- Title
- Thermal studies of chlorinated thiophenols
- Creator
- Dar, Tajwar
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2016
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- The formation of toxic pollutants formed during the thermal decomposition of chlorinated thiophenol and its related compounds (thiophenes and thioxane) have been investigated using combined combustion experiments and quantum chemical calculations. Computational calculations elucidated the kinetic and mechanistic aspects for the formation of two main pollutants, polychlorinated dibenzothiophene and polychlorinated thianthrene. In order to gain better understanding of thermal decomposition of chlorinated thiophenols, thermochemical parameters of a complete series of congeners of chlorothiophenols were derived. Standard enthalpies of formation of chlorothiophenols and thiophenoxy radicals were calculated using isodesmic work reactions. Thermodynamic scales of enthalpy and Gibbs energy enabled the highlighting of most stable isomers in each homologue group. Standard entropies and heat capacities were calculated with the treatment of the internal rotation of the S-H group as a hindered rotor. Values of solvation energies designate that the interaction of chlorinated congeners of thiophenols with water molecules decreases with the degree of chlorination; however, no apparent dependency can be deduced with regard to the pattern of chlorination. A thermodynamic cycle was constructed to estimate pKa values based on gas phase deprotonation free energies and calculated solvation energies for chlorothiophenol molecules and chlorinated thiophenolate anions. Calculated pKa values are in good agreement with the limited available experimental measurements. Density functional theory calculations were performed on 2-chlorothiophenol and 2,4,5-trichlorothiophenol to investigate the reaction pathways for the formation of polychlorinated dibenzothiophene and polychlorinated thianthrene and report relevant thermochemical and kinetic parameters. The results were compared with the analogous oxygenated system and it has been found that the presence of oxygen lowers the enthalpic profile of the systems, resulting in polychlorinated dibenzothiophene and polychlorinated thianthrene formation more favourable compared to their oxygenated counterparts (Polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofuran respectively). This is rationalized in terms of different influences of S-H and O-H moieties in chlorothiophenol and chlorinated phenol precursors. Furthermore, polychlorinated dibenzothiophene formation is found to be more likely than polychlorinated thianthrene formation based on thermochemical and kinetic parameters. In addition, degree and pattern of chlorination on chlorothiophenol precursor have subtle effects on polychlorinated dibenzothiophene and polychlorinated thianthrene formation mechanism. Experimental studies were also conducted under gas phase conditions over a temperature range of 300-800 °C. The thermal oxidation of 4-chlorothiophenol offered twelve toxic volatile organic compounds and seven congeners of polychlorinated dibenzothiophene and its oxygenated analogue polychlorinated dibenzofuran between 500-700 °C. Based on the results, the pathways for the formation of two important sulfur containing volatile organic compounds, chlorobenzothiophene and chloronaphthalene thiol were explained using quantum chemical calculations. It has also been concluded that chlorocyclopentadiene radical acts as key precursor for the formation of several dominant volatile organic compounds. For more comprehensive insights into the oxidative and pyrolytic thermal decomposition of cyclic organosulfur compounds, high level computational chemistry methods were used to deduce pathways for tetrahydrothiophene, 2-methyltetrahydrothiophene, 2,5-dimethyltetrahydrothiophene and 1,4-thioxane. Thiophene family has been found to decompose preferentially via C-S bond cleavage compared to C-C β bond scission on account of lower barrier heights. However, direct oxidation of thiophenes by ground state oxygen is exhibited to be only important in case of high temperature combustion processes. Similarly, 1,4-thioxane decomposition is favoured by β-CH₂-O scission.
- Subject
- dioxin; chlorinated; dibenzot hiophene; thianthrene; persistent organic pollutant; combustion; toxic pollutants; thermal decomposition
- Identifier
- http://hdl.handle.net/1959.13/1321950
- Identifier
- uon:24483
- Rights
- Copyright 2016 Tajwar Dar
- Language
- eng
- Full Text
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