Molecular Aluminum Additive for Burn Enhancement of Hydrocarbon Fuels

Additives to hydrocarbon fuels are commonly explored to change the combustion dynamics, chemical distribution, and/or product integrity. Here we employ a novel aluminum-based molecular additive, Al(I) tetrameric cluster [AlBrNEt₃]₄ (Et = C₂H₅), to a hydrocarbon fuel and evaluate the resultant single-droplet combustion properties. This Al₄ cluster offers a soluble alternative to nanoscale particulate additives that have recently been explored and may mitigate the observed problems of particle aggregation. Results show the [AlBrNEt₃]₄ additive to increase the burn rate constant of a toluene-diethyl ether fuel mixture by ∼20% in a room temperature oxygen environment with only 39 mM of active aluminum additive (0.16 wt % limited by additive solubility). In comparison, a roughly similar addition of nano-aluminum particulate shows no discernible difference in burn properties of the hydrocarbon fuel. High speed video shows the [AlBrNEt₃]₄ to induce microexplosive gas release events during the last ∼30% of the droplet combustion time. We attribute this to HBr gas release based on results of temperature-programmed reaction (TPR) experiments of the [AlBrNEt₃]₄ dosed with O₂ and D₂O. A possible mechanism of burn rate enhancement is presented that is consistent with microexplosion observations and TPR results.