Hydroxylammonium Nitrate: Synthesis, Cocrystals, and Properties

Abstract

Hydroxylammonium nitrate (HAN) is a high-performing, less-toxic alternative to hydrazine in liquid propellants. HAN also possesses many desirable qualities for use as an oxidizer in solid propellant formulations, where a chlorine-free replacement for ammonium perchlorate is urgently required; however, the extreme hygroscopicity of HAN precludes it from consideration for this role. Ionic cocrystallization, the combination of a salt and a neutral molecular solid within a crystal lattice, is demonstrated here to overcome this limitation for the first time. Solid HAN, synthesized via salt metathesis in ethanol and purified via sublimation, provides an ideal starting point for cocrystallization. Melt fusion at 50 °C readily generated the first examples of HAN cocrystals. The cocrystals feature dramatically reduced hygroscopicity, stabilizing HAN against ambient moisture for the first time. Three of nine cocrystals were formed between HAN and molecules with proposed or realized applications in propellants or explosives, and these possess calculated specific impulse and detonation velocity exceeding those of both HAN and the respective coformer molecules.