Combustion and pyrolysis reactions of alkylated polycyclic aromatic compounds: The decomposition of 13C methylarenes in relation to diesel engine emissions

The thermal decomposition of methylarenes labelled with 13C in the methyl group was investigated. This was conducted using both a direct injection diesel engine and a pyrolysis flow cell connected to a GC–MS. 2-[13C]methylnaphthalene and 9-[13C]methylphenanthrene were synthesized by means of the Corey– House coupling reaction and their identity and purity confirmed by mass spectrometry and NMR. GC–MS analysis of the aromatic fraction separated from the extract of the exhaust particulate collected from the engine operated with n-hexadecane doped with the labelled methylnaphthalene showed that the 13C was not redistributed among the methyl groups of higher PAH. However, with 9-[13C]methylphenanthrene in the fuel a significant amount was retained in the particulate, even though the principal in-cylinder reaction was dealkylation. Pyrolytic reactions of the 13C-labelled methyl arenes were studied in a micro-pyrolysis–GC–MS-apparatus and confirmed dealkylation as the predominant reaction. The detailed chemical mechanism of the pyrolysis was explained by a scheme involving two alternative radical transfer reactions.