydroxyethylhydrazine and 2-Hydroxyethylhydrazinium Nitrate Ionic Liquid

To develop chemical kinetics models for the combustion of ionic liquid-based monopropellants, identification of the elementary steps in the thermal and catalytic decomposition of components such as 2-hydroxyethylhydrazinium nitrate (HEHN) is needed but is currently not well understood. The first decomposition step in protic ionic liquids such as HEHN is typically the proton transfer from the cation to the anion, resulting in the formation of 2-hydroxyethylhydrazine (HEH) and HNO₃. In the first part of this investigation, the high-temperature thermal decomposition of HEH is probed with flash pyrolysis (<1400 K) and vacuum ultraviolet (10.45 eV) photoionization time-of-flight mass spectrometry (VUV-PI-TOFMS). Next, the investigation into the thermal and catalytic decomposition of HEHN includes two mass spectrometric techniques: (1) tunable VUV-PI-TOFMS (7.4-15 eV) and (2) ambient ionization mass spectrometry utilizing both plasma and laser ionization techniques whereby HEHN is introduced onto a heated inert or iridium catalytic surface and the products are probed. The products can be identified by their masses, their ionization energies, and their collision-induced fragmentation patterns. Formation of product species indicates that catalytic surface recombination is an important reaction process in the decomposition mechanism of HEHN. The products and their possible elementary reaction mechanisms are discussed.

Molecular Dynamics Simulations, Reaction Pathway and Mechanism Dissection, and Kinetics Modeling of the Nitric Acid Oxidation of Dicyanamide and Dicyanoborohydride Anions

Direct dynamics simulations of HNO₃ with dicyanamide anion DCA^- (i.e., N(CN)₂^-) and dicyanoborohydride anion DCBH^- (i.e., BH₂(CN)₂^-) were performed at the B3LYP/6-31+G(d) level of theory in an attempt to elucidate the primary and secondary reactions in the two reaction systems. Guided by trajectory results, reaction coordinates and potential energy diagrams were mapped out for the oxidation of DCA^- and DCBH^- by one and two HNO₃ molecules, respectively, in the gas-phase and in the condensed-phase ionic liquids using the B3LYP/6-311++G(d,p) method. The oxidation of DCA^- by HNO₃ is initiated by proton transfer. The most important pathway leads to the formation of O₂N-NHC(O)NCN^-, and the latter reacts with a second HNO₃ to produce O₂N-NHC(O)NC(O)NH-NO₂^-(DNB^-). The oxidation of DCBH^- by HNO₃ may follow a similar mechanism as that of DCA^-, producing two analogue products: O₂N-NHC(O)BH₂CN^- and O₂N-NHC(O)BH₂C(O)NH-NO₂^-. Moreover, two new, unique reaction pathways were discovered for DCBH^- because of its boron-hydride group: (1) isomerization of DCBH^- to CNBH₂CN^- and CNBH₂NC^- and (2) H₂ elimination in which the proton in HNO₃ combines with a hydride-H in DCBH^-. The Rice-Ramsperger-Kassel-Marcus (RRKM) theory was utilized to calculate reaction kinetics and product branching ratios. The RRKM results indicate that the formation of DNB^- is exclusively important in the oxidation of DCA^-, whereas the same type of reaction is a minor channel in the oxidation of DCBH^-. In the latter case, H₂ elimination becomes dominating. The RRKM modeling also indicates that the oxidation rate constant of DCBH^- is higher than that of DCA^- by an order of magnitude. This rationalizes the enhanced preignition performance of DCBH^- over DCA^- with HNO₃.

Photoinduced Intermolecular Electron Transfer in Gas Phase Ion Pairs of the 1-Ethyl-3-methylimidazolium Cation and the Bis(trifluoromethylsulfonyl)imide Anion

In this study, the UV photodissociation of gas phase ion pairs of the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [emim]⁺[tf2n]^-, is shown to proceed primarily through radical intermediates. [emim]⁺[tf2n]^- ion pairs have been shown previously to undergo two-photon-dependent dissociation, but the mechanisms of this have not been probed in detail. By employing a two-laser pump probe spectroscopy and time-dependent density functional theory (TD-DFT) calculations, we have illustrated that one of the major UV photodissociation pathways in [emim]⁺[tf2n]^- ion pairs is an intermolecular electron transfer wherein the anion transfers an electron to the cation resulting in two neutral open-shelled products. These products were observable for at least 1.6 μs post photodissociation, the experimental limit, via detection of the [emim]⁺ cation. This data demonstrates that the likely photoproducts of [emim]⁺[tf2n]^- UV photodissociation are two neutral species that separate spatially, demonstrated through lack of observed relaxation pathways such as electron recombination. TD-DFT and frontier molecular orbital analysis calculations at the MN15/6-311++G(d,p) level are employed to aid in identifying excited state characteristics and support the interpretations of the experimental data. The energetic onset of the intermolecular electron transfer is consistent with previously observed [emim]⁺[tf2n]^- absorption spectra in the bulk and gas phases. The similarities between bulk and gas phase UV spectra imply that this electron-transfer pathway may be a major photodissociation channel in both phases.

