Mechanistic and Kinetic Studies on the Homogeneous Gas-Phase Formation of PCTA/DTs from 2,4-Dichlorothiophenol and 2,4,6-Trichlorothiophenol

DFT and AIMD insights into heterogeneous dissociation of 2-chlorothiophenol on CuO(111) surface: Impact of H2O and OH

Copper oxides are vital catalysts in facilitating the formation of polychlorinated thianthrenes/dibenzothiophenes (PCTA/DTs) through heterogeneous reactions in high-temperature industrial processes. Chlorothiophenols (CTPs) are the most crucial precursors for PCTA/DT formation. The initial step in this process is the metal-catalyzed production of chlorothiophenoxy radicals (CTPRs) from CTPs via dissociation reactions. This work combines density functional theory (DFT) calculations with ab initio molecular dynamics (AIMD) simulations to explore the formation mechanism of the adsorbed 2-CTPR from 2-CTP, with the assistance of CuO(111). Our study demonstrates that flat adsorption configurations of 2-CTP on the CuO(111) surface are more stable than vertical configurations. The CuO(111) surface acts as a strong catalyst, facilitating the dissociation of 2-CTP into the adsorbed 2-CTPR. Surface oxygen vacancies enhance the adsorption of 2-CTP on the CuO(111) surface, while moderately suppressing the dissociation of 2-CTP. More importantly, water molecules and surface hydroxyl groups actively promote the dissociation of 2-CTP. Specifically, water directly participates in the reaction through "water bridge", enabling a barrier-free process. This research provides molecular-level insights into the heterogeneous generation of dioxins with the catalysis of metal oxides in fly ash from static and dynamic aspects, providing novel approaches for reducing dioxin emissions and establishing dioxin control strategies.

Occurrence, physicochemical properties and environmental behavior of polychlorinated dibenzothiophenes: A comprehensive review and future perspectives

Polychlorinated dibenzothiophenes (PCDTs) are a form of emerging pollutant that has attracted great attention due to their structural resemblance to dioxins, which cast detrimental influence on the ecosystem and human health. This review shows the current status of research on PCDTs, focusing on their environmental occurrence, physicochemical properties, environmental behavior, and toxicity. Studies have suggested that the steps leading to the formation of PCDTs resemble those generating polychlorinated dibenzo-p-dioxin/dibenzofurans (PCDD/Fs), indicating their probable origin from the same sources. Furthermore, they may undergo a dechlorination process as a result of their photodegradation in the environment and metabolic reaction occurring within organisms, which could result in the conversion of these substances into additional pollutants like dibenzothiophene. PCDTs exist widely in the environmental media and have high logKOW values (>4.0), indicating their tendency to bioaccumulate. Moreover, the prediction results of EPI (Estimation Program Interface) Suite demonstrated a strong accumulation capacity for tetra-CDTs in fish compared to other chlorinated PCDTs. The biotransformation half-life of PCDTs would prolong with an increasing number of substituted Cl atoms in fish. A limited number of studies have also suggested that PCDTs can cause damage to the liver and immune system in living organisms, and the toxicity of PCDTs depends on the number and position of substituted Cl atoms. Future studies should be conducted on processes causing PCDT toxicity as well as their behavior and fate in actual environments.

Periodic DFT calculations for the heterogeneous formation of 2-chlorothiophenoxy radical from 2-chlorothiophenol on Cu(111) surface in fly ash

Copper plays a crucial role in the heterogenous dissociation of chlorothiophenols (CTPs) to form chlorothiophenoxy radicals (CTPRs), which is the initial and critical step in the formation of polychlorinated thianthrenes/dibenzothiophenes (PCTA/DTs). Here, first-principles calculations were performed to investigate the activity of Cu(111) surface towards the formation of adsorbed 2-CTPR from 2-CTP. The interaction between 2-CTP and Cu(111) surface was explored to find stable adsorption configurations. Besides, the decomposition routes of 2-CTP on the Cu(111) surface were further explored. Moreover, the effects of water on the formation of absorbed 2-CTPR on the Cu(111) surface were examined. Our results demonstrate that the flat adsorption of 2-CTP on the surface with adsorption energy in the range of -33.21 kcal/mol to -28.37 kcal/mol is more stable than the vertical adsorption with adsorption energy ranging from -23.53 kcal/mol to -13.38 kcal/mol. The Cu(111) surface catalyzes the conversion of 2-CTP into the adsorbed 2-CTPR with a modest energy barrier of 9.46 kcal/mol. Furthermore, water molecules exhibit stronger catalytic activity in this process with a decreased energy barrier of 5.87 kcal/mol through "water bridge" and hydrogen bonding. Specifically, the water accepts the hydrogen atom from 2-CTP and donates another hydrogen to the surface via "water bridge". This research provides a molecular-level understanding of the heterogeneous formation of PCTA/DTs by fly ash, suggesting novel approaches for control strategy and legislation of dioxin analogues.

