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 degreesC 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 (D Ds) are formed as (cross)condensation products. Overall rates of formation, through (reversible) ortho-C, ortho-C combination of two (chloro)phenoxy r adicals, are discussed on a thermokinetic basis, using new data relating to the O-H bond strengths in the target phenols. Chlorinated DDs are the pred ominant condensation products only when the ortho positions are fully chlor inated, a situation insignificant during thermal combustion of real waste. The question of mechanism is merely of scientific interest, but thermokinet ic evaluation together with an experimental check on the behaviour of 'tric losan' - a model o-hydroxy-diphenyl ether intermediate - provides the concl usion that both radical/radical combination and radical/molecule reaction m ay be involved in this case, with displacement of o-Cl by a (chloro)phenoxy radical. As chlorinated DFs and unconverted (chloro)phenols in real incine rators are subject to further reaction - (oxy) chlorination of the DFs, and condensation of the phenols to DDs - catalysed by ashes in the pollution c ontrol devices, the mixture of polychlorinated DFS and DDs commonly found i n incinerator emissions can be viewed as arising from (chloro)phenols as th e only important precursors.