Temperature dependence of DCDD/F isomer distributions from chlorophenol precursors

The temperature dependence of the gas-phase, rate-limited formation of dich lorodibenzo-p-dioxin (DCDD) and dichlorodibenzofuran (DCDF) isomers from 2, 6-dichlorophenol and 3-chlorophenol, respectively, has been studied experim entally in an isothermal flow reactor over the range 300-900 degreesC under pyrolytic, oxidative and catalytic conditions and computationally using se mi-empirical molecular orbital methods. At high temperatures, distributions of sets of DCDD/F condensation products are consistent with the calculated thermodynamic distributions, indicating that the relative rates of formati on are governed by differences in symmetry and steric hindrance present in the isomer product structures. At low temperatures, however, this is not th e case. In the case of 1,6- and 1,9-DCDD formed from 2, 6-dichlorophenol vi a Smiles rearrangement, the 1,6 isomer is favored at low temperatures more than thermodynamically predicted. This result appears to be consistent with kinetic effects of either the expansion of the five-membered ring Smiles i ntermediate or a lower activation energy six-membered ring intermediate pat hway that produces only the 1,6 isomer. For formation of 1,7-, 3,7- and 1,9 -DCDF from 3-chlorophenol, the 1,7 isomer fraction increases at low tempera tures whereas thermodynamics predicts a decrease. This result can be attrib uted to steric effects in alternative "sandwich-type" approach geometries o f phenoxy radicals to form the o, o'-dihydroxybiphenyl (DOHB) intermediate via its keto-tautomers. Higher level molecular theory (ab initio) is needed to provide a more quantitative description of these kinetics. (C) 2001 Els evier Science Ltd. All rights reserved.