Effect of Water Molecule on the Complete Series Reactions of Chlorothiobenzenes with H/·OH: A Theoretical Study

The chlorothiobenzenes (CTBs) are the principal precursors for the formation of polychlorinated thianthrene/dibenzothiophenes (PCTA/DTs), which have high toxicity and wide distribution in the environment. Under the pyrolysis or combustion conditions, CTBs can react with H/·OH radicals to form the chlorothiobenzyl radicals (CTBRs) through abstraction of the chlorothiobenzyl-hydrogen. The water molecule can play an important role in this process. The coupling of CTBRs is the essential first step in forming PCTA/DTs. In this paper, quantum chemical calculations were carried out to investigate the formation of CTBRs from the complete series reactions of 19 chlorothiobenzene (CTB) congeners with H/·OH radicals in the presence of the water molecule. Using the MPWB1K/6-311 + G(3df,2p)//MPWB1K/6-31 + G(d,p) energy level, schematic energy profiles were constructed with the water molecule and then compared with the non-hydrated case. The present study shows that structural parameters and thermal data, as well as CTBRs formation potential from CTBs, are strongly dominated by the chlorine substitution at the ortho-position of CTBs. Meanwhile, the water molecule can promote the CTBR formation from CTBs abstracted by H/·OH, which has a stronger catalysis effect on the H abstraction from CTBs by OH than from CTBs by H. This study may provide reference parameters for future experimental research, which would enhance measures to reduce dioxin emission and establish dioxin control strategies.

Mechanism and kinetic properties for the complete series reactions of chloro(thio)phenols with O(3P) under high temperature conditions

Polychlorinated dibenzo-p-dioxins/dibenzofurans (PCDD/Fs) and polychlorinated dibenzothiophenes/thianthrenes (PCDT/TAs) are two groups of dioxin-like compounds with oxygen and sulfur substitution, respectively. Chlorophenols (CPs) and chlorothiophenols (CTPs) are direct precursors in PCDD/F and PCDT/TA formation. The formation of chlorophenoxy radicals (CPRs) and chlorothiophenoxy radicals (CTPRs) from chlorophenols (CPs) and chlorothiophenols (CTPs) with O(3P) is an important initial step for the formation of PCDD/Fs and PCDT/TAs, respectively. In this paper, the formation of CPRs/CTPRs from the complete series reactions of 19 CP/CTP congeners with O(3P) was studied using the density functional theory (DFT) method. The rate constants of each reaction were calculated using canonical variational transition state (CVT) theory along with a small-curvature tunneling (SCT) contribution over a wide temperature range of 600-1200 K. The effect of the chlorine substitution pattern on the structural parameters, thermochemical properties and rate constants in both CPs and CTPs was discussed. This study shows that the reactions between CPs and O(3P) can be affected by the chlorine substitution at the para-position, and the reactions between CTPs and O(3P) are mostly influenced by both ortho-substitutions. The thiophenoxyl-hydrogen abstraction from CTPs by O(3P) is more likely to occur than the phenoxyl-hydrogen abstraction from CPs by O(3P). Comparison of the reactivity of CP/CTPs with O(3P) with our previous work on CP/CTPs with H and OH shows that the order for phenoxyl-hydrogen abstraction potential is CP + OH > CP + O(3P) > CP + H, and the order for thiophenoxyl-hydrogen abstraction potential is CTP + O(3P) > CTP + H > CTP + OH.

The homogeneous gas-phase formation mechanisms of PCPTs/PCDTs/PCDFs from the radical/radical cross-condensation of 2-CPR and 2-CTPR: a theoretical, mechanistic and kinetics study

Polychlorinated phenoxathiins (PCPTs) are one group of dioxin-like compounds, which can be considered to be one-oxygen-substituted polychlorinated thianthrene (PCTA) compounds or one-sulfur-substituted polychlorinated dibenzo-p-dioxin (PCDD) compounds. Owing to their high toxicity and wide distribution, clarifying the formation and emission of PCPTs due to combustion and thermal processes can deepen our understanding of the dioxin formation mechanism and allow reduced-emission and dioxin-control strategies to be established. Chlorophenols (CPs) and chlorothiophenols (CTPs) are direct precursors in PCPT formation. In this paper, the homogeneous gas-phase formation mechanisms of PCPTs, as well as polychlorinated dibenzofurans (PCDFs) and polychlorinated dibenzothiophenes (PCDTs), from the cross-condensation of 2-chlorophenoxy radicals (2-CPRs) and 2-chlorothiophenoxy radicals (2-CTPRs) under thermal and combustion conditions were investigated theoretically using a density functional theory (DFT) method. The reaction priorities and effects of water molecules on the formation mechanisms were discussed. The rate constants of crucial elementary steps were calculated from 600-1200 K. The acute and chronic toxicities of the main products were predicted at three trophic levels. This study shows that routes starting with oxygen-carbon condensation are favored over those starting with sulfur-carbon condensation for PCPT formation, and routes ending with Cl loss can occur more easily than those ending with H loss. Water molecules have a negative catalytic effect on CH-S H-transfer steps but a positive catalytic effect on CH-O H-transfer steps.

