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

Abstract

Hypergolic ionic liquids (ILs) are a feasible green alternative to the toxic and volatile hydrazine-based fuel in propellant systems. However, most reported hypergolic ILs still have environmental and biological issues due to their petrochemical product sources. Theophylline is a plant-derived methylxanthine-type alkaloid with relatively high energy density. This work synthesizes a novel family of bio-renewable theophylline-derived cations for energetic ionic compounds, with dicyanamide as the hypergolic anion. Five with shorter cationic alkyl groups (less than or equal to four carbon atoms) exhibit spontaneous combustion upon contact with 100 % HNO3. Meanwhile, they possess high densities above 1.30 g·cm−3, and density-specific impulses varied in the range of 333.2–348.0 s·g·cm−3 (calculated by Gaussian 09 program), better than those of the hydrazine-based propellants. Notably, 9-butenyl-1,3,5-trimethylxanthinium dicyanamide exhibits excellent integrated properties, including the shortest ignition delay time (49 ms), most comprehensive liquid operating temperature range (−70 to 215 °C), and a high density-specific impulse (342.2 s·g·cm−3). This study is expected to pave the way for developing bio-renewable hypergolic ILs from natural sources.