Regioselective Synthesis of Methyl-Substituted Adamantanes for Promoting Oxidation Stability of High-Density Fuels

Alkyl-adamantane fuels have drawn tremendous attention for aerospace vehicles due to their high density and high oxidation stability. Here, we reported a regioselective route to synthesize methyl-substituted adamantanes including 1,3,5-trimethyl-adamantane and 1,3,5,7-tetramethyl-adamantane with single configuration via alkylation of 1,3-dimethyl-adamantane, with yields of ∼60 and ∼80%, respectively. The reaction kinetics of alkylation has been investigated, and the reaction rate constants and apparent activation energies were calculated based on the experimental kinetic data. The oxidation stability of methyl-substituted adamantane family were first evaluated via pressure differential scanning calorimetry and rapid small-scale oxidation test, and the results indicate that the oxidation stability is correlated to the carbon type available on the molecule in the order of 1,3,5,7-tetramethyl-adamantane > 1,3,5-trimethyl-adamantane > 1,3-dimethyl-adamantane > 1-methyl-adamantane. Notably, 1,3,5,7-tetramethyl-adamantane with four quaternary carbons and no tertiary carbon presents supreme oxidation stability than JP-10 and decalin. Finally, we used 1,3,5,7-tetramethyl-adamantane as an additive and achieved in improving the oxidation stability of JP-10. This work presents methyl-substituted adamantanes as good high-density and high-oxidation-stability fuel components and suggests that methyl groups substituted on tertiary carbon of adamantane has a beneficial effect on improving oxidation stability.