Integrating Hypergolic Metal-Organic Frameworks (HMOFs) into High-Energy Fuels Toward Dense Composite Solid Fuels

Hypergolic metal-organic frameworks (HMOFs), constructed by coordinating reactive metal centers with energetic ligands, are promising alternatives to carcinogenic, storage-demanding hydrazine fuels. However, the gravimetric energy densities (Eg) of reported HMOFs are generally below 30 kJ g-1. Here, a strategy is proposed to combine hypergolic HMOFs, [M(2-Mim)2(DCA)2]n (2-Mim = 2-methylimidazole, DCA = dicyanamide; M = Co 1, Ni 2), with high-energy yet nonhypergolic paraffin wax (PW), thereby achieving hypergolic materials with high energy density. The corresponding 20 wt % type II grains (type II = grain configuration containing an axial cavity filled with HMOF), 1-20%-PW-II and 2-20%-PW-II, achieved high energy densities of 42.29 and 42.30 kJ g-1, respectively, representing >72% increases over the neat HMOFs. Meanwhile, these composite materials retain reliable ignition and subsequent reignition capability. Compound 1 and its composite fuel both ignite faster than 2 and its corresponding composite. Calculations attribute this difference to the greater electronic responsiveness of the Co coordination node, stronger HNO3 binding, and enhanced charge transfer from 1 to HNO3. This strategy provides a reference for the design and preparation of hypergolic fuels with high energy density and reignition capability.