Theoretical Study On The Formation Mechanism Of Sulfur-substituted Dioxins From Chlorothiophenol In Gas-phase And On The Surface Of Cluster Under High Temperature

Dioxins are typical chlorinated aromatic compounds in the environment,including 75 congeners of polychlorinated dibenzo-p-dioxins(PCDDs)and 135 congeners of polychlorinated dibenzofurans(PCDFs),which can migrate and transform on a global scale throngh grasshopper effect,2,3,7,8-TeCDD is the most toxic compound known to humans.The sulfur-substituted dioxins can be regarded as dioxin-like substances in which the oxygen atoms on the oxygen-containing tricyclic rings of PCDDs and PCDFs are replaced by sulfur atoms to varying degrees,including PCTAs,PCPTs and PCDTs.Generally,they are considered as unintentional byproducts in high temperature processes,such as metal smelting and waste incineration.In addition,the sulfur-substituted dioxins have been extensively detected in the environment such as soil,water sediments,stack gases,and biological tissues.There are limitations in using experimental methods to study the formation mechanism of sulfur-substituted dioxins due to its large molecular weight and high toxicity,furthermore,it is difficult to capture the intermediates and transition states produced during the formation of PCTA/DT/PTs,which makes theoretical calculations important in the study of PCTA/DT/PTs formation mechanism.The formation mechanism of sulfur-substituted dioxins mainly includes homogeneous reactions(e.g.radical/radical,molecular/radical and molecular/molecular reactions),surface reaction(e.g.denovo synthesis and heterogeneous catalytic synthesis of precursors)and chlorination and dechlorination of PCTA/DT/PTs.Based on density functional theory,the PCTA/DT/PTs formation mechanism from 2-CTP as precursor has been studied in detail at the level of MPWB1K/6-311+G(3df,2p)//MPWB1K/6-31+G(d,p).In order to explain the relevant calculation results,this thesis carried out a topological analysis from the molecular level,using the CVT/SCT theory corrected by the small curvature tunnel effect to calculate the rate constants of the key elementary reactions in the temperature range of 600-1200 K,the aquatic toxicity of products was predicted.The main research results drawn from the paper are as follows1.Mechanistic Study of PCTA/DTs Generated by Molecule/Radical Recombination of 2-CTP and 2-CTPRPCDTs were observed in higher yields than PCTAs in pyrolysis or combustion condition,in order to explain the phenomenon,the formation mechanism of PCDT/TAs generated by radical/molecule recombination of 2-CTPR and 2-CTP is studied in this paper,topological analysis such as HOMO-LUMO analysis,LOL analysis and ESP analysis are in good agreement with theoretical canculation results.Moreover,the rate constants are calculated and the toxicity assessment of produts is done.The results show that:The radical/molecular reaction mechanism for the formation of PCDT/TAs is easier to carry out than the formation of oxygenated dioxins by chlorophenols;PCDTs are formed more competitively than PCTAs,which to some extent provides a plausible explanation for the high PCDT-to-PCTA ratios in the real environment;DT,TA,1-MCDT and 1-MCTA act as the predominant products in the homogeneous gas-phase formation of PCTA/DTs from radical/molecule reactions of 2-CTPR and 2-CTP.2.Mechanistic Study of PCPT/DT/DFs from Radical/Radical Cross-Condensation of 2-CPR and 2-CTPRThere are few literatures on the formation mechanism of PCPTs hitherto,the homogeneous gas-phase formation mechanism of PCPT/DT/DFs from radical/radical cross-condensation of 2-CPR and 2-CTPR is conducted in this paper using density functional theory,the influence of H2O on the formation process of PCDT/DFs was discussed.Besides,the rate constant calculations were performed over a wide temperature range of 600～1200 K in 100-K temperature increments,using the CVT/SCT theory,the Arrhenius formulas are fitted.This study indicates that:In the PCPTs formation from cross-coupling of 2-CPR and 2-CTPR,routes starting with oxygen-carbon condensation are favored over those starting with sulfur-carbon condensation,the main products are PT and 4-MCPT;For the PCDF/DTs formation,the dominant product is 4,6-DCDF;Water molecule acts as a bridge ring during H migration,which affects the reaction barrier,but does not change the optimal reaction path;The toxicities of PCPTs are higher than those of PCTA/DDs for a given form of chlorine substitution;For the PCTA/DT/DDs formation,the formation potential ranks for the self-and cross-condensation of 2-CPR and 2-CTPR is self-condensation of 2-CTPRs>cross-condensation of 2-CPR and 2-CTPR>self-condensation of 2-CPRs;For the PCDT/DFs formation,the formation potential rank is and cross-coupling of 2-CPR and 2-CTPR>self-coupling of 2-CTPRs>self-coupling of 2-CPRs3.Theoretical Study of PCTA/DTs Formation Mechanism from 2-CTP over Silica ClustersSilica is the main component of fly ash,which can provide numerous adsorption sites for CP and CTP compared with trace metals,thus,silica can have an inpact on the formation of PCTA/DTs.In this paper,the PCTA/DTs formation mechanism from 2-CTP over clusters(SiO2)3 and(SiO2)3O2H4 has been studied in detail at the level of MPWB1K/6-311+G(3df,2p)//MPWB1K/6-31+G(d,p)using the high-accuracy quantum chemistry calculations.The results prove that:The(SiO2)3 cluster is more active toward the attack of 2-CTP to form highly stable 2-chlorothiophenolate than(SiO2)3O2H4 cluster,probably because O-unsaturated sites are more reactive toward the attack of 2-CTP than OH-terminated sites.The formation of sulfur-substituted dioxins on the surface of silica clusters by 2-CTP follows two reaction mechanisms:In L-H mechanism,C-C coupling of two 2-CTP molecules adsorbed to the surface of the silica cluster only leads to the formation of PCDTs;In E-R mechanism,the first 2-CTP molecular adsorbed on the surface of silica cluaster to form a stable chlorothiophenolate,then can react directly with the second 2-CTP molecule which is in the gas phase,PCDTs can be formed via C-C coupling modes,PCTAs can be generated by S-C coupling modes.The calculated value of reaction barrier and reaction heat indicates that silica has a certain catalytic effect on the formation of sulfur-substituted dioxins from 2-CTP as the precursor,though the catalytic effect is relatively mild.