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^-.

Model development and design criteria of hypergolic imidazolium ionic liquids from ignition delay time and viscosity viewpoints

The relationships between ID time, viscosity and molecular structure of hypergolic imidazolium ILs are discussed to specify ideal structural characteristics.

First-Principles Molecular Dynamics of Monomethylhydrazine and Nitrogen Dioxide

The exploration of chemical reactions preceding ignition is essential for the development of ideal hypergolic propellants. Unexpected reaction pathways of a hypergolic mixture composed of monomethylhydrazine and nitrogen dioxide are predicted through a cooperative combination of (i) spin-unrestricted ab initio molecular dynamics (AIMD) and (ii) wave packet dynamics of protons. Ensembles of AIMD trajectories reveal a sequence of reaction steps for proton transfer and rupture of the C-N bond. The details of proton transfer are explored by wave packet dynamics on the basis of ab initio potential energy surfaces from AIMD trajectories. The possibility of spontaneous ignition of this hypergolic mixture at room temperature is predicted as a quantized feature of proton-transfer dynamics.

Oxidation of a Levitated Droplet of 1-Allyl-3-methylimidazolium Dicyanamide by Nitrogen Dioxide

Understanding the reaction mechanisms of ionic liquids and their oxidizers is necessary to develop the next generation of hypergolic, ionic-liquid-based fuels. We studied reactions between a levitated droplet of 1-allyl-3-methylimidazolium dicyanamide ([AMIM][DCA]), with and without hydrogen-capped boron nanoparticles, and nitrogen dioxide (NO₂). The reactions were monitored with Fourier-transform infrared (FTIR) and Raman spectroscopy. The emergence of new structures in the FTIR and Raman spectra is consistent with the formation of functional groups including organic nitrites (RONO), nitroamines (R¹R²NNO₂), and carbonitrates (R¹R²C=NO₂^-). Possible reaction mechanisms based on these new functional groups are discussed. The reaction rates were deduced at various temperatures by heating the levitated droplets with a carbon dioxide laser. We thereby determined an overall activation energy of 38.5 ± 2.3 kJ mol^-1 for the oxidation of [AMIM][DCA] for the first time.

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

A range of ionic liquids (ILs) have been synthesized and modeled to better understand the role of the cation in the ignition of hypergolic ionic liquids. Vogelhuber et al. have shown by density functional theory methods that the addition of sodium cations to an ionic liquid promotes ignition with white fuming nitric acid (WFNA) by lowering energy barriers. To validate this prediction, solid sodium dicyanamide (Na⁺DCA^-) was added at various weight percents to 1-butyl-3-methylimidazolium dicyanamide (BMIM⁺DCA^-). The ignition delay was measured for each mixture with WFNA. Overall, it was found that the Na⁺DCA^- lowered the ignition delay by 11 ms at 7 wt %. The calculations done by Vogelhuber et al. appear to be consistent with this observation. The sodium cation may play a role by orienting the anion with the WFNA resulting in the favorable reaction energetics observed.

Synthesis and Properties of Triaminocyclopropenium Cation Based Ionic Liquids as Hypergolic Fluids

A novel family of hydrophobic triaminocyclopropenium cation based ionic liquids have been synthesized, and their structures and physicochemical properties characterized by NMR and IR spectroscopy, elemental analysis, differential scanning calorimetry, and hypergolic tests. The experimental results showed that all of these ionic liquids exhibited the expected hypergolic reactivity with the oxidizer white fuming nitric acid. Among them, the hypergolic ionic liquid based on the cyanoimidazolylborohydride anion showed excellent integrated properties, including high decomposition temperature (194 °C), high density (0.95 g cm^-3 ), moderate viscosity (44 MPa s), ultrafast ignition delay time (6 ms), and high specific impulse (301.9 s); this demonstrates its potential as an environmentally friendly alternative to toxic hydrazine derivatives.

Theoretical Investigation of the Reactivity of Sodium Dicyanamide with Nitric Acid

There is a need to replace current hydrazine fuels with safer propellants, and dicyanamide (DCA^-)-based systems have emerged as promising alternatives because they autoignite when mixed with some oxidizers. Previous studies of the hypergolic reaction mechanism have focused on the reaction between DCA^- and the oxidizer HNO₃; here, we compare the calculated pathway of DCA^- + HNO₃ with the reaction coordinate of the ion pair sodium dicyanamide with nitric acid, Na[DCA] + HNO₃. Enthalpies and free energies are calculated in the gas phase and in solution using a quantum mechanical continuum solvation model, SMD-GIL. The barriers to the Na[DCA] + HNO₃ reaction are dramatically lowered relative to those of the reaction with the bare anion, and an exothermic exit channel to produce NaNO₃ and the reactive intermediate HDCA appears. These results suggest that Na[DCA] may accelerate the ignition reaction.

