Investigation of structural effects of aromatic compounds on sooting tendency with mechanistic insight into ethylphenol isomers

Small aromatic molecules with oxygen-containing functional groups are a promising class of fuel additives, as they can be readily sourced from depolymerized lignin. These oxygenated aromatic compounds (OAC) show a lower sooting tendency than aromatic hydrocarbons, but OAC having alkyl groups, e.g.,ethylphenol, show a higher sooting tendency than other OAC, i.e., phenol and anisole, despite the oxygen moiety. To address this, the relationship between chemical structure and soot precursor formation was studied to understand observed differences in the sooting tendency of OAC and to gain insight into how alkyl or oxygenated substituents on the aromatic ring affect soot precursor formation. The weakest bond for 15 aromatic compounds was identified and cleavage of these bonds was generated either benzyl or phenoxy radicals. A linear relationship between standard enthalpy of formation of these radicals and the yield sooting index (YSI) was found, and thus enthalpy of formation could be applied as a metric to estimate YSI of various aromatic compounds. Higher enthalpy of formation of a radical indicated an increase in the radical reactivity and led to more soot precursor formation. 2-Ethylphenol produces more oxygenated products than 3-ethylphenol since the ortho position had increased resonance stabilization of radical intermediates, which leads to lower YSI.