Energetic Bimetallic MOF: A Promising Promoter for Ionic Liquid Hypergolic Ignition

A bimetallic MOF, CoNi(EIM)₂(DCA)₂ (1), containing an energetic 1-ethylimidazole (EIM) ligand and a hypergolic linker, dicyandiamide (DCA), was synthesized via a facile method. A fascinating three-dimensional reticular architecture was observed by single-crystal X-ray diffraction in this bimetallic MOF, whereas the corresponding monometallic compounds Co(EIM)₄(DCA)₂ (2) and Ni(EIM)₄(DCA)₂ (3) were in the mononuclear coordination mode. Uniformly distributed Co and Ni were observed in the bimetallic MOF crystals by SEM-EDS elemental mapping. Bimetallic MOF 1 was thermally stable and insensitive to mechanical stimuli and possessed an excellent energetic density (22.37 kJ·g^-1). Using 1 as a hypergolic promoter, the ignition delay time of 1-butyl-3-methylimidazolium dicyanamide (BMIM DCA) was reduced from 53 to 37 ms, better than that of 2 and 3 as promoters, due to the synergistic catalysis of the bimetal. Furthermore, the thermal decomposition mechanisms of BMIM DCA with 1, 2, and 3 were studied by differential scanning calorimetry (DSC). 1 had the best catalytic performance in BMIM DCA thermolysis with a decrease in the decomposition temperature from 314.5 to 308.0 °C and a decrease in the activation energy by 16.3%. All results shed light on the better catalytic effect of the bimetallic MOF on ionic liquid hypergolic ignition than monometallic coordination compounds.