Effect of dichlorophenol substitution pattern on furan and dioxin formation

Abstract

The gas-phase formation of tetrachlorodibenzofurans (T 4 CDFs) and dichlorodibenzo- p -dioxins (DCDDs), and trichlorodibenzofuranols (T 3 CDF-ols) from combinations of 2,3-, 2,4-, and 2,5-dichlorophenols (DCPs), each of which have one ortho site chlorinated, was studied to assess the effect of chlorine substitution pattern on reactivity. Benzene containing DCP, dissolved individually and in mixtures of equal amounts, was vaporized when combined with a heated gas stream (1% total organic vapor, 8% oxygen) and passed through a 10s flow reactor. Maximum gas phase formation of furan and dioxin occurs at temperatures between 500 and 700 °CC. Results confirm that chlorine substitution at ortho sites it is necessary for dioxin formation via carbon-oxygen coupling, whereas formation of both dibenzofurans and dibenzofuranols occurs via carbon-carbon coupling at unsubstituted ortho sites Effects of the second chlorine substituent were complex. In single DCP precursor experiments, 2,3-DCP strongly favored T 4 CDF formation, whereas the greatest DCDD yield was from 2,4-DCP. T 3 CDF-ol was produced only from 2,5-DCP, consistent with a reaction mechanism that rquires a chlorine meta substitutent adjacent to an unsubstituted ortho site in the phenol precursor. In DCP mixture experiments, T 4 CDF yields were greatest from 2,3- and 2,5-DCP, two 2,3-DCP, and 2,3- and 2,4-DCP, consistent with these combinations of phenoxy radicals having at least steric hindrance in carbon-carbon coupling, DCDD yields were greater from combinations of 2,3- and 2,4-DCP and 2,3- and 2,5-DCP than from combinations of two 2,3-DCP, two 2,4-DCP, and two 2,5-DCP. In all cases, homogeneous gas-phase formation of furan was favored over dioxin from DCP containing one ortho chlorine substituent. Moreover, secondary furan products, but not dioxin products, were formed by reactions involving phenol, produced from benzene, and monochlorophenols, produced from DCP. This work contributes to a better understanding of factors that control the distribution of furan and dioxin products formed in the gas phase of combustion exhaust streams.