Fs from Chlorophenol

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New Understanding of the Formation of PCDD/Fs from Chlorophenol Precursors: A Mechanistic and Kinetic Study Yanfang Zhang, Dongju Zhang, Jun Gao, Jinhua Zhan, and Chengbu Liu J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 23 Dec 2013 Downloaded from http://pubs.acs.org on January 1, 2014

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The Journal of Physical Chemistry

New Understanding of the Formation of PCDD/Fs from Chlorophenol Precursors: A Mechanistic and Kinetic Study

Yanfang Zhang, Dongju Zhang,* Jun Gao, Jinhua Zhan, and Chengbu Liu

Institute of Theoretical Chemistry, Shandong University, Jinan, 250100, P. R. China

CORRESPONDING AUTHOR: Dr & Professor Dongju Zhang E-mail: [email protected] Phone: +86-531-88365833 Fax:

+86-531-88564464

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ACS Paragon Plus Environment


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ABSTRACT While chlorophenols (CPs) are considered to be the most important and direct precursors of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), our understanding of the formation mechanism of PCDD/Fs is exclusively limited to an invariable idea that chlorophenoxy radicals (CPRs) are only necessary intermediates. The present work presents a systematic theoretical study which aims at providing new insight into the homogeneous formation of PCDD/Fs from CPs. Two different types of radicals from CPRs, i.e. substituted phenyl radicals and phenoxyl diradicals, are proposed to serve as potential sources contributing to the formation of PCDD/Fs. The thermodynamic and kinetic properties of reactions of 2-chlorophenol (2-CP), as a representative of CP congeners, with atomic H to produce various potential radicals forming PCDD/Fs are studied by performing density functional theory calculations and direct kinetics studies. The newly proposed radicals, especially substituted phenoxyl diradicals (the most direct intermediates of PCDD/Fs), can be formed via reactions of 2-CP with atomic H with small barriers and large reaction energies. They should be expected to be responsible for the homogeneous formation of PCDD/Fs under high temperature. Several typical PCDD/F products are predicated through direct self- and crosscouplings of the newly proposed radicals. The radical coupling patterns proposed in the present work expands our understanding of the formation mechanism of PCDD/Fs from CP precursors. Keywords: Dioxins, chlorophenoxy radicals, phenyl radicals, phenoxyl diradicals, DFT, variational transition-state theory

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1. INTRODUCTION 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Polychlorinated dibenzo-p-dioxin (PCDDs) and polychlorinated dibenzofurans (PCDFs), as shown in Scheme 1, are notorious due to their extreme toxicity, persistence in the environment, and bioaccumulation.1 Hence, there is an obvious interest in understanding the formation mechanism of PCDD/Fs to provide fundamental physical insight into the related chemical processes.

Scheme 1. General structural formulas for PCDD/Fs. PCDD/Fs are formed as unwanted byproducts in various combustion processes, especially, the thermal treatment of municipal waste. In the past three decades, the formation mechanisms of PCDD/PCDFs (PCDD/Fs for short) have been extensively studied both experimentally2-5 and theoretically6-12. It has been well established that chlorophenols (CPs) are the predominant and direct precursors of PCDD/Fs under pyrolytic or oxidative conditions in thermal systems. CPs can be found ubiquitously in the environment due to their extensive and long-term use in industry and in daily life.13-14 It is widely accepted that the formation of chlorophenoxy radicals (CPRs) via two general pathways (homogeneous gas-phase reactions and heterogeneous surface-catalyzed processes) is the initial and key step in the formation of PCDD/Fs from CPs15-18. So previous studies by Zhang et al.12,15,19 and other groups6,20-23 exclusively emphasized the importance of dimerization of CPRs,2428

although, in an early work, Lomnicki et al.29 proposed the formation of PCDD/Fs through

condensation of the phenoxyl diradical (also called α-ketocarbene). Very recently, we stressed the great potential of two other radicals (structures 2 and 7 in Figure 1) rather than the well-known CPR (structure 1 in Figure 1) for the gas-phase formation of PCDD/Fs,30 because their formation was indicated to be energetically very favorable. It should be noted that there are some isomeric structures for the two radicals, as shown by a complete series of potential radicals from 2-CP in Figure 1. Similarly, analogous radicals may be formed from other CPs. In this sense, the radical precursors of PCDD/Fs should not be limited to CPRs. However, to our knowledge, no study 3



