Customized Green Energetic Tetra(Imidazole)Copper(II) Nitrate (Cu-Im) Complex/Ammonium Nitrate Co-crystal: A Novel Reactive Halogen-Free Oxidizer with Superior Stability and Decomposition Kinetics

Abstract

Ammonium nitrate (AN) is the most promising affordable green oxidizer for solid propellants; however, its endothermic phase transitions and endothermic decomposition restrict its applications. This work shaded the light on multifunctional energetic complex tetra(imidazole)copper (II) nitrate (Cu-Im) with combustion enthalpy of 15.57 KJ/g as a high energy dense material, phase stabilizer, and catalyst for AN-based propellant. The Cu-Im complex was synthesized via a green simple solvent-free melt-assisting technique. As-synthesized Cu-Im complex demonstrated a highly pure crystalline structure with thermal stability up to 200 °C. Cu-Im complex was integrated into AN via an evaporative crystallization technique. The phase transitions and the thermal behavior of the developed Cu-Im complex/AN cocrystal were investigated via DSC and TGA. The Cu-Im complex stabilized the phase transition of AN up to 88.48 °C via an evolved strong hydrogen bonding system within the Cu-Im complex/AN cocrystal. Moreover, the energetic Cu-Im complex offered a significant decrease in the endothermic phase transition peaks associated with IV↔II, and II↔I by 38.81%, and 25.28% respectively. Additionally, AN strong endothermic melting was decreased by 45.32%. The Cu-Im complex converts the AN endothermic thermal decomposition peak with + 1400 J/g into an intensive exothermic peak with the release of -2241 J/g. The Cu-Im complex exhibited a superior catalytic effect via the decrease in AN decomposition temperature by 48.85 °C. Cu-Im complex offered a remarkable reduction in apparent activation energy of AN decomposition by 56.24% and 48.12% via Kissinger’s model and integral iso-conversional nonlinear Vyazovkin method (VYA) respectively. The superior catalytic performance of the Cu-Im complex was attributed to the evolution of CuO nanoparticles during its decomposition. The electron-deficient Lewis acid copper ions have a large affinity to electron lone pair of nitrogen; this effect could support the conversion of NH3 to N2 and H2. This catalytic action could boost the decomposition enthalpy of AN. Consequently, the developed Cu-Im/AN cocrystal could be considered a potential green substitute for perchlorate-based oxidizers.