Temperature-dependent formation of polychlorinated naphthalenes and dibenzofurans from chlorophenols

To investigate the gas-phase formation of polychlorinated naphthalenes (PCNs) and dibenzofurans (PCDFs) from chlorinated phenols in combustion exhaust gas, experiments were performed with each of the three chlorophenols in a laminar flow reactor over the range of 550-750 degrees C under oxidative conditions. Maximum PCN and PCDF yields were observed between 625 and 725 degrees C. The degree of chlorination of naphthalene and dibenzofuran products decreased as temperature increased, and on average, the naphthalene congeners were less chlorinated than the dibenzofuran congeners. Congener distributions are consistent with proposed PCN and PCDF formation pathways, both involving phenoxy radical coupling at unchlorinated ortho-carbon sites to form a dihydroxybiphenyl keto tautomer intermediate. Tautomerization of this intermediate and subsequent fusion via H2O loss results in PCDF formation, whereas CO elimination and subsequent fusion with hydrogen and/or chlorine loss leads to PCN formation. PCDF isomer distributions were found to be weakly dependent on temperature. PCN isomer distributions were found to be more temperature sensitive, however, with selectivity to particular isomers decreasing with increasing temperature. These results contribute to the understanding of PCN and PCDF formation in combustion and provide information on how to predict and minimize these emissions.