systematically accounts for the formation of various distinct radicals of CPs, which is of intrinsically 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

importance for understanding the formation mechanism of PCDD/Fs. In this work, choosing 2-CP as a representative of 19 CP congeners, we present an exhaustive theoretical study on mechanistic and kinetic properties of a series of abstraction reactions of 2-CP by atomic H to form various radical precursors of PCDD/Fs. As shown in Figure 1, 2-CP can form 11 radicals, including 2-CPR (1), substituted phenyl radicals (2-6), phenoxyl diradicals (8-11). By comparing calculated barriers and rate constants, we evaluate their potential possibility contributing to the formation of PCDD/Fs. Further, we predicate possible PCDD/F products from these potential radical precursors. The present work is an extension of our recently published work30 that considered the coupling reactions of some radicals from 2-chlorophenol and only emphasized the mechanistic aspect forming PCDD/Fs. Here, we concentrate on a complete series of radical precursors of PCDD/Fs from 2-chlorophenol, and present a combined mechanistic and kinetic study of the formation of PCDD/Fs. The calculated results

may improve our understanding of the formation mechanism of PCDD/Fs from CP precursors and be informative to environmental scientists. 2. COMPUTATIONAL DETAILS Quantum chemistry calculations were carried out in the framework of density functional theory (DFT) using Gaussian 09 program package.31 The geometries of reactants, intermediates, and products were optimized using the hybrid meta functional, MPWB1K,32 which gives excellent performance for thermochemistry, thermochemical kinetics, hydrogen bonding, and weak interactions, with the standard 6-31+G(d,p) basis set. The reliability and accuracy of the MPWB1K functional for describing CP- and CPR- involving reactions have been confirmed in previous works.9-12 The vibrational frequencies were calculated at the same level of theory to determine the nature of minima and first-order saddle points and to provide the zero-point vibrational energy (ZPE). Intrinsic reaction coordinate (IRC) calculations were performed to verify that each transition state (TS) is connected to two desired minima. The single point-energy calculations were refined using the larger 6-311+G(3df,2p) basis set. The kinetic calculations were performed using the POLYRATE 9.3 program.33 The rate constants 4


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were calculated using canonical variational transition-state theory (CVT),34-36 and quantum tunneling 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

corrections were considered by the small curvature tunneling (SCT) method,37 which is extensively used in the studies of H-abstraction reactions.38 The rate constants of each reaction were obtained over a wide temperature range of 600-1200 K, which covers the possible formation temperature of PCDD/Fs in municipal waste incinerators. 3. RESULTS AND DISCUSSION Calculated results show that spin contamination is small for all the wave functions, implying the insignificant contribution from the energetically non-adiabatic processes. In this sense, the present work omitted the nonadiabatic effect of reactions. In addition, it is also difficult for performing nonadiabatic treatment of the system containing four or more atoms.39-40 2-CP has two distinct conformers, labeled as the syn and anti, depending on the hydroxyl H atom close or far away from the ortho-substituted Cl atom, respectively. The syn conformer was found to be about 3 kcal/mol more stable than the anti one owning to the intramolecular hydrogen bonding. The energy of 2-CP throughout this work involves the syn conformer. We calculated its various Habstraction reactions with atomic H, to form different radicals in Figure 1. In municipal waste incinerators, H radicals may arise from the following reactions at high temperature:41

2-C6H4OH + O2 → 2-C6H4O· + HO2 HO2 → H + O2 Calculated relative energies are given in Figure 1, and optimized geometries of H-abstraction transition states, denoted as TS1-TS11 and TS7′-TS11′, are shown in Figure 2. Radicals derived from 2-CP, 1-11 in Figure 1, can be classified into three categories, the phenoxy radical (1), substituted phenyl radicals (2-6), and substituted/pristine phenoxyl diradicals (7-11). In the following sections we first discuss their formation mechanism and relevant kinetics property, and then possible PCDD/F products obtained via self- and cross-couplings of these radicals. 3.1. The phenoxy radical, 2-CPR. As shown in a previous study by Zhang et al.,15 the direct cleavage of the O-H bond in 2-CP can result in the formation of 1. However, such a unimolecular 5



decomposition pathway is a highly endothermic process, involving an energy demand of 87.4 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

kcal/mol at 0 K. In contrast, with the assistance of activate radicals, such as atomic H, 2-CPR can be formed through the H-abstraction reaction with a barrier of 12.5 kcal/mol and a reaction energy of 13.3 kcal/mol (Figure 1), indicating that the formation of 2-CPR induced by H becomes an energetically feasible process. The transition state structure involved in the reaction is denoted as TS1 in Figure 2. The high stability of structure 1 is attributed to its electron delocalization via several resonance structures shown in Scheme 2. The delocalization energy was reported to be 16 kcal/mol.42 CPRs could build up in appreciable concentrations in the combustion environment of municipal waste incinerators, and are capable of self-condensation through phenolic oxygen, and ortho- and para-carbons.

