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

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 hyper-golic anion. Five with shorter cationic alkyl groups (less than or equal to four carbon atoms) exhibit spon-taneous combustion upon contact with 100 % HNO3. Meanwhile, they possess high densities above 1.30 g center dot cm-3, and density-specific impulses varied in the range of 333.2-348.0 s center dot g center dot cm-3 (calculated by Gaussian 09 program), better than those of the hydrazine-based propellants. Notably, 9-butenyl-1,3,5-tri methylxanthinium dicyanamide exhibits excellent integrated properties, including the shortest ignition delay time (49 ms), most comprehensive liquid operating temperature range (-70 to 215 degrees C), and a high density-specific impulse (342.2 s center dot g center dot cm-3). This study is expected to pave the way for developing bio-renewable hypergolic ILs from natural sources.(c) 2023 Published by Elsevier B.V.