Low-migratory ionic ferrocene-based burning rate catalysts with high combustion catalytic efficiency

Abstract

Alkyl substituted ferrocenes can catalyze the burning of composited solid propellants efficiently. These non-polar and volatile ferrocenes, however, migrate to the surface of the propellants during prolonged storage, which would alter the designed burning parameters, and more seriously cause unexpected explosions. To tackle this problem, twelve ionic ferrocene derivatives bearing methylimidazolium and methyltriazolium substituents with nitrate and picrate anions as new ionic ferrocene-based burning rate catalysts were synthesized. The compounds were fully characterized by ¹H NMR, ¹³C NMR and elemental analysis. Ten of them were characterized by single-crystal X-ray diffraction. Compounds 2 and 3 crystallize in the monoclinic space group P2(1)/n and P2(1)/c, respectively. Compounds 4·H₂O, 6·1/2H₂O, 8–10 and 12 crystallize in the triclinic space group P. Compounds 7 and 11 crystallize in the orthorhombic space group Pna21 and Pna2(1), respectively. Cyclic voltammetry investigations revealed that all the compounds exhibit quasi-reversible redox systems. Migration studies confirmed that the migration of these ionic ferrocenes is much slower than that of 2,2-bis(ethylferrocenyl)propane. The migration trend of the compounds is dependent on molecular structures of the ionic ferrocenes and that shorter alkyl chain lengths in the heterocyclic rings lead to slower migration rates. Their high thermal stability was determined by TG and DSC analyses (peak temperatures >172 °C). The thermal degradation of ammonium perchlorate (AP) catalyzed by the new burning-rate catalysts was evaluated by DSC methods. In the presence of nitrate (1–6) in 4 wt% or picrate (7–12) in 5 wt%, the peak decomposition temperature of AP shifts left significantly while the released heat increases dramatically. The catalytic activity of an ionic compound with a triazole ring in its cation is higher than its corresponding analog with an imidazole ring and the catalytic activity of a nitrate is generally higher than its picrate counterpart.