Mechanistic Study Of Dioxins Formation From Polychlorinated Biphenyls And Chlorothiophenols As Precursors In The Gas-phase And Heterogeneous-phase

Dioxins are typical POPs that cause carcinogenic,teratogenic and mutagenic effects.In addition to being the most toxic substances,they can be transported and transformed across the globe due to the grasshopper effect.Dioxins include oxygenated polychlorinated dibenzo-p-dioxins(PCDDs),polychlorinated dibenzofurans(PCDFs),sulfur-substituted polychlorinated thianthracenes(PCTAs)and polychlorinated dibenzothiophenes(PCDTs).Most PCDD/DF/TA/DTs are unintentionally produced,and never intentionally synthesized,as a result of industrial processes like high temperature incineration and pyrolysis.The formation mechanism of PCDD/DF/TA/DTs mainly involved ab initio synthesis and precursor synthesis under gas-phase and heterogeneous catalytic conditions.Chlorophenols(CPs),chlorothiophenols(CTPs)and polychlorinated biphenyls(PCBs)are among the most important precursors of dioxin formation,which are produced simultaneously with dioxin in incinerators with strongly correlated concentrations.Under high temperature conditions,CP/CTPs and PCBs can form radicals by single molecule dissociation or by bimolecular abstraction reactions with radicals such as H,OH,Cl,or O(3P).These radicals can initiate dioxin formation through radical-radical coupling,radical-molecule coupling,or self-isomerization reactions.Among others,water molecules and transition metal catalysts,are commonly found in incinerators and can catalyze the formation of dioxins.In the presence and absence of catalysts,there are large differences in the optimal pathways and product of dioxin.While much experimental and theoretical research has been done on the mechanism of dioxin formation,the mechanism of dioxin formation with water molecule and transition metal is currently unclear.Therefore,understanding the formation mechanism of dioxins can cover data support for dioxin formation and prediction models,and provide a theoretical basis for dioxin pollution control.It is significant to study the thermodynamic and kinetic properties of the formation mechanism by theoretical calculations,due to the high molecular weight and toxicity of dioxins,combined with the difficulty of capturing the intermediates and transition states produced by experimental methods during their formation.In view of this,this thesis investigated the detailed mechanism of the formation of PCDD/DF/TA/DTs using CTPs and PCBs as precursors by employing a density functional theory approach,identified the energy optimal pathway and the major products,and explored the effect of water molecule transition metal catalysis on dioxin formation.The rate constants of key elementary reactions in the temperature range 600～1200 K were also calculated using the regular variational transition state CVT/SCT theory corrected for small curvature tunneling effects,and the main findings obtained are as follows:1.Mechanistic and kinetic studies on the generation of PCDD/DFs from PCB77 as precursors under high temperature gas-phase conditionsPolychlorinated biphenyls are a kind of substances with similar structure and toxicity to dioxins.It was found that PCBs could be used as precursors to generate PCDFs during high temperature incineration and pyrolysis,although their formation mechanism remains unclear.In this paper,the formation mechanism of PCDFs from 3,3’,4,4’-tetracochlorinated biphenyl(PCB77)at high temperature gas phase was studied by density functional theory at M06-2X/6311++G(3df,2p)//M06-2X/6-311+G(d,p)level.The small-curvature tunneling effect corrected canonical variational transition(CVT/SCT)method was used to calculate the rate constants of the key elementary reactions in the formation pathways of PCDFs,and the Arrhenius equation was fitted.In addition,the influence of water molecules on the reaction mechanism was explored,comparing the dominant pathways of the reactions in the presence and absence of water.In this study,it was found that PCB77 generates PCDFs through H abstraction,O2 addition reactions,ring-forming,O-O single bond breaking,coupling of unsaturated O and C atoms to furan rings,C-O single bond breaking of oxygen-containing ternary rings,H migration to form hydroxyl groups(rate-controlling step),and hydroxyl group desorption.The main PCDF products and their formation potentials in non-hydrated cases are ranked as follows:2,3,6,7-TeCDF>2,3,7,8-TeCDF>3,4,6,7-TeCDF,which agrees well with the experimental phenomena.It should be noted that the addition of water molecules can reduce the energy barrier,and change the dominant path and main products of the reaction.Moreover,the catalytic effect of water molecules on the six-element H migration is more obvious than that of the four-element H migration.The main PCDF products and their formation potentials in hydrated cases are ranked as follows:2,3,7,8-TeCDF>3,4,6,7-TeCDF>2,3,6,7-TeCDF.This study can provide a theoretical basis and data support for the investigation of the environmental fate of dioxin and its analogs,and even the legislation of dioxin control environment.2.Computational study of the effect of water molecules on the homogeneous phase formation of CTPRs in the full series reaction of CTPs with H/OH radicalsCTPs are the main precursors of PCTA/DTs,which are highly toxic and widely distributed in the environment.CTPs can react with H/OH radicals to form chlorobenzenethiol radicals(CTPRs)by extracting H atoms under pyrolytic or combustion conditions,where water molecules play an important role in this process.The coupling of CTPRs is the initial and critical step in the formation of PCTA/DTs.In this thesis,quantum chemical calculations were performed to study the complete series of reactions of 19 CTPs homologs reacting with H/OH radicals with water to form CTPRs,using MPWB1K/6-31 1+G(3df,2p)//MPWB1K/631+G(d,p).We figure out the similarities and differences of configuration parameters,potential barriers,reaction heats and reaction mechanisms under water and dry conditions.Our results show that the structural parameters,thermodynamic data,and the formation potential of CTPRs are largely affected by the ortho chloro substitution of CTPs,which will lead to a longer S-H bond;water molecules can weaken the thiol-hydrogen reaction in CTPs It can catalyze H/OH radical extraction of thiol-hydrogen atoms of CTPs to form CTPR,and has a stronger catalytic effect on OH extraction;water catalyzes H/OH extraction of CTPs.The effect of sulfhydrylhydrogen is stronger than that of H/OH extraction of hydroxyl-hydrogen of CPs.3.Mechanistic and kinetic studies on the formation of PCDT/TAs from 2chlorothiophenol on copper surfacesSurface reactions account for about 70%formation mechanism of dioxins,including de novo synthesis and surface-mediated catalysis of precursors.At temperatures of 300℃ and 400℃,Cu(0)accounts for 29%and 20%of the total copper content,respectively.Therefore,it is important to investigate the catalytic effect of Cu(0)on dichlorobenzenethiols.In this study,the adsorption mechanism of 2-chlorothiophenol at each high symmetry site on the Cu(111)surface was investigated using the VASP simulation package.In this paper,the adsorption energies of 2-CTP on the Cu(111)plane perpendicular and parallel to four high symmetry sites of top,bridge,hcp and fcc were calculated.Meanwhile,the SH dissociation,charge transfer and DOS of the most stable adsorption configuration were analyzed.Our results indicate that the adsorption energy of surface parallel adsorption is stronger than that of vertical adsorption,and there is no significant difference in the adsorption energy of the same adsorption state at different adsorption sites due to the symmetry of the Cu(111)surface.The SH group on the 2CTP molecule has strong ionic interaction with the surface of Cu(111),which is more prone to H dissociation than the H atom on the benzene ring,and is superior to the sulfhydryl-hydrogen dissociation in the gas phase reaction.Cu(111)surface has an obvious catalytic effect on thiolhydrogen dissociation in the 2-CTP molecule.