Green Bipropellants: Hydrogen‐Rich Ionic Liquids that Are Hypergolic with Hydrogen Peroxide

Abstract

Researchers working in the area of rocket propulsion strive for environmental friendliness, low toxicity, and overall operability, as well as a performance level comparable with current propellant combinations such as hydrazine and N 2 O 4 . Maintaining high performance while lowering hazards is extremely difficult.All rocket oxidizers are hazardous by their very nature, and so reduction of those hazards, even though the resulting materials might not be completely harmless, is at the heart of green initiatives in propulsion. The corrosivity of nitric acid is well known, and, although N 2 O 4 is much less corrosive, it combines high toxicity with high vapor pressure. A significant step to a lower-toxicity bipropulsion system would be the demonstration of hypergolicity (spontaneous ignition) between an ionic liquid (IL), which is a paragon of low vapor toxicity, and a safer oxidizer. Apart from cryogens, hydrogen peroxide seems to be especially promising because of its high performance, less-toxic vapor and corrosivity, and its environmentally benign decomposition products, [1] which make handling this oxidizer considerably less difficult than N 2 O 4 or nitric acid.A high fuel performance can be fostered by light metals with large combustion energies and relatively light products. Elements with considerable performance advantages and nontoxic products are aluminum and boron. The need for light combustion products through the production of hydrogen gas and water vapor is fulfilled by a high hydrogen content. Aluminum and boron are well known for their ability to serve as hydrogen carriers in neutral and ionic molecules. Defense research in the 1960s focused extensively on the development of hydrogen-containing fuels with boron, aluminum, and other metals, [2] but was mainly concerned with neutral compounds that have high vapor toxicity. Their rich anionic chemistry combined with the design flexibility of ILs presage novel materials that have the potential to overcome problems that caused these promising propellants to be abandoned.To date, no IL has been reported to be hypergolic with H 2 O 2 , and first-generation hypergolic ILs based on dicyanamide, nitrocyanamide, and azide anions lack high hydrogen content. [3] We tested ILs from each class with 90 % and 98 % H 2 O 2 , and all failed to ignite. This result is hardly surprising since fuels that are hypergolic with nitric acid vastly outnumber those that ignite with N 2 O 4 . For many years, hydrazine was the only fuel hypergolic with H 2 O 2 . [4] Since solutions of lithium aluminum hydrides and LiBH 4 in ethers have demonstrated H 2 O 2 hypergolicity, [5] the same behavior from ILs with metal hydride anions might be expected. However, the development of energetic roomtemperature ILs (RTILs) with metal hydride anions involves a number of technical challenges. Simple metal hydride anions are poor liquefying agents. Furthermore, heterocyclic, unsaturated salts that feature imidazolium, triazolium, pyridinium, and other common IL cations are reduced by BH 4 À ions,...

Solid Aluminum Borohydrides for Prospective Hydrogen Storage

“…Thus, Al(BH 4 ) 3 stabilized in a solid form as [Al(BH 4 ) 4 ] − anions can serve as a platform either for safe Al(BH 4 ) 3 storage, as it recently proposed by Popczun et al., or for the design of novel RHCs. This beneficial feature can be used not only for hydrogen storage, but also in technological processes such as the fabrication of the light‐emitting wide‐gap Al 1− x B x semiconductor technologies (e.g., Al 1− x B x PSi 3 ), as well as a prospective green bipropellant rocket fuel …”

Synthesis and Improved Properties of Hypergolic Boronium‐Based Ionic Liquids

“…[4] Considerable efforts have been made in the design and synthesis of HILs from the known hypergolic fuels such as hydrazine-based ILs, [12] amine-based ILs, [4] B-H bonds-rich ILs, [1,[14][15][16][17][18] and so forth. [4] Most of these ILs could be ignited upon contact with white fuming nitric acid (WFNA) and could be used as "green" fuels.…”

“…As such, much attention has recently been paid to ionic liquids (ILs) owing to the promising advantages of low vapor pressure, low toxicity, low phase transition temperature and high thermal stability. [3][4][5][6] Since the first hypergolic IL (HIL) family of dicyanamide salt was reported by Schneider et al, [7] the chemistry of HIL has been rapidly developed with several series, including the nitrocyanamideanion based ILs, [8] azide-functionalized liquid salts, [9][10][11] dialkyltriazanium-based ILs, [12] [Al(BH 4 …”

Ionic Liquid Propellants: Future Fuels for Space Propulsion

“…Apart from hazardous WFNA and N 2 O 4 , less hazardous H 2 O 2 can also be used as the oxidizer in a propellant formulation, although there are relatively few studies. Recently, a hydrogen‐rich ionic liquid ( 11 ), which can spontaneously ignite with 98 % H 2 O 2 , was reported (Scheme ) 17. In the design strategy of this molecule, the Al(BH 4 ) 4 − anion was chosen to combine with a phosphonium cation to form a hydrogen‐rich low‐melting salt.…”