Mechanistic and Kinetic Study on Self-/Cross- Condensation of PCTA/DT Formation Mechanisms from Three Types of Radicals of 2,4-Dichlorothiophenol

Chlorothiophenols (CTPs) are known to be key and direct precursors of polychlorinated thianthrene/dibenzothiophenes (PCTA/DTs). Self/cross-coupling of the chlorothiophenoxy radicals (CTPRs), sulfydryl-substituted phenyl radicals and thiophenoxyl diradicals evolving from CTPs are initial and important steps for PCTA/DT formation. In this study, quantum chemical calculations were carried out to investigate the homogenous gas-phase formation of PCTA/DTs from self/cross-coupling of 2,4-dichlorothiophenoxy radical (R1), 2-sulfydryl-3,5-dichlorophenyl radical (R2) and 3,5-dichlorothiophenoxyl diradical (DR) at the MPWB1K/6-311+G(3df,2p)//MPWB1K/6-31+G(d,p) level. The rate constants of crucial elementary steps were deduced over 600-1200 K, using canonical variational transition state theory with a small curvature tunneling contribution. For the formation of PCTAs, the S•/σ-C• condensation with both thiophenolic sulfur in one radical and ortho carbon in the other radical bonded to single electron is the most efficient sulfur-carbon coupling mode, and the ranking of the PCTA formation potential is DR + DR > R2 + DR > R1 + DR > R1 + R2 > R1 + R1. For the formation of PCDTs, the σ-C•/σ-C• coupling with both ortho carbon in the two radicals bonded to single electron is the energetically favored carbon-carbon coupling mode, and the ranking of the PCDT formation potential is: R2 + DR > R2 + R2 > R1 + DR > R1 + R2 > R1 + R1. The PCTA/DTs could be produced from R1, R2 and DR much more readily than PCDD/DFs from corresponding oxygen substituted radicals.

Quantum Chemical and Kinetic Study on Radical/Molecule Formation Mechanism of Pre-Intermediates for PCTA/PT/DT/DFs from 2-Chlorothiophenol and 2-Chlorophenol Precursors

Polychlorinated phenoxathiins (PCPTs), polychlorinated dibenzothiophenes (PCDTs), and polychlorinated thianthrenes (PCTAs) are sulfur analogues of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/DFs). Chlorothiophenols (CTPs) and chlorophenols (CPs) are key precursors for the formation of PCTA/PT/DTs, which can react with H or OH to form chloro(thio)phenoxy radical, sulfydryl/hydroxyl-substituted phenyl radicals, and (thio)phenoxyl diradicals. However, previous radical/radical PCTA/DT formation mechanisms in the literature failed to explain the higher concentration of PCDTs than that of PCTAs under the pyrolysis or combustion conditions. In this work, a detailed thermodynamics and kinetic calculations were carried out to investigate the pre-intermediate formation for PCTA/PT/DTs from radical/molecule coupling of the 2-C(T)P with their key radical species. Our study showed that the radical/molecule coupling mechanism explains the gas-phase formation of PCTA/PT/DTs in both thermodynamic and kinetic perspectives. The S/C coupling modes to form thioether-(thio)enol intermediates are preferable over the O/C coupling modes to form ether-(thio)enol intermediates. Thus, although the radical/molecule coupling of chlorophenoxy radical with 2-C(T)P has no effect on the PCDD/PT formation, the radical/molecule coupling of chlorothiophenoxy radical with 2-C(T)P plays an important role in the PCTA/PT formation. Most importantly, the pre-PCDT intermediates formation pathways from the couplings of sulfydryl/hydroxyl-substituted phenyl radical with 2-C(T)P and (thio)phenoxyl diradicals with 2-C(T)P are more favorable than pre-PCTA/PT intermediates formation pathways from the coupling of chlorothiophenoxy radical with 2-C(T)P, which provides reasonable explanation for the high PCDT-to-PCTA ratio in the environment.