Scheme 2. Resonance structures of 2-chlorophenoxy radical. 3.2. Substituted phenyl radicals. Structures 2-6 in Figure 1 are classified as substituted phenyl radicals. These structures should be considered to add to the environmental load of PCDD/Fs during oxidation and pyrolysis of CPs, because their parent structure, the phenyl radical (⋅C6H5), has been proved to be highly active and plays an important role in the formation of various pollutants such as polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and PCDD/Fs.43,44 Furthermore, Schuler et al.45 reported the conversion of 2-hydroxyphenyl radical (⋅C6H4-OH, structure 6 in Figure 1) to phenoxyl radical (C6H5-O⋅) in aqueous solution by ESR spectroscopy, and Nataga et al 46 determined the infrared spectrum of 2-hydroxyphenyl radical. These facts imply that the substituted phenyl radicals 2-6, derivatives of the phenyl radical, could appear as intermediates during the oxidation of 2-CP, and hence should be expected responsibly for the formation of PCDD/Fs. Unfortunately, the relevant topic is never considered in previous publications. Here, we calculated the formation of these five substituted phenyl radicals from 2-CP through an 6


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abstraction reaction of H-atom in benzene ring by atomic H. Note that similar to 2-CP, each of these 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

five radicals has two conformers, called the syn and anti depending on the conformer of their parent structure, 2-CP. Both syn- and anti-conformers of these five radicals are found to be planer. Their relative energies are summarized in Table 1. It is found that the syn conformer is more stable than the corresponding anti one, and that the energy difference of the syn- and anti-conformers is small (1.6-3.4 kcal/mol), implying the easy transformation between two conformers by hydrogen tunneling like hydroquinone derivatives.47 On the other hand, the energy difference of the syn- or anticonformers of structures 2-5, which are isomers of 1, is less than 2 kcal/mol, indicating that the formation of these radicals are competing from a thermodynamic point of view. For structure 6, the calculated relative energy difference of syn- and anti-conformers is slightly smaller than that given by Nagata et al46 (1.55 vs. 1.84 kcal/mol). In the present work, we focuse on their more stable synconformers. These five substituted phenyl radicals may arise via the reaction of 2-CP with various free radicals, such as H, OH, O, and Cl, in municipal waste incinerator. We here consider their formation through the reactions of 2-CP with atomic H. The formations of radicals 2-6 involve the breakages of aromatic C-H and C-Cl bonds, which are stronger than the O-H bond in the hydroxyl group. So it is not surprising that the calculated barriers (16.1-17.5 kcal/mol for the H-abstraction and 14.5 kcal/mol for the Cl-abstraction) are higher than that (12.5 kcal/mol) involved for the formation of 1. The H-abstraction reactions are predicated to be endothermic by 9.9-11.7 kcal/mol, and the Clabstraction reaction is exothermic by 2.7 kcal/mol. However, such energy demands are easily met under high temperature in municipal waste incinerator, where the highly endothermic process of the direct cleavage of the O-H bond in 2-CP is even considered to be initial step forming PCDD/Fs.48 On the other hand, as will be seen in the following section, once formed, these active substituted phenyl radicals may immediately convert themselves into the much more stable phenoxyl diradicals, 7-11. Thus they should be considered as precursors of PCDD/Fs. However, from a structural point of view, only 2 and 6 can directly contribute to the formation of PCDD/Fs. 2-5 are isomeric structure of 1. They are estimated to be about 23-25 kcal/mol less stable than 1 7



