Gas-Phase Chemistry of Chlorinated Phenols − Formation of Dibenzofurans and Dibenzodioxins in Slow Combustion

The effects of the introduction of chlorine substituents onto phenol on the rates of, and products from, their slow combustion at 500−550 °C are described. Competitive experiments showed that phenol, ortho-chlorophenol, 2,4,6-trichlorophenol and pentachlorophenol differ little in their overall rates of conversion − mainly into CO/CO2. Provided that an ortho-H moiety is available, chlorinated dibenzofurans (DFs) rather than dibenzodioxins (DDs) are formed as (cross)condensation products. Overall rates of formation, through (reversible) ortho-C, ortho-C combination of two (chloro)phenoxy radicals, are discussed on a thermokinetic basis, using new data relating to the O−H bond strengths in the target phenols. Chlorinated DDs are the predominant condensation products only when the ortho positions are fully chlorinated, a situation insignificant during thermal combustion of real waste. The question of mechanism is merely of scientific interest, but thermokinetic evaluation together with an experimental check on the behaviour of ′triclosan’ − a model o-hydroxy-diphenyl ether intermediate − provides the conclusion that both radical/radical combination and radical/molecule reaction may be involved in this case, with displacement of o-Cl by a (chloro)phenoxy radical. As chlorinated DFs and unconverted (chloro)phenols in real incinerators are subject to further reaction − (oxy)chlorination of the DFs, and condensation of the phenols to DDs − catalysed by ashes in the pollution control devices, the mixture of polychlorinated DFs and DDs commonly found in incinerator emissions can be viewed as arising from (chloro)phenols as the only important precursors.