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Formation of polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), and chlorinated phenols on CuCl2 from unsubstituted phenol and three monochlorophenols was studied in a flow reactor over a temperature range of 100-425 °C. Heated nitrogen gas streams containing 8.0% oxygen were used as carrier gas. The 0.00024 mol of unsubstituted phenol and 0.00039 mol of each monochlorophenol were passed through a 1 g and 1 cm SiO2 particle containing 0.5% (Cu by mass) CuCl2. Chlorination preferentially occurred on ortho-(2, 6) and para-(4) positions. Chlorination increased up to 200 °C, and thereafter decreased as temperature increased. Chlorination of phenols plays an important role in the formation of the more chlorinated PCDD/Fs. Chlorinated benzenes are formed possibly from both chlorination of benzene and chlorodehydroxylation of phenols. Chlorinated phenols with ortho chlorine formed PCDD products, and major PCDD products were produced via loss of one chlorine. For PCDF formation, at least one unchlorinated ortho carbon was required.

The homogeneous, gas-phase oxidative thermal degradation of 2-chlorophenol was studied in a 1 cm i.d., fused silica flow reactor at a concentration of 88 ppm, reaction time of 2.0 s, over a temperature range of 300 to 1000 °C. Observed products in order of yield were as follows: 4,6-dichlorodibenzofuran (4,6-DCDF) > dibenzo-p-dioxin (DD) > 1-monochlorodibenzo-p-dioxin (1-MCDD), 4-chlorodibenzofuran (4-MCDF), dibenzofuran (DF), naphthalene, chloronaphthalene, 2,4-dichlorophenol, 2,6-dichlorophenol, phenol, chlorobenzene, and benzene. In contrast to pyrolysis, 4,6-DCDF is the major product rather than DD, and 1-MCDD and naphthalene are formed at temperatures as low as 400 °C. Under oxidative conditions, ?OH and Cl? are the major carriers, which favors 4,6-DCDF formation over DD or 1-MCDD through abstraction of H? through diketo- and ether- intermediates. It is proposed that below 500 °C, unimolecular tautomerization/HCl elimination and CO elimination/isomerization reactions result in the formation of 1-MCDD and naphthalene, respectively.

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 °C. 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.

In this work, the role of phenoxy radicals in polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F) formation was investigated by studying the slow oxidation of 2-chlorophenol (2-CP) and 2-chloroanisole (2-CA) at a gas-phase concentration of 4 ppm (～2.1 × 104 μg/m3) over a temperature range of 400-800°C. Residence times were maintained at 2.0 ± 0.10 s. PCDD/F reaction products were dibenzofuran, dibenzo-p-dioxin, 4-chlorodibenzofuran, 1-chlorodibenzo-p-dioxin, 4,6-dichlorodibenzofuran, and 1,6-dichlorodibenzo-p-dioxin (1,6-DCDD). Major products observed in these experiments were 2,6-dichlorophenol, 3-phenyl-2-propenal, 1-indanone, 1,3-isobenzofurandione, and 3-phenyl-2-propenoyl chloride. The 2-CP and 2-CA experiments, along with the variable concentration 2-CA experiments, showed that the concentration of radicals present in the oxidation system has a significant effect on the PCDD/F product distribution and ultimately the PCDD/PCDF ratio. Also, the observation of dichlorinated phenoxy phenol and dichlorinated dihydroxybiphenyl, the proposed intermediate species in the radical-radical mechanism, suggests that radical-radical mechanism dominates gas-phase PCDD/F formation. This information will be helpful in constructing a detailed kinetic mechanism of PCDD/F formation/destruction in combustor postcombustion zone.

Correlations between products of incomplete combustion (PIC), e.g., chloroaromatic compounds, can be used to characterise the emissions from combustion processes, like municipal or hazardous waste incineration. A possible application of such relationships may be the on-line real-time monitoring of a characteristic surrogate, e.g., with Resonance-Enhanced Multiphoton Ionization-Time-of-Flight Mass Spectrometry (REMPI-TOFMS). In this paper, we report the relationships of homologues and individual congeners of chlorinated benzenes (PCBz), dibenzo-p-dioxins (PCDD), dibenzofurans (PCDF) and phenols (PCPh) to the International Toxicity Equivalent (I-TEQ) of the PCDD/F (I-TEQ value) in the flue gas and stack gas of a 22 MW hazardous waste incinerator (HWI). As the REMPI detection sensitivity is decreasing with the increase of the degree of chlorination, this study focuses on the lower chlorinated species of the compounds mentioned above. Lower chlorinated species, e.g., chlorobenzene (MCBz), 1,4-dichlorobenzene, 2,4,6-trichlorodibenzofuran or 2,4-dichlorophenol, were identified as I-TEQ surrogates in the flue gas. In contrast to the higher chlorinated phenols, the lower chlorinated phenols (degree of chlorination 4) were not reliable as surrogates in the stack gas. The identified surrogates are evaluated in terms of their detectability by REMPI-TOFMS laser mass spectrometry. The outcome is that MCBz is the best suited surrogate for (indirect) on-line measuring of the I-TEQ value in the flue gas by REMPI-TOFMS. The correlation coefficient r of the MCBz concentration to the I-TEQ in the flue gas was 0.85.

The temperature dependence of the gas-phase, rate-limited formation of dichlorodibenzo-p-dioxin (DCDD) and dichlorodibenzofuran (DCDF) isomers from 2,6-dichlorophenol and 3-chlorophenol, respectively, has been studied experimentally in an isothermal flow reactor over the range 300-900°C under pyrolytic, oxidative and catalytic conditions and computationally using semi-empirical molecular orbital methods. At high temperatures, distributions of sets of DCDD/F condensation products are consistent with the calculated thermodynamic distributions, indicating that the relative rates of formation are governed by differences in symmetry and steric hindrance present in the isomer product structures. At low temperatures, however, this is not the case. In the case of 1,6- and 1,9-DCDD formed from 2, 6-dichlorophenol via Smiles rearrangement, the 1,6 isomer is favored at low temperatures more than thermodynamically predicted. This result appears to be consistent with kinetic effects of either the expansion of the five-membered ring Smiles intermediate or a lower activation energy six-membered ring intermediate pathway that produces only the 1,6 isomer. For formation of 1,7-, 3,7- and 1,9-DCDF from 3-chlorophenol, the 1,7 isomer fraction increases at low temperatures whereas thermodynamics predicts a decrease. This result can be attributed to steric effects in alternative sandwich-type approach geometries of phenoxy radicals to form the o, o′-dihydroxybiphenyl (DOHB) intermediate via its keto-tautomers. Higher level molecular theory (ab initio) is needed to provide a more quantitative description of these kinetics.

Total homologue concentrations and select congener concentrations from amongst the mono- to tri-chlorinated dibenzodioxins (CDDs) and dibenzofurans (CDFs) are used to model both Total (mono- to octa-) CDD + CDF emissions and the toxicity equivalent (TEQ) of the 2,3,7,8-chlorine-substituted emissions. Analysis of emission data from two facilities indicates that use of total homologue concentrations shows limited, facility-specific correlations with Total CDDs/CDFs and TEQ. Concentrations of select mono- to tri-CDD/CDF congeners show promising correlation with CDD/CDF TEQ across facilities, suggesting that these compounds can act as TEQ indicators. (C) 2000 Elsevier Science Ltd.