(Table 1), implying that these radicals might be transient and active intermediates and can be 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

transformed into the chlorinated and pristine phenoxyl diradicals via abstracting the H atom in the hydroxyl group by atomic H. 3.3 The pristine and chlorinated phenoxyl diradicals. Structure 11 in Scheme 3 is the phenoxyl diradical, and 7-10 are its chlorinated derivatives. These structures are collectively termed “diradicals” in the present work. They may arise from either 1 or 2-6 through a second H/Cl abstraction reaction by atomic H (Figure 1). The reason we consider these radicals as potential intermediates of PCDD/Fs is that structure 11 is the resonance structure of the α-ketocarbene (Scheme 3), which has been observed experimentally with a long lifetime in the order of microseconds by laser flash photolysis of 2-bromophenol in aqueous solution49 and proposed to be an intermediate forming PCDD/Fs from the pyrolytic thermal degradation of catechol.29 Structure 7 is the chlorinated derivative of 11, and its resonance structure is the chlorinated α-ketocarbene (Scheme 3). Species 8-10 are isomeric structures of 7. The ground states of all these five diradicals are found to be in triplet states, confirming their diradical character. Table 2 compares the relative stability of four chlorinated phenoxyl diradicals.

Scheme 3. Resonance structures of the pristine (11) and chlorinated (7) phenoxyl diradicals. We here calculated two potential formation pathways of 7-11: arising from 2-6 by abstracting the H atom of the hydroxyl group, and from 1 by abstracting the H/Cl atom of the benzene ring. The optimized transition state structures, denoted as TS7-TS11 (arising from 1) and TS7′-T11S′ (arising 8


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from 2-6), are gathered in Figure 2, and calculated barriers and reaction heats are shown in Figure 1. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

It is found that abstracting an aromatic H or Cl atom from 2-6, resulting in the chlorinated or pristine phenoxy diradicals, requires overcoming a barrier of 11.0-13.1 kcal/mol with an estimated exoergicity of 12.8-16.8 kcal/mol. The small barriers and the exothermicity of these reactions indicate the facile formation of diradicals, where two unpaired electrons are delocalized over the phenyl ring. Alternatively, structures 7-11 can be produced from 1 by abstracting the H atom in the hydroxyl group. The calculated barriers are in the ranges of 14.2-17.5 kcal/mol, and the H-abstract reactions are estimated to be endothermic by 7.4-11.4 kcal/mol while the Cl-abstract reaction is calculated to exothermic by -5.6 kcal/mol. These diradicals are direct precursors of PCDD/Fs. Their self/cross-coupling reactions as well as their cross-coupling reactions with 1-6, can result in various C-C and C-O-C links between two aromatic rings, which is an intrinsically key step in the formation of PCDD/Fs. However, it should be noted that only structures 7 and 11 are responsible for the direct formation of a PCDD/F molecule because only in these two structures two unpaired electrons located on the O and C centers are in ortho position, which is necessary for building a PCDD/F framework via the radical-radical coupling reaction. 3.4. Formation of PCDD/Fs. Based on the above discussion, it is clear that structures 1, 2, 6, 7, and 11 can directly contribute to the formation of PCDD/Fs, while 3-5, and 8-10 are not direct precursors for the formation of PCDD/Fs due to the lack of the active site ortho to the hydroxyl group/phenoxy radical although they have potential to form other organic pollutants.43-44 The selfdimerizations and crossing-couplings of radicals 1, 2, 6, 7, and 11 can produce various PCDD/Fs. Taking diradical 7 as a representative, we examined its self-coupling and cross-couplings with 1, 2, 6, and 11. As shown in Figure 3, the reaction of 7 with 1 can result in the formations of 1monochlordibenzo-p-dioxin (1-MCDD) and 1,6-dichlordibenzo-p-dioxi (1,6-DCDD) through four different pathways that involve initial barrierless coupling of the two radicals followed by 9



cyclization and elimination of Cl or H atom. Pathways I and II result in 1-MCDD, and pathways III 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

