Decomposition of 2-chlorophenol on surfaces of neat alumina and alumina supported iron (III) oxide catalysts

Chlorophenols constitute a group of the most important precursors for the formation of polychlorodibenzo-p-dioxin and polychlorinated dibenzofurans (PCDD/F, aka dioxins) in catalytic (250-400 °C)1-4 and non-catalytic (550-700 °C)5, 6 processes. Two catalytic processes exist7, comprising the formation of PCDD/F from precursor molecules, such as 2-chlorophenol, and from oxidation of complex polycyclic soot matrices in the so-called de novo synthesis. In the former mechansism, chlorophenols react with the terminal hydroxyl groups of active sites on select surfaces, followed by an electron transfer from the chemisorbed molecule of chlorophenolate to an active site8. Subsequent reduction of the catalyst active site, and formation of physisorbed chlorophenoxy and chlorohydroxyphenoxy radicals9, result in the formation of PCDD or PCDF via either Eley-Rideal (a reaction comprising an adsorbed radical and a gas-phase molecule) or Langmuir-Hinshelwood (a reaction involving two adsorbed radicals) mechanisms, respectively. Oxides of redox-active metals such as copper, nickel, iron, and zinc effectively catalyse the formation of PCDD/F from chlorophenols1. Iron oxide, alumina and silica occur in abundance in fly ash, with the latter two forming zeolites. These compounds catalyse generation of environmental pollutants under suitable conditions. In our earlier articles, we presented the decomposition of 2-chlorophenol (2-CPh) on the surface of silica-supported iron (III) oxide10, and studied adsorption of hydrogen chloride gas on the surface of CuZSM-5 zeolite11. In this contribution, we report the formation of volatile organic compounds (VOC, such as higher chlorinated phenols and chlorobenzenes) as well as PCDD/F from oxidation of 2-CPh on the surface of neat alumina and alumina-supported iron (III) oxide at a reaction temperature of 350 °C. In particular, this study concentrates on gaining an understanding of the role of iron oxide and alumina on formation of PCDD/F and VOC and has direct application to emissions of these pollutants from combustion systems