Formation of dibenzo-p-dioxins and dibenzofurans in oxidation of 2-chlorophenol over iron oxide/silica surface

S. Mosallanejad; B.Z. Dlugogorski; E.M. Kennedy; M. Stockenhuber; M. Altarawneh

Chlorophenols constitute one of the most important precursors for formation of polychlorodibenzo-pdioxins and polychlorinated dibenzofurans (PCDD/F, dioxins), both under pyrolytic and oxidative conditions. PCDD/F emerge from condensation of chlorophenol, chlorophenoxy and chlorohydrophenoxy species, in catalytic (250 to 400 °C) and non-catalytic (550 - 700 °C) processes. The catalytic process involves reaction of chlorophenols with the terminal hydroxy groups on the active surfaces, followed by an electron transfer from the chemisorbed molecule of chlorophenolate to an active site, subsequent reduction of the active site on the catalyst with concurrent formation of physisorbed chlorophenoxy and chlorohydroxyphenoxy radicals, to conclude with either Langmuir-Hinshelwood (i.e., a reaction involving two adsorbed radicals) or Eley-Rideal (i.e., a reaction comprising an adsorbed radical and a gas-phase molecule) type formation of PCDF and PCDD, respectively. Oxides of redox-active metals such as copper, nickel, iron, and zinc effectively catalyse the formation of PCDD/F from chlorophenols. In this contribution, for convenience, the term PCDD/F also includes monochlorinated and nonchlorinated dibenzo-p-dioxins and dibenzofurans. Although iron represents the most abundant transition metal found in the fly ash produced in combustion systems, the role of its oxides on the formation of PCDD/F remains unresolved. Some experimental studies have reported the destruction of PCDD/F on surfaces of iron oxides, whereas the recent study by Nganai et al. described significant yields of PCDD/F from chlorophenols, including dibenzo-p-dioxin (DD), 1-monochlorodibenzo-p-dioxin (1-MCDD), dibenzofuran (DF) and 4,6-dichlorodibenzofuran (4,6-DCDF), in addition to large amounts of highly chlorinated benzenes. Nganai et al.’s detection of non-chlorinated DF (which was undetected in other studies), no detection of 4-MCDF, 2-MCDD and dichlorinated DD, and lower ratios of CDD/CDF comprise the most intriguing differences between their results and the measurements of the previous investigators, in particular Born et al. and Ryu who studied, respectively, the oxidation of 2-CP on fly ash and CuCl2 surfaces. Hence, different reaction pathways might operate on surfaces of Cu and Fe catalysts. The present study was aimed at investigating the formation of volatile organic compounds (VOC, such as chlorophenols and chlorobenzenes) as well as PCDD/F from oxidation of 2-CP on silica-supported iron (III) oxide catalyst, under experimental conditions nearly similar to those of Nganai et al., to gain insight into the role of iron-rich fly ash on pollutant emissions from combustion systems. In particular, we intended to confirm Nganai et al.’s findings of significantly different effects of copper and iron surfaces on the formation of dibenzop-dioxins and dibenzofuran congeners from 2-CP. We used a more sensitive mass spectrometer than that featured in the work of Nganai et al., affording identification of additional reaction products. Our results indicate a similar distribution of PCDD/F products to those reported by Born et al. and Ryu; in other words, we are unable to confirm the measurements of Nganai et al. The reason for the observed discrepancy remains unclear. We also carried out the density functional theory calculations involving a cluster of iron oxide to elucidate the energetics of the formation of the initial phenoxy radicals on Fe2O3 surfaces. Although these calculations are preliminary, they indicate profound similarities in the roles of copper and iron surfaces in formation of PCDD/F from 2-CP, supporting the experimental findings reported in this contribution.

Formation of dibenzo-p-dioxins and dibenzofurans in oxidation of 2-chlorophenol over iron oxide/silica surface

Files and links (1)

Abstract

Details

Metrics