and IV give 1,6-DCDD. From calculated barriers, pathways I and III are dominant for the formations of 1-MCDD and 1,6-DCDD. In pathway I, the phenolic oxygen of 1 initially attaches to the radical character carbon of 7 to form an intermediate that then cyclizes into 1-MCDD via attack of the phenolic oxygen on the chlorine-bearing carbon atom with the elimination of Cl radical. This process is calculated to be exothermic by 11.7 kcal/mol and encounter a barrier of 27.7 kcal/mol. Pathway III involves the attack of phenolic oxygen of 7 on the ortho-carbon of the carbon-centred mesomer of 1 followed by H elimination and ring closure. Note that the barrier involved in pathway III is only 2.9 kcal/mol, which is in distinct contrast to that involved in pathway I, 27.7 kcal/mol. This result indicates that the coupling between 7 and 1 may preferably results in the formation of 1,6-MCDD. The reaction of 7 with 2 can also produce 1,6-MCDD via pathway V, where the phenolic oxygen of 7 attaches to the radical character carbon of 2. However, this process is found to involve a barrier of 42.4 kcal/mol. Alternatively, the linkage between two radical carbons in 7 and 2 can form 4,6dichlordibenzofuran (4,6-DCDF) via pathway VI, in which the barrier of rate-determining step is calculated to 31.6 kcal/mol. Similarly, the reaction of 7 with 6 can proceed via pathways VII and VIII, leading to 1-MCDD and 4-monochlordibenzofuran (4-MCDF). These two reactions involve very similar mechanisms and also similar kinetic and thermodynamic properties to pathways V and VI, respectively. The self-dimerization reaction of 7 also produces 1,6-DCDD via pathways IX and X, which involves synchronous and successive couplings of two pairs of radicals. Similarly, the crossingcoupling between diradicals 7 and 11 via pathways XI-XIII results in the formation of 1-MCDD. All these pathways are barrierless and highly exothermic. The above discussion indicates that the self-coupling of 7 and its crossing-couplings with 1, 2, 6, and 7 can result in the formation of 1-MCDD, 1,6-DCDD, 4,6-DCDF, and 4-MCDF, which are major PCDD/F products observed experimentally in the oxidation and pyrolysis of 2-CP.2,4,28 Importantly, it is noted that two PCDD products can be directly formed via the direct self-coupling 10


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of 7 and its crossing-coupling with 11. Such dimerization patterns have never been reported in 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

previous studies. 3.5. Kinetics Calculations. To evaluate the kinetics properties of the reactions forming radicals 111 from 2-CP precursor, based on CVT theory with SCT contribution we calculated the rate constants of 16 elementary reactions over the entire temperature range of interest (600-1200 K). Previous study by Zhang et al.10 has confirmed the good performance of the CVT/SCT method for calculating rate constants of CPR radical formation. In specific calculations, we found that the lowest frequency mode of each transition state corresponds to a hindered internal rotation rather than a small-amplituded vibrational. Thus, in the calculations of rate constants, it has been removed from the vibration partition function of the transition state, and alternatively included in the expression of the hindered rotor partition function, as proposed by Truhlar er al50. The calculated Arrhenius formulas are summarized into Table 3. It is found that rate constants for reactions abstracting the aromatic H/Cl to form 2-6 are generally comparable with that for the reaction abstracting the phenoxyl H to form 1. For example, at 1000 K, the CVT/SCT rate constant for the reaction forming 1 is 4.05×10-14 cm3 molecule-1 s-1, while they are 9.36×10-15 and 2.67×10-14 cm3 molecule-1 s-1 for reactions forming 2 and 6. For the reaction forming 5, the calculated CVT/SCT rate constant is even larger than that for the reaction forming 1, (1.90×10-13 vs 4.05×10-14 cm3 molecule-1 s-1). These data indicates that the aromatic C-H and C-Cl bond dissociations from 2-CP may be competing with the O-H bond dissociation. Therefore, substituted phenyl radicals (2-6) should be considered as potential precursors contributing to the formation of PCDD/Fs. Further, we examine the rate constants for reactions forming diradicals 8-11 from 1 or 2-6. The calculated rate constants at 1000 K for reactions of 1 with H to form 7, 9, and 11 are 1.68×10-14, 2.59×10-14, and 2.62×10-14 cm3 molecule-1 s-1, which are 1~2 order of magnitude larger than the reactions with H to form 8, and 10. The same regularity also applies to the reactions forming 7-11 from 2-6, i.e., the rate constants forming 7, 9, and 11 are larger than those forming 8-10. From these calculated CVT/SCT rate constants, it seems that 7, 9, and 11 should have higher concentrations than 11



8 and 10 during the oxidation of 2-CP in thermal systems. The Arrhenius formulas summarized in 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table 3 are expected to be useful for evaluating PCDD/F products from 2-CP precursor. 4. SUMMARY AND CONCLUSIONS By performing DFT calculations, we have carried out a systematic theoretical study on the mechanism and kinetics of the homogeneous formation of PCDD/Fs from 2-CP precursor. Two new types of radicals from 2-CP, i.e. substituted phenyl radicals and the phenoxyl diradicals, are proposed to serve as potential sources contributing to the formation of PCDD/Fs. These radicals can be easily formed via reactions of 2-CP with atomic H with small barriers and large reaction energies. Direct self- and cross-couplings of the newly proposed radicals can results in the formation of PCDD/Fs, including 1-MCDD, 1,6-DCDD, 4,6-DCDF, and 4-MCDF. The present results expand our understanding of the formation mechanism of PCDD/Fs from CP precursors.

