Exploring Homolytic Aromatic Substitution as a Driver for Fuel Deposition with Quantum Chemistry and Experiments

•Insolubles formation in fuel facilitated by a newly identified homolytic aromatic substitution (HAS) mechanism.•Hydroperoxides play a key role in facilitating the HAS pathway.•Kinetic values for a simplified surrogate can predict differences in deposition tendency of different aromatics.•Excluding the HAS pathway breaks down predicted deposition trends.•Formation of hydroperoxides and homolytic coupling strongly influence final deposit mass. 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. Thesixcompounds chosen were phenol, toluene, naphthalene, pyrrole, quinoline, and indole. Using a combination of DFT calculations and gravimetric and petroxy experimental work,a novelhomolytic aromatic substitution (HAS)coupling pathwaywasidentified 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 mechanismswere then used to predicttrends indeposition behavior of the compounds tested in a simplen-dodecane surrogate.Thenovel HAS mechanism suggested for fuels was proposed to start with thereaction of an aromatic radical (Ar·) to an aromatic (ArH), which then formed a radical(ArHAr·) σ-intermediate. It wasthenfound 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.