Hypergolic Ionic Liquids Research Articles (Page 1)

Multicyclopropyl-functionalized hypergolic ionic liquids with enhanced energetic performance

Octahydrotriborate (B3H8-) based hypergolic ionic liquids as ultra-fast ignition propellant fuels with superior decomposition efficiency

Hypergolic ionic liquids (HILs) represent a critical pool of reactive ionic liquids which ignite spontaneously in absence of oxygen when mixed with an oxidizer such as white fuming nitric acid (WFNA, HNO3) or hydrogen peroxide (H2O2). These HILs have emerged as greener alternatives to the toxic hydrazine family of fuels for operations in space under anaerobic conditions. Here, we report on the unusual atmospheric ignition chemistry of the 1-ethyl-3-methylimidazolium cyanoborohydride ([EMIM][CBH])-H2O2 bipropellant while comparing with the parent hypergolic reaction by exploiting a chirped-pulse triggered droplet merging technique in an ultrasonic levitation apparatus under controlled environment. Significant enhancements of the ignition performance and noticeable differences of product distribution in the simulated atmosphere containing molecular oxygen are revealed. The critical mechanistic role of the surrounding oxygen plausibly involves the reaction with a suitable intermediate (iminomethyl)boronic acid, [(HO)2BCH=NH] formed in the initial reactions of the IL and H2O2 which eventually generates catalytic hydroperoxyl (⋅HO2) and hydroxyl (⋅OH) radicals - further promoting the ignition reaction. This systematic case study conceptually demonstrates that under atmospheric conditions, well-defined HIL - oxidizer combinations provide excellent ignition performance and effectively be used as universal fuel both in space and in the terrestrial atmosphere simplifying the multistage propulsion system.

Hypergolic ionic liquids (HILs) have emerged as promising self-igniting green space propellants in combination with an oxidizer, replacing toxic hydrazine family rocket fuels. Despite numerous new HILs being reported in the literature, there is no systematic study addressing the key reaction mechanism of such hypergolic ignition. Here, the first comprehensive molecular level understanding of this ignition reaction is revealed, exploring a 1-ethyl-3-methylimidazolium cyanoborohydride-hydrogen peroxide ([EMIM][CBH]-H2O2) green bipropellant pair by a novel chirped-pulse droplet-merging technique in a controlled environment. Mechanistically, the anion [CBH]- triggers the hypergolic ignition through facile exoergic oxidation of the boron center yielding boron dioxide (BO2) in a barrierless termolecular reaction with two molecules of H2O2, followed by an enhanced reactivity of the cation [EMIM]+, as evidenced from the excess yield of carbon dioxide (CO2) and evaluated decay rate constants of [EMIM]+ in situ during the droplet-merging reaction.

Tension ring-functionalized bicyclic ammonium ionic liquids as hypergolic fuels with superior energy density

Abstract Hydrazine and its derivatives have served as a conventional bipropellant fuel for several decades. However, their extremely acute toxicity and carcinogenicity, high volatility, and environmental impact have attracted significant attention. In order to synthesize green bipropellant fuels with high density, high specific impulse, and good thermal stability, three novel N-alkyl-1,2,4-triazole–borane complexes were successfully synthesized by reacting alkylated 1,2,4-triazole coordinated with sodium borohydride in the presence of ammonium sulfate. During the droplet test with white fuming nitric acid, there was a relatively short ignition delay time of 98 ms. Additionally, these hypergolic fuels possessed a high density exceeding 1.10 g cm–3, and the specific impulse is ranging from 187 to 199 s, and the highest decomposition temperature reaches 153.4 °C. These results demonstrate their great potential as hypergolic fuels or hypergolic ionic liquid additives in the field of hypergolic materials.

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.

The development of liquid energetic fuels with improved properties is important topic in space propulsion technologies. In this manuscript, a series of energetic ionic liquids incorporating a 1,2,5-oxadiazole ring and nitrate, dicyanamide or dinitramide anion was synthesized and their physicochemical properties were evaluated. The synthesized compounds were fully characterized and were found to have good thermal stabilities (up to 219 °C) and experimental densities (1.21-1.47 g cm-3). Advantageously, 1,2,5-oxadiazole-based ionic liquids have high combined nitrogen-oxygen contents (up to 64.4%), while their detonation velocities are on the level of known explosive TNT, and combustion performance exceeds those of benchmark 2-hydroxyethylhydrazinium nitrate. Considering the established hypergolicity with H2O2 in the presence of a catalyst, and insensitivity to impact, synthesized ionic liquids have strong application potential as energetic fuels for space technologies.

The hypergolic ignition behaviors of three ionic liquids [BMIm][DCA], [EMIm][DCA], and [EMIm][CDB] reacting with white fuming nitric acid (WFNA) were investigated using the drop test approach. Two high-speed cameras with/without a long-distance microscope and a time-resolved infrared camera were used to simultaneously record the hypergolic ignition process and the reaction region temperature evolution. Results showed two distinct hypergolic ignition modes. Specifically, for [BMIm][DCA] and [EMIm][DCA], some “vapor smoke” is generated after the fuel droplet contacts the WFNA pool, followed by sudden ejection of liquids induced by microexplosion. The measured temperature in the observation window is found to first increase, then decrease, and rise again. For [EMIm][CDB], no microexplosion is observed and the temperature monotonically increases before hypergolic ignition. It is inferred that microexplosion is caused by sufficient accumulation of gaseous intermediates or products underneath the liquid surface, and the monotonic or nonmonotonic increasing temperature is a characteristic of ignition modes. A conceptual model is proposed to illustrate the two ignition modes. In addition, the droplet impact velocity (U0) was varied from 1.1 to 2.1 m/s in the experiment. It is found that the vapor delay time (VDT), explosion delay time (EDT), and ignition delay time (IDT) are all reduced with U0 by at most 50%. For a given U0, the three time scales decrease in the order [BMIm][DCA], [EMIm][DCA], and [EMIm][CDB] (IDT as low as 14.4 ms).

