Synthesis of Bimetallic Metal-Organic Frameworks: A Strategy to Enhance Water Stability for Flame Retardants

Low-characteristic-signal propellants are critical for aviation and missile technology, but existing flame retardants suffer from poor water stability and undesirable combustion interference. A bimetallic MOF strategy is proposed to enhance water stability by immobilizing K⁺ through coordination interactions, while synergistically integrating flame retardancy with catalytic performance. Two bimetallic MOFs, [Me₂NH₂][KPb₂(TATAB)₂]·DMF (MOF 1) and [Me₂NH₂]₂[KNi₂(TATAB)₂(H₂O)₆]₂·3DMF (MOF 2) (H₃TATAB = 4,4,4-(1,3,5-triazine-2,4,6-triyl)tris(azanediyl)tribenzoic acid), were successfully synthesized and structurally characterized by single-crystal X-ray diffraction (SC-XRD). The composition and valence states of the bimetallic MOFs were determined by elemental analysis (EA) and X-ray photoelectron spectroscopy (XPS). Their excellent water stability, thermal stability, and nitrocellulose (NC) compatibility were confirmed by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), thermal analysis (DTA), and other characterization methods. Through a combustion observation device, the burning characteristics of each formulation were recorded, revealing the flame-retardant and catalytic mechanisms of the MOFs. Compared with KNO₃ or K-TATAB controls, the formulation containing MOF 1/2 produced narrower, shorter, and darker flames, with shortened ignition delay and combustion duration. Pb/Ni-TATABs further shortened combustion duration, confirming the catalytic activity of Pb²⁺/Ni²⁺. Thus, the distinctive architecture of these bimetallic MOFs offers superior multifunctional additives, integrating water stability, flame retardancy, and catalytic performance.