Exploring homolytic aromatic substitution as a driver for fuel deposition with quantum chemistry and experiments

Liquid phase insoluble formation in fuels can cause performance and safety issues. To understand the formation of insolubles in fuels from first principles, a series of density functional theory (DFT) calculations were run to calculate the energetic barriers of the autoxidation and coupling reactions for several common fuel aromatics/heteroatoms. The six compounds chosen were phenol, toluene, naphthalene, pyrrole, quinoline, and indole. Using a combination of DFT calculations and gravimetric and petroxy experimental work, a novel homolytic aromatic substitution (HAS) coupling pathway was identified for each compound. While previous studies have treated deposition steps implicitly, our detailed calculations of HAS reactions and bulk fuel (RH) oxidation reaction barriers afforded the development of bespoke pseudo-detailed mechanisms for each aromatic compound with explicit reaction steps. These mechanisms were then used to predict trends in deposition behavior of the compounds tested in a simple n-dodecane surrogate. The novel HAS mechanism suggested for fuels was proposed to start with the reaction of an aromatic radical (Ar·) to an aromatic (ArH), which then formed a radical (ArHAr·) σ-intermediate. It was then found that hydroperoxides (ROOH) could re-aromatize the radical intermediate (ArHA·), forming a deposit dimer (ArAr). Although our sensitivity analysis revealed that alkyl fuel radical and fuek alkoxyl radical abstraction steps influenced the final mass of the deposit, the Ar. + ArH HAS coupling step was found to have the largest influence. Finally, an aromatic/heteratom model containing phenol and toluene was built, which showed that phenol suppressed deposition from toluene, and peaked in deposit mass at a phenol:toluene ratio of 25:75. Although our study was limited to Ar self-reactions, we hypothesize that bulk fuel – aromatic coupling could also be governed by HAS reactions, allowing researchers to move towards a more first-principles based deposition model.