The characterization of polycyclic aromatic hydrocarbons produced in combustion and pyrolysis environments: Laboratory-generated products of model compounds

Laboratory and computational techniques have been developed to characterize polycyclic aromatic hydrocarbons (PAH), presumed soot precursors and potentially harmful by-products of a variety of pyrolysis and combustion processes. Newly synthesized reference standards and the application of high-pressure liquid chromatography (HPLC) with ultraviolet-visible (UV) absorption spectroscopy have led to the unequivocal identification, among combustion and pyrolysis products, of several new PAH, many of which belong to the two newly recognized PAH classes, ethynyl-PAH and cyclopenta-fused PAH (CP-PAH). Empirical rules have also been formulated for the UV spectra of ethynyl- and CP-PAH; these rules allow preliminary identification of candidate compounds in combustion products, prior to labor-intensive synthetic procedures necessary for identity confirmation.; Pyrolysis products have been analyzed in two sets of experiments: benzene droplet combustion and gas-phase catechol (ortho-dihydroxybenzene) pyrolysis. In the first, benzene droplets are ignited and then captured by a phase-discriminating sampling probe; gas-phase pyrolysis products transported into the liquid phase of the droplet are identified and quantified. In the second set of experiments, catechol is pyrolysed in a laminar-flow reactor, at 700–1000°C and 0.4–1 sec, producing a range of aromatic products; the 30 most abundant are quantified.; Compositional analysis of the pyrolysis products by HPLC reveals a wide variety of PAH which have never before been identified as products of these fuels. In general, most products appear to be the result of multiple ring-buildup steps. The data reported here for catechol products represent one of the most extensive quantifications of aromatic products from any fuel, and the only one for catechol.; Semiempirical quantum chemical computations have been performed in order to examine the potential energy surfaces and equilibrium distributions of several compounds. The observed preference for cyclization to CP-PAH over formation of ethynyl-PAH can be explained by the significantly lower energy barrier. Comparison of computed PAH equilibrium distributions to those found experimentally reveals close agreement only for the C₁₆H₁₀ isomers—corroborating previous evidence of a facile route for interconversion of internally and externally fused five-membered rings in this isomer group.