Non-equilibrium plasma treatment of C1 and C2 chlorinated hydrocarbons under non-oxidative conditions and characterisation of the resultant polymers

Research Doctorate - Doctor of Philosophy (PhD)This thesis encompasses an investigation of the efficacy of a dielectric barrier discharge for treatment of chlorinated hydrocarbons in a non-oxidative environment by converting them to environmentally benign and potentially valuable products, most notably polymers. The approach for this research is rather unconventional in the sense that unlike the majority of research in this field, we have not singularly targeted the destruction of the chemical, but have rather focussed on its conversion to the aforementioned value added products. The four chlorinated hydrocarbons investigated in this research are: 1,2-dichloroethane (C₂H₄Cl₂), chloroform (CHCl₃), dichloromethane (CH₂Cl₂) and carbon tetrachloride (CCl₄). The thesis also investigates the effect of methane on the reaction of these chlorinated hydrocarbons as well as on the structure of the resultant polymer. By employing a non-oxidative atmosphere we have been able to successfully obviate the formation of a number of unwanted and hazardous by-products such as COCl₂ and CO, a notorious hindrance widely noted in the literature for treatment of waste chlorinated hydrocarbons. Qualitative and quantitative analysis of gas phase products was afforded by utilising analytical equipment such as a (micro) GC, GC-MS, FT-IR and Ion Chromatograph (IC). For each chapter in this thesis, a comprehensive mass balance accounting for both gaseous and solid products is provided at a representative applied voltage. There exists a significant knowledge gap in understanding the structure of polymers or solids that can be obtained from non-equilibrium plasma treatment of the target hazardous chlorinated hydrocarbons; and hence a substantial part of this thesis is directed towards characterisation of these polymers. Analytical techniques such as NMR (1-D and 2-D), GPC and FT-IR were utilised for this purpose. The research pertaining to C₂H₄Cl₂ and CHCl₃ is predominantly experimental in nature. However in case of CH₂Cl₂ and CCl₄, the research also has a computational chemistry aspect to it, used to gain insight into the reaction mechanism. Quantum chemical calculations performed on the Gaussian09 software suite were utilised in conjunction with the experimental data to determine the most probable routes of decomposition and also to explain the formation of some of the major gas phase products. In general, the conversion levels of the investigated chlorinated hydrocarbons and methane were found to increase with increasing applied voltage. The maximum conversion levels attained for 1,2-dichloroethane, dichloromethane ,chloroform and carbon tetrachloride were 91.5 % (18.5 kV), 80.8 % (16 kV), 66.8 % (16 kV), and 37 % (18 kV) respectively. In case of C₂H₄Cl₂, CHCl₃ and CH₂Cl₂, it was found that addition of methane had a net inhibiting effect on the conversion levels of the chlorinated hydrocarbons. For example the EDC conversion level at 16 kV in absence of methane is 87.9 %, while decreasing to 78.6 % upon methane addition. However, in case of carbon tetrachloride a very interesting phenomenon was observed; the addition of methane promoted the conversion of CCl₄ to gaseous and polymeric products. An additional study was conducted to examine the effect of hydrogen on the reaction of CHCl₃ in the non-equilibrium plasma. The results indicated that hydrogen, like methane, had an inhibiting effect on CHCl₃ conversion and had no significant effect on the structure of the polymer. An important attribute of the polymers obtained from individual studies of the chlorinated hydrocarbons in this thesis is that they are predominantly non-crosslinked in nature and are comprised of –(CH₂-CHC1)- as a major component of their polymeric chain structure, which is also the repeating main chain group in poly (vinyl chloride). NMR analyses of the polymers obtained from the reaction of methane and the chlorinated hydrocarbons revealed that bulk of the polymer is similar to the polymers obtained from the reaction of the respective chlorinated hydrocarbons in absence of methane. Nonetheless, the most significant outcome of methane addition with respect to the resultant polymer was evidenced in case of C₂H₄Cl₂ where the addition of methane resulted in the virtual elimination of unsaturation in the polymer, which is basically a structural defect. The peaks which indicate presence of unsaturation in the polymer are evidenced between 5.6-5.9 ppm chemical shift in the ¹H NMR for the polymer obtained from reaction of EDC in absence of methane. Conversely, these peaks are absent in the ¹H NMR of the polymer obtained from C₂H₄Cl₂ + CH₄ reaction. This effect however, was not observed in case of any other chlorinated hydrocarbons investigated in this study. The computational study in case of CH₂Cl₂ was performed by employing the B3LYP/6-31 G(2df,p) level of theory for initial optimisation of geometries and ZPVE calculations. However, for increased accuracy of energy calculations we subjected the optimised structures to a G4MP2 level of theory. This computational study was mainly directed towards explaining the formation of two major species formed during CH₂Cl₂ decomposition, viz. CHCl₃ and C₂HCl₃. Transition state structures for this study were determined using the Synchronous Transit-Guided Quasi-Newton (STQN) method, more specifically the QST3 function. IRC calculations were also employed in this study