Cyclopenta-fused polycyclic aromatic hydrocarbons from brown coal pyrolysis

Abstract

To examine certain aspects of coal tar composition, we have pyrolyzed acid-washed Yallourn brown coal under nitrogen at temperatures of 600 to 1000°C in a fluidized-bed reactor. Analysis of the product tar by reverse-phase high-performance liquid chromatography with diode-array ultraviolet-visible absorption detection reveals that the tars are composed of a large number of polycyclic aromatic compounds, many of which are polycyclic aromatic hydrocarbons (PAH) with peripherally fused cyclopenta rings (CP-PAH). Among PAH, CP-PAH are of particular interest because of their proneness to oxidation in the en vironment, their relatively high biological activity, and their postulated role in soot formation. Of the 10 CP-PAH identified in our tar samples, 4 of the most abundant are acenaphthylene (C₁₂H₈), acephenanthrylene and aceanthrylene (C₁₆H₁₀), and cyclopental [cd]pyrene (C₁₈H₁₀)—all of which have been detected previously in products of coal pyrolysis and/or combustion. The recent synthesis of several new CP-PAH reference standards, however, has enabled us to also identify, in the brown coal tars, six additional CP-PAH-cyclopent[hi]acephenanthrylene and cyclopenta[cd]fluoran thene (C₁₈H₁₀), dicyclopenta[cd, mn]pyrene and dicyclopental[cd, jk]pyrene (C₂₀H₁₀), benzo[ghi]cyclopenta[cd]perylene (C₂₄H₁₂), and cyclopenta[bc]coronene (C₂₆H₁₂)—none of which has ever before been identified in coal products. The mass fractions of individual CP-PAH span a range of four orders of magnitude—from 0.000062 for cyclopenta[bc]coronene to 0.265 for acenaphthylene in the 1000°C tar smaple. Accounting for approximately one-third of the mass of the tar produced at 1000°C, the CP-PAH yields show a monotonic increase with pyrolysis temperature—confirming that the CP-PAH are not primary products of coal devolatilization but instead result from secondary pyrolytic reactions in the gas phase. Possible reaction mechanisms are explored.