Robert Louw

Formation of polychlorinated benzenes during the catalytic combustion of chlorobenzene using a Pt/γ-Al2O3 catalyst

Abstract Pt/γ-Al2O3 has been used for the complete catalytic oxidation of chlorobenzene, a model for chlorinated aromatic compounds present in flue gases. Complete conversion of chlorobenzene is reached at ca. 440°C, but at this temperature substantial amounts of polychlorinated benzenes are formed. Only at 600°C complete selectivity to carbon dioxide is achieved. The addition of 1.75% of water to the carrier gas reduces formation of polychlorinated benzenes and improves conversion. The oxygen partial pressure has a remarkable effect on byproduct formation: the amounts of polychlorinated benzenes rise sharply with increasing oxygen concentration. Total conversion of chlorobenzene remains stable, hence the selectivity to carbon dioxide increases with decreasing oxygen pressure. When the γ-Al2O3 support alone is applied, complete conversion of chlorobenzene is reached only at ca. 550°C, without production of polychlorinated benzenes. The selectivity to carbon oxides however is poor. So platinum seems to be responsible for the formation of polychlorinated benzenes, which we propose is brought about by chlorination of adsorbed (chloro)benzene-species through platinum (oxy) chlorides.

Formation of Dibenzodioxins and Dibenzofurans in Homogenous Gas-Phase Reactions of Phenols

The operation of Municipal Waste Incinerators (MWI’s) results in the emission of organochlorine compounds including trace amounts of hazardous polychlorinated dibenzodioxins and dibenzofurans. Although the presence of PCDDs and PCDFs in stack gases and on fly ash is well established, little is known about the mechanisms and kinetics of formation and the phase(s) — e.g. pyrolysis, burning, or fly-ash catalysis — in which these compounds and/or their precursors are formed. Proper insight into these chemical features may learn how to improve a MWI installation so as to reduce, or eliminate, these emissions.

The role of the support and dispersion in the catalytic combustion of chlorobenzene on noble metal based catalysts

Polychlorinated benzenes (PhClx) are formed as byproducts in the combustion of chlorobenzene on Pt supported on γ-Al2O3, SiO2, SiO2–Al2O3, or ZrO2. The congener and isomer distribution of the PhClx differs for the various supports. The amounts of PhClx correlate with the dispersion of platinum. Thus, a Pt/γ-Al2O3 catalyst calcined at 500°C to yield very small Pt crystallites was more active in PhClx formation than Pt/γ-Al2O3 calcined at 800°C. In all cases T50% for chlorobenzene conversion is close to 300°C and appears to be independent of the crystallite size of the platinum. Replacing platinum by palladium led to lower rates of combustion and to more byproducts. These results lead us to propose that, in the presence of Cl and higher oxygen concentrations, small Pt crystallites are converted more easily into Pt(IV) species. These are less efficient in combustion, but can be more active in chlorination.

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

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.

Catalytic combustion of chlorobenzene on Pt/γ-Al2O3 in the presence of aliphatic hydrocarbons

During the catalytic combustion of chlorobenzene on a 2% Pt/γ-Al2O3 catalyst, considerable amounts of polychlorinated benzenes are formed as by-products. The co-feeding of heptane practically eliminates this unwanted side-reaction. Moreover, the conversion of chlorobenzene occurs at much lower temperatures (the T50%drops from 305 to 225°C). Simultaneously, the conversion of heptane is retarded. The addition of other hydrocarbons have a similar effect. Water and heat produced by the combustion of the added hydrocarbon cannot explain the increase in destruction rate of chlorobenzene. Removal of Cl from the surface by the alkane appears to be the ruling factor.

Radical/radical vs radical/molecule reactions in the formation of PCDD/Fs from (chloro)phenols in incinerators

Pathways from chlorinated phenols as precursors to PCDD/Fs are discussed with focus on the effect of (poly)chlorination on thermochemistry and rate in the displacement of chlorine from a chlorophenol molecule by a (chloro)phenoxy radical (reaction (A) as a key example). Through measurements on the respective methylethers (anisoles) the O-H bond of 2,4,6-TCP turns out to be 5 kcal/ mol, and that in PCP 4 kcal/mol, less strong than O-H in phenol itself. On this basis it is concluded that-in contrast with earlier proposals--displacements such as in reaction (A) are at least as slow as reaction (B) of phenoxy radical with chlorobenzene. PhO. + PhCl -->PhOPh + Cl. reaction B Compared with condensation of two (chloro)phenoxy radicals, such radical/molecule reactions are therefore an insignificant pathway to dioxins in incinerators.

