Molecular Design and Characterization of High-Cetane Alkyl Diamondoid Fuels

Four alkyl diamondoids, 1-pentyladamantane (PA), 2-butyladamantane (BA), 2-propyladamantane (2-PrA), and 2-ethyladamantane (EA), were synthesized on a preparative scale, and key fuel properties, including density, net heat of combustion, low-temperature viscosity, and derived cetane number (DCN), were measured. The fuel molecules had densities up to 17% higher than conventional jet fuel and volumetric net heats of combustion comparable to or exceeding that of the synthetic missile fuel JP-10. Remarkably, the alkyl diamondoid fuels had DCNs in the range of 42–49, which should allow for their efficient combustion in diesel engines. PA, BA, and 2-PrA all had essentially the same DCN (∼49), while the short alkyl chain in EA lowered the value to 42.7. Bond dissociation energies for the formation of diradicals from 2-PrA and PA were calculated with density functional theory methods to gain a qualitative understanding of potential combustion mechanisms. Ring opening of the adamantane core during combustion is pr...