Polycyclic aromatic hydrocarbons from the co-pyrolysis of catechol and propyne

In order to investigate C3 species as potential participants in aromatic-ring-growth reactions of polycyclic aromatic hydrocarbons (PAH) from solid fuels, we have performed pyrolysis experiments in an isothermal laminar-flow reactor (at temperatures, 700–1000 °C; residence time, 0.3 s) with the C3 hydrocarbon propyne, the model fuel catechol (representative of aromatic moieties in coal and biomass), and with propyne and catechol together (in a catechol-to-propyne molar ratio of 0.938). Analysis of the 1000-°C reaction products of propyne – by high-pressure liquid chromatography with diode-array ultraviolet-visible absorbance detection – has led to the identification of 58 two- to eight-ring PAH that have never before been reported as products of propyne pyrolysis or combustion. At all temperatures except 1000 °C, however, propyne-only pyrolysis produces very low yields of PAH – a consequence of low propyne (and propadiene) conversion and high production of benzene, which remains relatively invulnerable to ring-growth reactions. Catechol-only pyrolysis, in contrast, produces fairly high yields of PAH (up to 5.6% on a %-fed-carbon basis) – a result of catechol’s high production of effective C2 and C4 growth species as well as a rich radical pool that accelerates fuel conversion and aromatic-ring-growth reactions. When catechol and propyne are co-pyrolyzed, the radical pool provided by catechol causes a dramatic acceleration of propyne’s decomposition to methyl and acetylene – shifting the small-hydrocarbon product distribution from one that is rich in C3 species to one that is rich in C2 species. The 1- and 2-ring aromatic hydrocarbon product distribution also shifts from one dominated by unreactive benzene to one having more substituted species, more reactive to ring-growth. The net result of these shifts is a tremendous boost in PAH production: At all temperatures >800 °C, PAH yields from the co-pyrolysis experiments are 2.5–2.8 times higher than what would result from the two fuels being pyrolyzed independently.