C16H10 Ethynyl-substituted Polycyclic Aromatic Hydrocarbons from the Pyrolysis of Coal, Coal Volatiles, and Anthracene

Abstract

The acquisition of several specially synthesized reference standards of ethynyl-substituted three-ring polycyclic aromatic hydrocarbons (PAH) has enabled us to identify, for the first time, two C16H10 ethynyl-PAH among the pyrolysis products of coal and coal-derived fuels. The fuel product mixtures are analyzed by high-pressure liquid chromatography (HPLC) with diode-array ultraviolet (UV) absorbance detection, and the identifications are made by matching product components' HPLC elution times and UV absorption spectra with those of the reference standards. 2-ethynylanthracene and 2-ethynylphenanthrene are identified among the pyrolysis products of brown coal, pyrolyzed at 1000°C in a fluidized-bed reactor, and of bituminous coal volatiles, pyrolyzed at 1000°C in a tubular flow reactor. 2-ethynylanthracene is observed as a pyrolysis product, at 1300 to 1500 K, of anthracene, a three-ring model compound representative of the aromatic moieties in coal. The identification of these ethynyl-PAH provides important experimental evidence that acetylene addition to aryl radicals indeed takes place in these fuel reaction environments, as is customarily assumed in modelling of PAH growth during combustion. 2-ethynylanthracene and 2-ethynylphenanthrene are similar in that their proposed precursors, 2-vinylanthryl and 2-vinylphenanthryl, are configurationally precluded from undergoing cyclization reactions to form cyclopentafused PAH. Other isomers of ethynylanthracene and ethynylphenanthrene – whose precursors can undergo this cyclization – do not appear among our pyrolysis products, but the cyclization products, aceanthrylene and acephenanthrylene, do. Our experimental observations are thus consistent with theoretical calculations showing that the formation of cyclopenta-fused PAH by cyclization, when allowed. is energetically favored over the production of ethynyl-PAH.