Synthesis and properties of bio-renewable ionic salts derived from theophylline as green hypergolic fuels

The ignition delay time of the hypergolic ionic liquids, 1-ethyl-3-methylimidazolium dicyanamide [EMIM][C2N3] and 1,3-dimethyl imidazolium dicyandiamide [DMIM][C2N3], can be controlled to approximately 20 ms by adding 1-amino-4-methylpiperazine while keeping the vapor pressure below 1 torr at 298 K.

Hypergolic propellants are widely used in liquid rocket propulsion. Instantaneous control of combustion, lack of requirement of an ignition system, and storability for long durations are the major advantages of hypergolic propellants. The propellant community is on the lookout for replacements for the currently utilized toxic hydrazine and its derivatives. Energetic hypergolic ionic liquids are excellent candidates for such replacement. However, their high viscosity and surface tension render atomization difficult, and hence, increase the ignition delays in the combustion chamber and reduce combustion efficiency. The current study focuses on the geometrical aspects of injector design and other injection conditions that control the ignition process and combustion efficiency. These mainly depend on the extent of mixing and atomization. Shadowgraphy and patternator-based experiments were performed on doublet and triplet impingement injectors under variable injection conditions, such as impingement angle, jet diameters, and injection velocities. A blend of 50% unsymmetrical dimethylhydrazine in the energetic ionic liquid hydroxyethylhydrazinium nitrate (UHN50) shows promise as a candidate for a hydrazine replacement. A blend of 38% ethanol in glycerol (GE6238) was used as a surrogate for UHN50, whereas water was used as a surrogate for nitrogen tetroxide. The effect of variation of injection conditions was found to be contradictory for achieving atomization and mixing. An optimum criterion was obtained for both atomization and mixing.

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.

Hypergolic ionic liquids have come under increased study for having several desirable properties as a fuel source. One particular ionic liquid, 1-ethyl-3-methylimidazolium/cyanoborohydride (EMIM+/CBH-), and oxidant, nitric acid (HNO3), has been reported to be hypergolic experimentally, but its mechanism is not well-understood at a mechanistic level. In this computational study, the reaction is first probed with ab initio molecular dynamics simulations to confirm that anion-oxidant interactions likely are the first step in the mechanism. Second, the potential energy surface of the anion-oxidant system is studied with an in-depth search over possible isomerizations, and a network of possible intermediates are found. The critical point search is unsupervised and thus has the potential of identifying structures that deviate from chemical intuition. Molecular graphs are employed for analyzing 3000+ intermediates found, and nudged elastic band calculations are employed to identify transition states between them. Finally, the reactivity of the system is discussed through examination of minimal energy paths connecting the reactant to various common products from hypergolic ionic liquid oxidation. Eight products are reported for this system: NO, N2O, NO2, HNO, HONO, HNO2, HCN, and H2O. All reaction paths leading to these exothermic products have overall reaction barriers of 6-7 kcal/mol.

Experimental characterization of the electrospray propulsive performance for ionic liquid propellants [EMIm][DCA] and [BMIm][DCA]

Research on Hypergolic Ionic Liquid Propellants

Hydrazine and its derivatives have been used as standard propellants for spacecraft propulsion systems since the 1960s, despite being highly toxic and carcinogenic. The propellant synthesis community has constantly been looking for green alternatives for the same. Hypergolic ionic liquids (HILs) with several attractive properties, such as high energy content, high bulk density, low vapor pressure, and low toxicity, have been proposed as an alternative to hydrazine and its derivatives. In the present study, the theoretical performance of sixty-eight HILs was studied at a combustion chamber pressure of 3 MPa and a nozzle expansion ratio of 40. The specific impulse and density specific impulse of the HILs were calculated with white fuming nitric acid (WFNA), inhibited red fuming nitric acid (IRFNA), and nitrogen tetroxide (NTO) as oxidizers. The specific impulse of 2,2-dimethyltriazanium nitrate (HIL-1) was found to be 23 s higher than monomethylhydrazine (MMH), whereas its density-specific impulse was found to be 123 g-s/cm3 higher than MMH. The gains in the specific impulse and density specific impulse coupled with other desirable “green” properties for several HILs are expected to establish them as potential replacements for hydrazine and its derivatives.

Synthesis and characterization of hypergolic salts based on bis(1H-1,2,3-triazole-1-yl) dihydroborate anion

A new family of HILs, based on substituted 1H-1,2,4-triazol-4-ium, pyrrolidinium, ammonium and pyridinium cations and a cyanoborohydride anion, is introduced, with higher heats of formation, heats of combustion and specific impulse compared to dimethylhydrazine.

Microfluidic assisted 90% loading CL-20 spherical particles: Enhancing self-sustaining combustion performance