C16H10 ethynyl-substituted polycyclic aromatic hydrocarbons from the pyrolysis of coal, coal volatiles, and anthracene

The acquisition of several specially synthesized reference standards of eth ynyl-substituted three-ring polycyclic aromatic hydrocarbons (PAH) has enab led us to identify, for the first time, two C16H10 ethynyl-PAH among the py rolysis 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-ethynylphe nanthrene are identified among the pyrolysis products of brown coal, pyroly zed at 1000 degreesC in a fluidized-bed reactor, and of bituminous coal vol atiles, pyrolyzed at 1000 degreesC in a tubular flow reactor. 2-ethynylanth racene is observed as a pyrolysis product, at 1300 to 1500K, of anthracene, a three-ring model compound representative of the aromatic moieties in coa l. The identification of these ethynyl-PAH provides important experimental evidence that acetylene addition to aryl radicals indeed takes place in the se fuel reaction environments, as is customarily assumed in modelling of PA H growth during combustion. 2-ethynylanthracene and 2-ethynylphenanthrene a re similar in that their proposed precursors, 2-vinylanthryl and 2-vinylphe nanthryl, are configurationally precluded from undergoing cyclization react ions to form cyclopenta-fused PAH. Other isomers of ethynylanthracene and e thynylphenanthrene- whose precursors can undergo this cyclization-do not ap pear among our pyrolysis products, but the cyclization products, aceanthryl ene and acephenanthrylene, do. Our experimental observations are thus consi stent with theoretical calculations showing that the formation of cyclopent a-fused PAH by cyclization, when allowed, is energetically favored over the production of ethynyl-PAH.