Products, Rates, and Mechanism of the Gas-Phase Condensation of Phenoxy Radicals between 500-840 K

Abstract

Phenols are demonstrated precursors of “dioxins” ‐ polychlorinated dibenzo‐p‐dioxins (DDs) and dibenzofurans (DFs) ‐ in thermal processes, especially incineration. Heterogeneous catalysis, depending on conditions, can play an important role, but mere gas‐phase combination of phenolic entities to ultimately DD and/or DF is always possible. The present paper addresses the fundamental role of phenol itself. Phenol has long been known to give DF upon pyrolysis and in similar thermal reactions. In the liquid phase under oxidative conditions it yields five condensation products (A‐E); this clearly occurs through the dimerization of two phenoxy (PhO) radicals, followed by enolisation/rearomatisation. Our study shows that in the gas phase, at the lower T end, such dimers are also formed, but still with very little DF. That DF, indeed, is almost the only condensation product at elevated temperatures is substantiated by thermochemical‐kinetic analysis (favouring the pathway of ortho‐C/ortho‐C combination of two PhO radicals), as well as by results obtained with two plausible intermediates, viz. 2,2′‐dihydroxybiphenyl (A) and 2‐phenoxyphenol (C). Mechanisms for the requisite enolisation and dehydration steps leading to DF are discussed.

Formation of Bromochlorodibenzo-p-dioxins and Furans from the High-Temperature Pyrolysis of a 2-Chlorophenol/2-Bromophenol Mixture

“…With the exception of DF, formation of PCDD/Fs and PBDD/Fs occurs at low temperatures. The higher temperature formation of DF, which was also observed in the studies of pure 2-MCP and 2-MBP (29)(30), suggests it is formed by recombination of nonhalogenated phenoxyl radicals (46) formed by the displacement of chlorine or bromine from 2-MCP or 2-MBP. Scheme 1 depicts the possible reaction pathways for the brominated phenoxyl radical to form DD and 1-MBDD.…”

Oxidation kinetics of pentachlorophenol by manganese dioxide

“…Pathway II of Figure 6 is the coupling of PCP itself to release HCl and yield dimers as the major products. The coupling reactions most likely occur via C–C or C–O coupling at the ortho or para positions [38,39]. In this study, only the dimeric product via C–O, not C–C, was detected.…”

“…According to the observed surface reaction kinetics, identified products, and the published information on the oxidation of phenolic and substituted phenolic compounds mentioned previously [30,38–40], we propose a tentative reaction scheme for the PCP oxidation by MnO 2 (Fig. 6).…”

Toxic pollutants emitted from thermal decomposition of phthalimide compounds

“…Fission of the O-O bond gives phenoxy radical, which is a crucial intermediate in the formation of DD and DF in the oxidation of THPI and PI. The coupling of oxygen centred phenoxy radical to carbon centred phenoxy radical produces DD while the recombination between carbon centred phenoxy radical with itself or phenyl results in DF [36]. In comparison with the results of PI, the higher yields of DD and DF in the oxidation of THPI are consistent with the expected higher amounts of phenyl radicals and hence benzene in the case of THPI.…”