Elucidating the Chemical Pathways Responsible for the Sooting Tendency of 1 and 2-phenylethanol

Yield Sooting Index (YSI) measurements have shown that oxygenated aromatic compounds (OAC) tend to have lower YSI than aromatic hydrocarbon (AHC) compounds. However, this trend is not always true as was observed for the structural isomers 1-phenylethanol (1PE, YSI = 142) and 2-phenylethanol (2PE, YSI = 207), where 2PE contains a YSI more representative of AHC than OAC. Flow reactor experiments and density functional theory (DFT) calculations were performed to examine how oxygen functionality present in 1PE and 2PE alters the reaction pathways leading to the observed difference in soot formation. The proximity of the oxygen functional group to the aromatic ring determines whether the oxygen remains attached to the primary reacting species (for 1PE) or was eliminated early in the combustion sequence (for 2PE). For these alcohols, preservation of the oxygen in the molecule led to further OAC, while loss of the oxygen ledsto AHC and benzyl radical. The direct pathways to AHC and benzyl radical resulted in the higher YSI observed for 2PE.