Products, rates, and mechanism of the gas-phase condensation of phenoxy radicals between 500-840 K

Phenols are demonstrated precursors of "dioxins" - polychlorinated dibenzo- p-dioxins (DDs) and dibenzofurans (DFs) in thermal processes, especially in cineration. Heterogeneous catalysis, depending on conditions, can play an i mportant role, but mere gas-phase combination of phenolic entities to ultim ately DD and/or DF is always possible. The present paper addresses the fund amental role of phenol itself. Phenol has long been known to give DF upon p yrolysis and in similar thermal reactions. In the Liquid phase under oxidat ive conditions it yields five condensation products (A-E); this clearly occ urs through the dimerization of two phenoxy (PhO) radicals, followed by eno lisation/rearomatisation. Our study shows that in the gas phase, at the low er T end, such dimers are also formed, but still with very Little DF. That DF, indeed, is almost the only condensation product at elevated temperature s is substantiated by thermochemical-kinetic analysis (favouring the pathwa y of ortho-C/ortho-C combination of two PhO radicals), as well as by result s obtained with two plausible intermediates, viz. 2,2'-dihydroxybiphenyl (A ) and 2-phenoxyphenol (C). Mechanisms for the requisite enolisation and deh ydration steps leading to DF are discussed.