A detailed mechanism of the surface-mediated formation of PCDD/F from the oxidation of 2-chlorophenol on a CuO/silica surface

The formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) via a Cu(II)O-mediated reaction of 2-chlorophenol (2-MCP) has been studied in a packed bed reactor over a temperature range of 200-500 degreesC. Under oxidative conditions, the principle PCDD/F products were 1-monochlorodibenzo-p-dioxin (MCDD) > 4,6-dichlorodibenzofuran (DCDF) > dibenzo-p-dioxin (DD). EPR studies indicated the presence of a carbon-centered phenoxyl radical on the surface, which is attributed to chemisorption of 2-MCP at a copper oxide site followed by electron transfer to Cu(II) to form Cu(I) and a phenoxyl radical. The presence of a surface bound phenoxyl radical and the formation of MCDD, DCDF, and DD, which were also observed as the principle products of the gas-phase oxidation of 2-MCP, strongly suggest a surface-mediated mechanism involving many of the same radical and molecular species involved in the gas-phase formation of PCDD/F from 2-MCP. Reaction orders of 0.5-1.0 were observed for MCDD and DD formation, indicating an Eley-Rideal formation mechanism. Negative reaction orders were observed for DCDF formation, indicating a Langmuir-Hinshelwood formation mechanism. No highly chlorinated PCDFs were observed, suggesting a mechanism in which DCDF is desorbed from the surface before it can undergo additional chlorination. Highly chlorinated PCDDs were observed, which were consistent with a mechanism in which DD remained adsorbed to the surface and underwent additional chlorination. Chloro-o-quinone and chlorocatechol, which are precursors to semiquinone radicals, were also observed products. A detailed reaction mechanism accounting for all reported products is proposed.