■ AUTHOR INFORMATION Corresponding Authors *(D.Z.) E-mail: [email protected]; phone: +86-531-88365833; fax: +86-531-88564464 Notes The authors declare no competing financial interest.

■ ACKNOWLEDGEMENTS This work was jointly supported by National Basic Research Program of China (973 Program, 2013CB934301) and National Natural Science Foundation of China (Grant Nos: 21273131 and 91127014).

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Direct Kinetic Study of the Polychlorinated Dibenzo-p-dioxin and Dibenzofuran Formations from 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Thermochemical Kinetics, and Noncovalent Interactions: the MPW1B95 and MPWB1K Models and 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Chlorination, 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Dechlorination and

Destruction

of

Polychlorinated

Dibenzo-p-dioxins and

Dibenzofurans (PCDD/Fs). Prog. Energy Combust Sci. 2009, 35, 245-274. (42) Afeefy, H. Y,; Liebman, J. F.; Stein, S. E. Neutral Thermochemical Data. In: Linstrom PJ, Mallard WG, editors. NIST chemistry WebBook, NIST standard reference database. Gaithersburg, MD: National Institute of Standards and Technology, http://webbook.nist.gov; 2005. (43) Kaiser, R. I.; Parker, D. S. N.; Zhang, F. PAH Formation under Single Collision Conditions: Reaction of Phenyl Radical and 1,3-Butadiene to form 1,4-Dihydronaphthalene. J. Phys. Chem. A 2012, 116, 4248–4258. (44) Shukla, B.; Susa, A.; Miyoshi, A.; Koshi, M. Role of Phenyl Radicals in the Growth of Polycyclic Aromatic Hydrocarbons. J. Phys. Chem. A 2008, 112, 2362–2369. (45) Schuler, R. H.; Neta, P.; Zemel, H.; Fessenden, R. W. Conversion of Hydroxyphenyl to Phenoxyl Radicals: a Radiolytic Study of the Reduction of Bromophenols in Aqueous Solution. J. Am. Chem. Soc. 1976, 98, 3825–3831. (46) Nagata, M.; Futami, Y.; Akai, N.; Kudoh, S.; Nakata, M. Structure and Infrared Spectrum of 2Hydroxyphenyl Radical. Chem. Phys. Lett. 2004, 392, 259–264. (47) Akai, N.; Kudoh, S.; Nakata, M. Photoisomerization and Tunneling Isomerization of Tetrachlorohydroquinone in a Low-Temperature Argon Matrix. J. Phys. Chem. A 2003, 107, 3655– 3659. (48) Evans, C. S.; Dellinger, B. Mechanisms of Dioxin Formation from the High-Temperature Pyrolysis of 2-Chlorophenol. Environ. Sci. Technol. 2003, 37, 1325–1330. (49) Bonnichon, F.; Richard, C.; Grabner, G. Formation of an α-Ketocarbene by Photolysis of Aqueous 2-Bromophenol. Chem. Comm. 2001, 73–74. (50) Truhlar, D. G. A Simple Approximation for the Vibrational Partition Function of a Hindered Internal Rotation. J. Comput. Chem. 1991, 12, 266–270.

17



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Page 18 of 25

TABLE 1. Calculated Relative Energies (in kcal/mol) of Substituted Phenyl Radicals syna

anti

∆Eb

2

1.77 (25.04)

3.50

1.73

3

0.00 (23.27)

3.39

3.39

4

0.89 (24.16)

4.13

3.24

5

0.88 (24.16)

4.22

3.34

1.55

1.55 (1.84)c

6 a

0.00

The values in parentheses are the relative energies with respect to 2-

chlorophenoxy radical. bEnergy difference between the syn- and anti-conformers. c

ref. 42.

