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

•Green hypergolic fuels were developed from bio-renewable theophylline.•They have good thermal stability, density specific impulse, and ignition activity.•Their molecular structures were investigated by single crystals.•The change mechanism of properties was studied based on quantum chemistry theory.•The theophylline-derived fuels can be alternatives to conventional toxic propellants.