This paper attempts to expand mechanistic understanding of the fundamental chemistry involved in jet fuel thermal oxidative deposit formation. The proposed mechanisms are a synthesis of ideas and results from the fuel community at large; a few new experiments are also reported. In addition, it is suggested that the available literature is consistent with similar chemistry for deposit formation for both storage and thermal oxidative degradation for middle distillates in general.

The U.S. Air Force (USAF) has committed to use 1/1 volumetric blends of conventional jet fuels with Fischer-Tropsch (FT) derived fuels by 2016. Previous work by Balster et al. (Balster, L. M.; Zabarnick, S.; Striebich, R. C.; Shafer, L. M.; West, Z. Energy Fuels 2006, 20, 2564-2571) examined the relationship between thermal oxidative deposit and the concentration of various polar compounds present in 20 petroleum jet fuels. The thermal oxidative stability of FT blends, derived from four conventional jet fuels selected from the study of Balster et al. (Balster, L. M.; Zabarnick, S.; Striebich, R. C.; Shafer, L. M.; West, Z. Energy Fuels 2006, 20, 2564-2571), was examined with the Penn State University (PSU) flow reactor. Excellent linear correlations were found between fuel thermal oxidative deposit and indigenous fuel phenol, indole, and carbazole concentrations. This data is consistent with a mechanism previously proposed for the thermal oxidative degradation of both jet and diesel fuels (Beaver, B.; Gao, L.; Burgess-Clifford, C.; Sobkowiak, M. Energy Fuels 2005, 19, 1574-1579).

The oxidation of 1,2,5-trimethylpyrrole (TMP) in aqueous and organic solvents is studied by various techniques. Heating oxygenated chlorobenzene solutions of TMP results in autoxidation that is initiated via reaction of TMP with O2 and partly propagated via oxidation of TMP by a TMP-derived peroxyl radical. In radiolytic experiments, TMP is oxidized rapidly by Br2 •- (k = 2.3 × 109 L mol-1 s-1), I2 •- (k = 5.1 × 108 L mol-1 s-1), CCl3O2 • (k = 5 × 108 L mol-1 s-1), and N3 • radicals in aqueous solutions and by peroxyl radicals in organic solvents. One-electron oxidation forms the radical cation, which exhibits significant absorption in the UV (λmax ∼ 270 nm, ε ∼ 1300 L mol-1 cm-1) and weaker absorptions in the visible range. This species undergoes rapid dimerization (2k ∼ 5 × 108 L mol-1 s-1), and the dimer is very easily oxidized to a stable product absorbing around 460 nm. NMR analysis of the product formed in irradiated CH2Cl2 solutions is in accord with a dication of dimeric TMP. Other products are also formed under different conditions, probably resulting from addition of peroxyl radicals to the pyrrole ring. In cyclic voltammetry experiments at low scan rates, an irreversible peak at a potential of 0.72 V vs SCE is found for oxidation of TMP in acetonitrile solutions, and a stable product absorbing at 460 nm is formed. The formation of this product involves the transfer of more than one electron per TMP monomer. At very high scan rates, a reversible oxidation step is observed, from which a redox potential of 0.87 V vs SCE is derived for the couple TMP/TMP•+. Several mechanisms are suggested for the consumption of O2 by TMP in organic solvents, including electron transfer and σ-bonding via peroxyl radical addition.