Theoretical Study Of The Formation Mechanism Of PCDD/Fs From Chlorophenol Precursors

Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) are notorious as the typical persistent organic pollutants for their biochemical and high toxic effects, which are included in the persistent organic pollutants (POPs). PCDD/Fs are by-products associated with human activities. By studying the formation mechanism and the factors, we can find the methods to reduce or avoid the formation and emission of PCDD/Fs.The formation mechanism of PCDD/Fs in municipal waste incineration process is very complicated and paid more and more atteniion to by environmental chemist nowadays. Although, there are still some controversy on the formation mechanism of PCDD/Fs, it is widely accepted that the formation of PCDD/Fs via two general pathways:de novo synthesis mechanisms and precursor synthesis mechanisms.In this dissertation, a series of theoretical studies have been carried out on the the formation mechanism of PCDD/Fs in municipal waste incineration process. we proposed that the chlorinated phenoxy radical, the chlorinated phenyl radical and the phenoxyl diradicals are all the important intermediates to form PCDD/Fs and designe the fomation mechanistic of PCDD/Fs through direct self- and cross-couplings of the newly proposed radicals, the calculated results improve our understanding for the formation mechanism of PCDD/Fs from chlorophenol precursors. We also made a detailed study in PCDD/Fs formation from chlorophenols through surface-mediated reactions. By comparing the reactions of chlorophenols via CuO and SiO2 surface-mediated with the the direct intermolecular condensation of two chlorophenol molecules, we can derive the catalytic mechanism and the catalytic effect of CuO and SiO2 in the generation process of PCDD/Fs.The main contents of the paper and the results from this dissertation are listed as follows:1. Chlorophenols are known as precursors of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), however, it is still not very clear for the formation mechanisms of dioxin. Based on the results of density functional theory calculations, our work presents a systematic theoretical study which aims at providing new insight into the homogeneous formation of PCDD/Fs from CPs. Using 2-chlorophenol as a model compound of chlorophenols, we find that apart from the chlorinated phenoxy radical, the chlorinated phenyl radical and the phenoxyl diradicals also have great potential for PCDD/F formation, which has scarcely been considered in previous literature. The thermodynamic and kinetic properties of reactions of 2-chlorophenol to produce various potential radicals forming PCDD/Fs are studied by performing density functional theory calculations and direct kinetics studies. We also predicated the fomation mechanistic of PCDD/Fs through direct self- and cross-couplings of the newly proposed radicals. The calculated results improve our understanding for the formation mechanism of PCDD/Fs from chlorophenol precursors, also provides some theoretical guidance for experimental research.2. It is widely accepted that homogeneous gas-phase reactions and heterogeneous surface-catalyzed processes are the two general pathways to form the PCDD/Fs from CPs precursors, and more than 70% of the dioxin is generated by heterogeneous surface-catalyzed processes. Transition-metal species, such as silica, copper compounds, especially copper oxides are the main component of combustion-generated fly ash and have a strong catalytic effect on the formation of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) in municipal waste incinerators. In this paper, we theoretically studied the reactions of 2,4,5-Trichlorophenol(2,4,5-TCP) over the clusters (SiO2、and CuO. The results show that the clusters (SiO2)2 is active towards the attack of 2,4,5-TCP to form highly stable 2,4,5-TCP, but the further dissociation of 2,4,5-TCP to form PCDD/Fs is calculated to be very difficulty, because the calculated energy barrier is very high. However, copper oxides have the largest catalytic effect for production of PCDD/Fs. The results presented here may have main impact to understand the mechanism of PCDD/Fs surface-catalyzed processes in waste combustion.