Thermal growth and decomposition of methylnaphthalenes

Diesel particulate matter (DPM) has been recognized as carcinogenic due to its respirable sizes and toxic compositions. It is essential to understand its formation mechanisms to effectively reduce DPM emissions. Studies have indicated that resonance stabilize radicals can result in more soot and PAC (polycyclic aromatic compounds) formation in combustion processes. As the largest single polycyclic aromatic constituent in petroleum fuels, 1- and 2-methylnaphthalenes can readily form resonance-stabilized radicals in combustion, and yet the reaction mechanisms are not well understood. In this paper, product formation mechanisms of these compounds are experimentally investigated in a flow reactor. The pyrolysis of 1- and 2-methylnaphthalene was studied from 800 to 1000 degrees C, and the oxidation of 2-methylnaphthalene was studied from 650to 950 degrees C. The PACs were quantified by GC/MS and HPLC, and soot was quantified gravimetrically. Naphthalene is the most abundant product and is formed from methyl loss by H displacement. Isomerization in pyrolytic conditions results in the second most abundant product. The methyl radicals recombine with the reactants to form ethylnaphthalenes and dimethylnaphthalenes. 2-Naphthaldehyde is formed from the oxidation of the naphthylmethyl radicals. 1-Methylnaphthalene is of slightly higher reactivity in comparison to its isomer. The experimental results indicate that the PAC formation from the pyrolysis and oxidation of the two methylnaphthalenes are likely to be governed by similar mechanisms. The lack of carbon dimerization products in significant quantities may be due to their further growth into soot.