18


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TABLE 2. Calculated Relative Energies of Diradicals 7

8

9

10

2.22

2.75

0.00

3.99

Relative energy （in kcal/mol）

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

TABLE 3. Arrhenius Formulas (in cm-3 Molecule-1 s-1) for the Formation of Radicals 1-11 from the Reactions of 2-Chlorophenoxy Radical with Atomic H over the Temperature Range of 600∼ ∼1200 K. reactions

Arrhenius Formulas

→1

k(T)=(6.50×10-11)exp(-7380.9/T)

→2

k(T)=(1.46×10-10)exp(-9654.9/T)

→3

k(T)=(1.46×10-10)exp(-9025.5/T) + H2 (HCl)

2-CP + H →4

k(T)=(1.39×10-10)exp(-9195.9/T)

→5

k(T)=(3.86×10-11)exp(-5312.7/T)

→6

k(T)=(8.38×10-11)exp(-8051.5/T)

→7

k(T)=(1.07×10-10)exp(-8760.4/T)

→8

k(T)=(1.88×10-12)exp(-8212.7/T)

→ 9 + H2 (HCl)

k(T)=(9.02×10-11)exp(-8154.5/T)

→ 10

k(T)=(7.99×10-11) exp(-9457.9/T)

→ 11

k(T)=(1.05×10-10)exp(-8295.6/T)

2

→7

k(T)=(1.62×10-11)exp(-4563.4/T)

3

→8

k(T)=(7.04×10-11)exp(-7584.8/T)

4 + H

→ 9 + H2

k(T)=(7.38×10-12)exp(-4139.2/T)

5

→ 10

k(T)=(4.20×10-11)exp(-7748.9/T)

6

→ 11

k(T)=(1.82×10-11)exp(-4318.3/T)

1 + H

20


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


O ∆E a =12.45

H

Cl

∆E r = -13.33

H 2 HCl

TS1

1

O

OH ∆E a =17.47

Cl

∆E r=11.71 TS2

∆E a=11.62

H2 7

∆E a=13.07

Cl

H2

3

H 2 HCl

8

OH ∆E a=16.58

O ∆E a=11.24

Cl

∆Er =10.83

Cl

∆E r = -16.77

TS4

TS9'

H

∆E a =14.20 ∆E r=7.39 TS9

H2 9

4

O

OH ∆E a=16.74

∆E a =13.02

Cl

∆E r =10.82 TS5

Cl

∆E r = -12.77

H

TS10'

∆E a =17.48 ∆E r=11.38 TS10

H2

5

∆E a=14.52

∆E a =16.65 ∆E r=10.13 TS8

TS8'

H

H

Cl

∆E r= -13.13

TS3

Cl

2-CP

O

OH

∆E r=9.94

OH

∆E a =15.72 ∆E r=9.61 TS7

TS7'

H 2

∆E a =16.11

Cl

∆E r = -15.43

10

O

OH

∆Er = -2.73 TS6

∆Ea =10.95

∆E a =14.29

∆E r= -16.15

∆E r= -5.55

H

TS11'

TS11

H2

6

11

Figure 1. Schematic formation of various radicals from the reactions of 2-CP with H with calculated barriers (∆Ea, the relative energy of the transition state with respect to separated reactants, in kcal/mol) and the reaction energies (∆Er, the relative energy of the total energy of products with respect to separated reactants, in kcal/mol).

21



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Page 22 of 25

1.341

1.496 0.834

TS1 （2220i）

TS2 （896i）

TS3 （981i）

TS4 （924i）

TS5 （946i）

TS6 （910i）

TS7 （764i）

TS7′′ （2209i）

TS8 （1003i）

TS8′′ （2239i）

TS9 （1064i）

TS9′′ （2208i）

TS10 （950i）

TS10′′ （2224i）

TS11 （915i）

TS11′′ (2182i）

Figure 2. Optimized structures of H-abstraction transition states. The key geometrical parameters are given in Å. Values in parentheses are imaginary frequencies of transition states (in cm-1). Red, green, gray, and white balls denote O, Cl, C and H atoms, respectively.

22


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23



Figure 3. Self-coupling reaction of the chlorinated phenoxyl diradical (7) and its cross-coupling 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

reactions with 2-chlorophenoxy radical (1), substituted phenyl radicals (2 and 6), and the phenoxyl diradical (11) to form various PCDD/F products. Calculated data of ∆Ea and ∆Er are given in kcal/mol.

24


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TOC

25


Fs from Chlorophenol

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