Theoretical study on the reactions of a series of polybromobenzenes with OH radicals: mechanism, kinetics, and QSAR

Polybromobenzenes are a kind of monocyclic aromatic flame retardants that are used as a substitute for polybrominated diphenyl ethers and hexabromocyclododecane. In this paper, the reaction mechanism and rate constants for the reaction of OH radicals with a series of polybromobenzenes such as hexabromobenzene (HBB), 1,2,4,5-tetrabromobenzenes (1,2,4,5-TeBB), pentabromobenzene (PEBB), pentabromoethylbenzene (PBEB), pentabromotoluene (PBT), and 2,4,5-tribromotoluene (2,4,5-TrBT) have been investigated by quantum chemical method. The reaction mechanism was obtained at the MPWB1K/6-311+g(3df,2p)//MPWB1K/6-31+g(d,p) level of theory and the rate constants were deduced over the temperature range of 200–370 K using canonical variational transition state (CVT) theory with the small curvature tunneling (SCT) method. The rate constants of OH radicals with HBB, 1,2,4,5-TeBB, PEBB, PBEB, PBT, and 2,4,5-TrBT are determined to be 5.72 × 10−13, 1.23 × 10−12, 8.78 × 10−13, 9.23 × 10−13, 6.46 × 10−13, and 1.69 × 10−12, respectively, at 298 K and 1 atm. The estimated atmospheric lifetimes of HBB (20.08 days), 1,2,4,5-TeBB (9.65 days), PEBB (13.5 days), PBEB (12.9 days), PBT (18.4 days), and 2,4,5-TrBT (7.0 days) determined by OH radicals indicate that polybromobenzenes have the potential for long-range transport. The genetic function approximation is used to study the quantitative structure–activity relationship. The coefficients indicate that the ELUMO has the highest correlation to logkOH.

Theoretical study on the formation mechanism of polychlorinated dibenzothiophenes/thianthrenes from 2-chlorothiophenol molecules

Homogeneous formation of polychlorinated dibenzothiophenes/thianthrenes (PCDT/TAs), sulfurated compounds analogous to polychlorinated dibenzo-p-dioxin/dibenzofurans (PCDD/Fs), has been well-documented to occur via radical-radical coupling reactions from chlorinated thiophenol precursors. However, the current understanding of the formation mechanism of PCDT/TAs is exclusively limited to the inherent point of view that chlorothiophenoxy radicals act as the only required intermediates for PCDT/TAs. This study investigates reaction pathways for the formation of PCDT/TAs involving two new types of radical species, i.e., substituted phenyl radicals and substituted thiophenoxyl diradicals. Taking 2-chlorothiophenol (2-CTP) as a model compound for chlorothiophenols, we found that apart from the mostly discussed chlorothiophenoxy radicals, substituted phenyl radicals and substituted thiophenoxyl diradicals could also be readily formed via the reaction of 2-CTP with H radicals. Furthermore, direct self- and cross-coupling of these radicals can result in the formation of PCDT/TAs, including 1-monochlorothianthrene (1-MCTA), 1,6-dichlorothianthrene (1,6-DCTA), 4,6-dichlorodibenzothiophene (4,6-DCDT) and 1,6-dichlorodibenzothiophene (1,6-DCDT). The pathways proposed in this work are proven to be both thermodynamically and kinetically favorable. Particularly, comparisons were made between the formation mechanisms of sulfurated and oxygenated dioxin systems from an energetic point view, showing that replacing oxygen with sulfur atoms greatly reduces the activation barriers of the rate-controlling steps involved in the PCDT/TA formation processes compared with those involved for PCDD/Fs. The calculated results in this work may improve our understanding of the formation mechanism of PCDT/TAs from chlorothiophenol precursors and should be informative to environmental scientists.

Formation of persistent organic pollutants from 2,4,5-trichlorothiophenol combustion: a density functional theory investigation

Polychlorinated dibenzothiophene (PCDT) and polychlorinated thianthrene (PCTA) are sulfur analogues of dioxins, such as polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F). In this work, we present a detailed mechanistic and kinetic analysis of PCDT and PCTA formation from the combustion of 2,4,5-trichlorothiophenol. It is shown that the formation of these persistent organic pollutants is more favourable, both kinetically and thermodynamically, than their analogous dioxin counterparts. This is rationalised in terms of the different influences of the S-H and O-H moieties in the 2,4,5-trichlorothiophenol and 2,4,5-trichlorophenol precursors. Kinetic parameters also indicate that the yield of PCDT should exceed that of PCDD. Finally, we demonstrate here that the degree and pattern of chlorination on the 2,4,5-trichlorothiophenol precursor leads to subtle thermodynamic and kinetic changes to the PCDT/PCTA formation mechanisms. Graphical abstract Formation mechanisms of persistant organic pollutants, PCDT and PCTA, from 2,4,5-trichlorothiophenol combustion, has been investigated using density functional theory.