Formation of PCDFs during chlorination and oxidation of chlorobenzene in chlorine/oxygen mixtures around 340 °C

Homogeneous gas-phase chlorination of chlorobenzene in the presence of oxygen at 330–350°C gives substantial amounts of PCDFs, up to 6% on the major product, the dichlorobenzenes. Chlorine atoms abstract H from chlorobenzene, the resulting chlorophenyl radicals reacting rapidly with Cl2, or with O2. The latter reaction leads to chlorophenoxyl radicals. Part of these appear to be further chlorinated before condensation occurs to mainly D2CDF, T3CDF and T4CDF. The simultaneous production of CO and (extra) HCl shows that (chlorine initiated) slow combustion also takes place, presumably by reaction of chlorinated phenylperoxyl- and/or phenoxyl radicals with oxygen.

Vapour phase chemistry of arenes. Part II. Thermolysis of chlorobenzene and reactions with aryl radicals and chlorine and hydrogen atoms at 500

Pyrolysis of chlorobenzene is interpreted as a radical (chain) process with the radicals ·C6H4Cl, ·Cl, and H· as carriers. Chlorobenzene, with added sources of R·(Cl·, H·, or Ph·) gives the biaryls chlorobiphenyl and bichlorophenyl, the ratio being highly dependent on R·. Thus, Cl· mainly abstracts H from PhCl to give o, m, and p-·C6H4Cl radicals (20 : 55 : 25) forming bichlorophenyls as the main product, whereas H· replaces and abstracts chlorine; Ph· leads to chlorobiphenyls as the principal products. Isomer distributions have been determined and are discussed. It is also shown that ·C6H4Cl radicals are isomerically stable under our conditions (e.g.,m-·C6H4Cl + PhH → 3-ClC6H4Ph only). With o-·C6H4Cl, naphthalene is formed in addition to 2-chlorobiphenyl. However, benzyne is rejected as an intermediate. ortho-Dichlorobenzene and biphenyl are found to react with H· at comparable rates.

Formation of dibenzodioxins and chlorobenzenes in fly ash catalyzed reactions of monochlorophenols

Abstract Partial autoxidation of monochlorophenols to carbon dioxide and carbon monoxide proceeds at 350 – 400 °C mediated by M unicipal W aste I ncinerator (MWI) fly ash. Moreover a wide range of (poly)chlorinated benzenes, mono benzofurans and dibenzo-p-dioxins is produced, with a considerable fraction of the original organic chlorine concentrated into these remaining aromatic rings.

Gas-Phase Chemistry of Chlorinated Phenols − Formation of Dibenzofurans and Dibenzodioxins in Slow Combustion

The effects of the introduction of chlorine substituents onto phenol on the rates of, and products from, their slow combustion at 500−550 °C are described. Competitive experiments showed that phenol, ortho-chlorophenol, 2,4,6-trichlorophenol and pentachlorophenol differ little in their overall rates of conversion − mainly into CO/CO2. Provided that an ortho-H moiety is available, chlorinated dibenzofurans (DFs) rather than dibenzodioxins (DDs) are formed as (cross)condensation products. Overall rates of formation, through (reversible) ortho-C, ortho-C combination of two (chloro)phenoxy radicals, are discussed on a thermokinetic basis, using new data relating to the O−H bond strengths in the target phenols. Chlorinated DDs are the predominant condensation products only when the ortho positions are fully chlorinated, a situation insignificant during thermal combustion of real waste. The question of mechanism is merely of scientific interest, but thermokinetic evaluation together with an experimental check on the behaviour of ′triclosan’ − a model o-hydroxy-diphenyl ether intermediate − provides the conclusion that both radical/radical combination and radical/molecule reaction may be involved in this case, with displacement of o-Cl by a (chloro)phenoxy radical. As chlorinated DFs and unconverted (chloro)phenols in real incinerators are subject to further reaction − (oxy)chlorination of the DFs, and condensation of the phenols to DDs − catalysed by ashes in the pollution control devices, the mixture of polychlorinated DFs and DDs commonly found in incinerator emissions can be viewed as arising from (chloro)phenols as the only important precursors.