- Title
- Studies on the oxidative and non-oxidative decomposition of alpha-cypermethrin and related molecules of non-chlorinated biphenyl and dibenzo-p-dioxin
- Creator
- Summoogum, Sindra Lutchmee
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2012
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- In this thesis, we have investigated the thermal decomposition of the pyrethroid, alpha-cypermethrin, and two cognate molecules of biphenyl and dibenzo-p-dioxin (DD), into toxic products including polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F, dioxins). Alpha-cypermethrin has widespread indoor and outdoor applications while biphenyl is mostly used as a fungicide and termiticide. Based on these results, a mechanism has been developed in which the key reaction steps occurring during pyrolysis and oxidation of alpha-cypermethrin are outlined. With the aim of gaining better and more comprehensive understanding of the formation of PCDD/F and destruction of DD, we examined a range of reaction conditions using a bench type tubular reactor facility, with particular focus on varying oxygen levels, temperatures and residence times. Stable reaction products were identified and quantitated using a range of analytical techniques and equipment, including micro gas chromatograph (µGC), triple quadrupole mass spectrometer (QQQMS), ion trap mass spectrometer (ITMS), and Fourier transform infrared (FTIR) spectrometer. For some key reaction steps, density functional theory (DFT) calculations were deployed to help unravel the reaction mechanism. Initially, we studied the pyrolysis of alpha-cypermethrin in the temperature range of 300 to 600 °C. We identified five main gaseous species including hydrogen chloride, hydrogen cyanide, methyl cyanide, acetaldehyde and crotonaldehyde. We identified two major pathways, one at low temperatures (two species form only at 450 °C) and the other at higher temperatures (formation of naphthalene and ethylbenzene, possibly by reaction with radicals, expulsion of CO and internal rearrangement, rather than by breaking of ether bridges). The unimolecular rearrangement of alpha-cypermethrin was observed at low temperatures, dominating over the alternative pathway which involves the recombination of the initial products. To gain further understanding of this pathway, we performed DFT computations, at the B3LYP/6-31G(d) level of theory, to understand the role of the CN group in O-CH(CN) bond fission in alpha-cypermethrin, in comparison to fission of the same bond in permethrin. There is a significant lowering of the bond energy which is due to considerable stabilisation of the CH(CN)C₆H₅ radical compared with the benzyl radical formed in the permethrin model case. Consequently, the activation energy for O—CHCN fission in cypermethrin is relatively similar to the activation energy for internal rearrangement and aromatisation. Hence, bond fission in alpha-cypermethrin should predominate at a much lower temperature than in permethrin itself, exactly as observed in the present experiments. The oxidative pyrolysis of alpha-cypermethrin generates substantially more PCDF than PCDD under all experimental conditions (temperatures, residence times and equivalence ratio), with the maximum emission factor of PCDD/F being observed at 550 °C, a residence time of 5 s and an equivalence ratio of 0.03. As indicated by the homologue distribution of PCDD/F, all the chlorinated congeners were detected with the exception of octachlorinated dibenzo-p-dioxins and octachlorinated furans (OCDD/F) in our measurements. The VOC analysis revealed the production of benzene, phenol, chlorotoluenes, trichlorophenols, tetrachlorophenols. The formation of PCDD/F from the gas-phase oxidation of alpha-cypermethrin may proceed through the coupling of chlorophenoxy radicals to benzene or chlorinated benzenes, and the self-condensation of chlorophenoxy radicals. This study combines the results of experimental measurements and theoretical computations to investigate the initial steps in the oxidation of dibenzo-p-dioxin (DD). The analyses of VOC, performed on a high resolution gas chromatograph-triple quadrupole mass spectrometer (HRGC-QQQMS), identified 2-methylbenzofuran and 2,3-dihydro-2-methylenebenzofuran as the initial products. This has been confirmed by injection of authentic standards and the application of collision induced dissociation that fragmented the isolated parent ions into specific product ions affording the identification of parent species. The oxidative decomposition of DD initiated at around 450 °C, with the evolution of VOC being maximum between 650 and 700 °C. At temperatures in excess of 750 °C, all VOCs were completely oxidised. The potential energy surface, based on the density functional theory of B3LYP, mapped the initial steps involved in the oxidation of DD, and yielded a detailed reaction scheme for the onset of oxidation of DD that results in the formation of 2-methylbenzofuran and 2,3-dihydro-2-methylenebenzofuran. Finally, we discuss the feasibility of the oxidation of biphenyl at low temperatures. Although it has been known for several years that combustion of polychlorinated biphenyl (PCB) in accidental fires of electrical equipment and in municipal waste can lead to significant emissions of dioxins, understanding of the mechanism of the oxidation process is quite limited. Oxidation of biphenyl (as a prototype model compound for PCB) in an alumina reactor at 490 ºC yields the initial products dibenzofuran and benzaldehyde which have been confirmed in GC/MS studies. It is postulated that O₂ (¹Δg) whose formation is catalysed by the reactor surfaces, initiates the reaction at this relatively low temperature. Quantum chemical computations of the reaction potential energy surfaces suggest low energy pathways to the observed initial products.
- Subject
- thermal decomposition; pyrethroid; biphenyl; dibenzo-p-dioxin; dioxins; GC-MS; quantum chemical computations
- Identifier
- http://hdl.handle.net/1959.13/932043
- Identifier
- uon:11243
- Rights
- Copyright 2012 Sindra Lutchmee Summoogum
- Language
- eng
- Full Text
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