Oxygen‐Balanced Energetic Ionic Liquid

Abstract

Energetic ionic liquids (EILs) are of great interest. They offer enhanced stability, higher densities, no vapor pressure, and, hence, no vapor toxicity. As a general principle, the stability of energetic ionic compounds can be greatly enhanced by making the cation the fuel and the anion the oxidizer. The formal positive charge increases the ionization potential of the fuel cation, and the formal negative charge decreases the electron affinity of the anion. In this manner, the fuel cation becomes more oxidizer-resistant, and the oxidizer anion is protected against premature reduction by the cation. For environmental reasons, it is also desirable to avoid halogen-containing ingredients, such as perchlorates. The previously known EILs consist of small oxidizing anions, such as ClO4 , NO3 , or N(NO2)2 , and large fuel cations containing quaternary nitrogen heterocycles with long, asymmetric, poorly packing side chains. The most serious drawback of these EILs is that they are underoxidized. The small anions do not carry sufficient oxygen for complete oxidation of the large fuel cations to carbon monoxide, resulting in poor performance. In rocket propulsion, a low molecular weight of the exhaust products is very important. 5] Furthermore, at high flame temperatures CO2 is dissociated almost completely to CO and O2 (Boudouard equilibrium). Therefore, it is often sufficient to oxidize the carbon content only to CO and not to CO2 to achieve nearmaximum performance. The aim of this study was the preparation of halogen-free, CO-balanced, EILs. In 1998, the concept of oxidizer-balanced EILs was proposed, and in 2002, its practicability was shown by the preparation of 1-ethyl-3-methylimidazolium tetranitratoborate, a compound that turned out to be indeed an ionic liquid with a freezing point of 25 8C. However, its energy content and thermal stability were marginal. Herein, we report on a significantly improved compound using the tetranitratoaluminate anion as a thermally more stable highoxygen carrier and the 1-ethyl-4,5-dimethyltetrazolium cation as a more energetic counterion (imidazole, DH o f = + 49.8 kJmol ; tetrazole, DH o f =+ 237.1 kJmol 1 ). These are the first CO-balanced EILs. Although an oxygenbalanced tetrazolium salt, 5-aminotetrazolium nitrate, was recently reported, its melting point of 173 8C does not classify it as an ionic liquid. Polynitratoaluminates were first studied in the 1960s in the USA and, subsequently, during the 1970s in the USSR. Several examples of alkali metal, NO2 , and ethylammonium salts of tetra-, penta-, and hexanitratoaluminate anions are known. The tetranitratoaluminate anion contains 12 oxygen atoms; of these, 10.5 are available to oxidize a fuel cation. Alkylated tetrazolium cations were used in this work because of their large positive heats of formation and their potential to form ionic liquids. Ionic salts of the tetranitratoaluminate anion can be prepared in essentially quantitative yields in one-pot reactions in nitromethane solution. The starting materials are the chloride salt of the cation, aluminum trichloride, and dinitrogen tetroxide. The synthesis of 1-ethyl4,5-dimethyltetrazolium tetranitratoaluminate (3) is shown in Scheme 1. The starting material 1-ethyl-4,5-dimethyltetrazo-