Formation of furans by gas-phase reactions of chlorophenols

Abstract

The gas-phase formation of dibenzofurans and other furan products from the three monochlorophenols, as well as phenol and four dichlorophenols, was studied in a 10-s flow reactor with and without oxygen. Results support a mechanism of dibenzofuran formation by coupling of chlorophenoxy radicals at ortho carbon atoms without chlorine substituents. Maximum yields of primary dibenzofuran products from chlorophenol pyrolysis were observed between 600 and 650°C. In the presence of oxygen, the same distribution of primary dibenzofuran product isomers was observed, but the formation of dibenzofurans occurred at lower temperatures. Secondary dibenzofuran products containing fewer chlorine substituents were produced at higher temperatures, corresponding with an increased conversion of chlorophenol to phenol. Statistical factors and steric effects associated with chlorine substitution at bay sites were found to control the distribution of dibenzofuran product isomers. The distributions of dibenzofuran isomers from gas-phase reactions of 3-chlorophenol and 3,4-dichlorophenol were found to be in close agreement with thermodynamic distributions calculated using semiempirical molecular orbital methods. Pathways are proposed for the formation of one monochlorodibenzofuranol produced in 3-chlorophenol pyrolysis and four benzonaphthofuran isomers produced in 2-chlorophenol pyrolysis. Further investigation is needed to confirm these proposed formation pathways. This work contributes to the development of a predictive understanding of furan product formation at high temperatures in incinerators.