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

Many ionic liquids containing the dicynamide anion (DCA^-, formula N(CN)₂^-) 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 B₁₀H₁₄ or B₉H₁₄^- 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.

Versatility and remarkable hypergolicity of exo-6, exo-9 imidazole-substituted nido-decaborane

The tunability of azoles combined with boranes provides a platform for controlling the physicochemical properties of new energetic materials.

Identification of multiple conformers of the ionic liquid [emim][tf2n] in the gas phase using IR/UV action spectroscopy

We have identified three families of conformers in gas phase ion pairs of [emim][tf2n] using IR/UV spectroscopy.

Solid-state Reaction of Azolium Hydrohalogen Salts with Silver Dicyanamide--Unexpected Formation of Cyanoguanidine-azoles, Reaction Mechanism and Their Hypergolic Properties

Cyanoguanidines as well as azoles are important bioactive groups, which play an important role in the medical application; meanwhile, the high nitrogen content makes them excellent backbones for energetic materials. A Novel and simple method that combined these two fragments into one molecular compound was developed through the transformation of dicyanamide ionic salts. In return, compounds 4–11 were synthesized, and fully characterized by IR, MS, NMR and elemental analysis. Meanwhile, the structures of compounds 4, 8 and 11 were confirmed by X-ray crystal diffraction. Detailed reaction mechanisms were studied through accurate calculations on the reaction energy profiles of the azolium cations and DCA anion, which revealed the essence of the transformation proceeding. Meanwhile, compound 8 exhibits excellent hypergolic property, which could be potentially novel molecular hypergolic fuel.

Binding of alkenes and ionic liquids to B-H-functionalized boron nanoparticles: creation of particles with controlled dispersibility and minimal surface oxidation

The interaction of B-H-functionalized boron nanoparticles with alkenes and nitrogen-rich ionic liquids (ILs) is investigated by a combination of X-ray photoelectron spectroscopy, FTIR spectroscopy, dynamic light scattering, thermogravimetric analysis, and helium ion microscopy. Surface B-H bonds are shown to react with terminal alkenes to produce alkyl-functionalized boron particles. The interaction of nitrogen-rich ILs with the particles appears, instead, to be dominated by boron-nitrogen bonding, even for an ILs with terminal alkene functionality. This chemistry provides a convenient approach to producing and capping boron nanoparticles with a protective organic layer, which is shown to protect the particles from oxidation during air exposure. By controlling the capping group, particles with high dispersibility in nonpolar or polar liquids can be produced. For the particles capped with ILs, the effect of particle loading on hypergolic ignition of the ILs is reported.

Insensitive energetic 5-nitroaminotetrazolate ionic liquids

Combustible 5-nitroaminotetrazolate-based energetic ionic liquids with high stability as well as environmentally friendly decomposition gases were prepared and fully characterized.

Polymeric ionic liquids: a strategy for preparation of novel polymeric materials

A novel polymeric ionic liquid (PIL), bearing high C-N and N-N content, potentially suitable for new safe energetic materials and catalyst supports was introduced. The PIL was prepared by way of radical co-polymerisation of 1-vinyl-3-p-nitrobenzylimidazolium bromide and 1-vinylimidazole at 80◦C using azobisisobutyronitrile (AIBN) as an initiator. The PIL thus produced was successfully transformed into NO

Thermophysical properties of energetic ionic liquids/nitric acid mixtures: insights from molecular dynamics simulations

Molecular dynamics (MD) simulations of mixtures of the room temperature ionic liquids (ILs) 1-butyl-4-methyl imidazolium [BMIM]/dicyanoamide [DCA] and [BMIM][NO3(-)] with HNO3 have been performed utilizing the polarizable, quantum chemistry based APPLE&P(®) potential. Experimentally it has been observed that [BMIM][DCA] exhibits hypergolic behavior when mixed with HNO3 while [BMIM][NO3(-)] does not. The structural, thermodynamic, and transport properties of the IL/HNO3 mixtures have been determined from equilibrium MD simulations over the entire composition range (pure IL to pure HNO3) based on bulk simulations. Additional (non-equilibrium) simulations of the composition profile for IL/HNO3 interfaces as a function of time have been utilized to estimate the composition dependent mutual diffusion coefficients for the mixtures. The latter have been employed in continuum-level simulations in order to examine the nature (composition and width) of the IL/HNO3 interfaces on the millisecond time scale.