Fs from 2

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New Insight into the Formation Mechanism of PCDD/Fs from 2‑Chlorophenol Precursor Wenxiao Pan,† Dongju Zhang,*,† Zhe Han,‡ Jinhua Zhan,*,† and Chengbu Liu† †

Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, Institute of Theoretical Chemistry, Shandong University, Jinan, 250100, P.R. China ‡ New Materials Institute of Shandong Academy of Sciences, Jinan 250014, P.R. China S Supporting Information *

ABSTRACT: Chlorophenols are known as precursors of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/ Fs). The widely accepted formation mechanism of PCDD/Fs always assumes chlorophenoxy radicals as key and important intermediates. Based on the results of density functional theory calculations, the present work reports new insight into the formation mechanism of PCDD/Fs from chlorophenol precursors. Using 2-chlorophenol as a model compound of chlorophenols, we find that apart from the chlorinated phenoxy radical, the chlorinated phenyl radical and the chlorinated αketocarbene also have great potential for PCDD/F formation, which has scarcely been considered in previous literature. The calculations on the self- and cross-coupling reactions of the chlorophenoxy radical, the chlorinated phenyl radical and the chlorinated α-ketocarbene show that the formations of 1-MCDD, 1,6-DCDD, 4,6-DCDF, and 4-MCDF are both thermodynamically and kinetically favorable. In particular, some pathways involving the chlorinated phenyl radicals and the chlorinated α-ketocarbene are even energetically more favorable than those involving the chlorophenoxy radical. The calculated results may improve our understanding for the formation mechanism of PCDD/Fs from chlorophenol precursors and be informative to environmental scientists.

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INTRODUCTION Polychlorinated dibenzo-p-dioxin (PCDDs) and polychlorinated dibenzofurans (PCDFs) are two groups of chlorinated materials found in the environment which have caused great concern owing to their extreme toxicity, persistence in the environment, and bioaccumulation.1 They are considered to be among the twelve initial most dangerous persistent organic pollutants (POPs) under the Stockholm Convention2 and can be found in nearly every corner of the earth. Unlike other POPs, such as polychlorinated biphenyls (PCBs) and dichlorodiphenyltrichloroethane (DDT), PCDD/Fs have never been intentionally synthesized or used and they are generated as unwanted byproducts in a variety of industrial and thermal processes.3 The formation mechanisms of PCDD/Fs have been extensively studied both experimentally4−7 and theoretically8−13 in the past three decades. It has been well established that PCDD/Fs are formed through two general catalytic processes: the de novo route and the precursor route. The former refers to the combination and heterogeneous reactions of carbon, oxygen, hydrogen, and chorine on transition metal surfaces at lower temperatures (200−600 °C). The latter relates both the surface-catalyzed processes at lower temperatures (200−600 °C) and the gas-phase processes at higher temperatures (> 600 °C) of chemical precursors, especially © 2013 American Chemical Society

chlorophenols (CPs). It is generally accepted that the surfacecatalyzed formation of PCDD/Fs from these two routes and in particular from the surface-catalyzed precursor condensation is the major contributor of the total PCDD/F output from combustion systems.14,15 In contrast, the gas-phase route is generally considered to be a less signification reaction pathway.16−18 However, some authors believed that contribution of the gas-phase pathway for PCDD/F formation was underestimated in the previous kinetic model proposed by Shaub and Tsang in 1983 due to the neglect of the selfrecombination of chlorophenoxyl radicals,18−20 and proposed that the gas-phase contribution is as high as 50%.21 A careful examination of the literature shows that many fundamental issues related to the gas formation of PCDD/Fs remain uncertain or at least are not well understood. In particular, our knowledge of the detailed molecular mechanism is still far from complete. For example, previous research related to the formation of PCDD/Fs from the gas-phase reactions of CP precursors mainly assumed that the chlorinated/nonchlorinated phenoxy radical (PhO·) as key intermediates,22 Received: Revised: Accepted: Published: 8489

February 7, 2013 May 15, 2013 June 14, 2013 June 14, 2013 dx.doi.org/10.1021/es400632j | Environ. Sci. Technol. 2013, 47, 8489−8498

Environmental Science & Technology

Article

bonding in the syn form. So, throughout this paper, 2-CP denotes the syn conformer. The calculations were carried out in the framework of density functional theory (DFT) using the Gaussian 09 program package.27 The geometries of reactants, intermediates, and products involved in this work were optimized using the newly developed BB1K functional by Zhao et al.28 with the standard 6-311G(d,p) basis set. The BB1K functional was adopted because of its excellent performance on saddle point geometry and barrier height calculations compared to all other DFT and hybrid DFT methods.28 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 zeropoint 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 at the BB1K/6-311+G(3df,2p) level. The profiles of the potential energy surface (PES) were constructed at the BB1K/6-311+G(3df,2p)// BB1K/6-311G(d,p) level, including ZPE correction.

which can be readily formed from CPs through the phenolic hydroxyl H-atom abstraction reaction by active radicals, such as OH and H, et al. However, it should be stressed that from CP precursors, such as 2-chlorophenol (2-CP), other intermediates besides 2-chlorophenoxy radical (denoted as R1 in Scheme 1), Scheme 1. Optimized Structures of syn- and anti-2Chlorophenols (2-CP), 2-Chlorophenoxy Radical (R1), Chlorinated Phenyl Radicals (R2−R5), and the Chlorinated α-Ketocarbene (R6)a

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RESULTS AND DISCUSSION To clarify the accuracy of the functional and basis set described above, we compared the calculated geometry parameters of 2CP, 2-chlorophenoxy radical (R1), and phenyl radical (Ph·) with the available theoretical results in the literature.29,30 As shown in Scheme 1, the calculated results at the BB1K/6311G(d,p) level are in good agreement with the reference values. Moreover, we also calculated the reaction enthalpies for the following two reactions for which the experimental data are available for comparison.

a

The distances are in angstroms. The values in parentheses are theoretical results from refs 29 and 30.

including various substituted phenyl radicals (denoted as R2− R5 in Scheme 1) and the chlorinated α-ketocarbene (denoted as R6 in Scheme 1) also have great potential for the gas-phase formation of PCDD/Fs.7,23,24 The substituted phenyl radicals R2−R5 can be considered as the derivatives of the phenyl radical (Ph·), which 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.23,24 The chlorinated α-ketocarbene R6 is a derivative of α-ketocarbene (2oxocyclohexa-3,5-dienylidence, in Scheme 1), which has also been detected by laser flash photolysis of 2-bromophenol in aqueous solution.25 In an early work, Lomnicki et al.26 have proposed the formation of dibenzo-p-dioxin and dibenzofuran through condensation of the α-ketocarbene (also called phenoxyl diradical) resulted from the pyrolytic thermal degradation. Therefore, radical species, R2−R6, should be considered as active or stable intermediates contributing to the formation of PCDD/Fs. However, to our knowledge, there is no detailed information on the formation of PCDD/Fs from these radical species. In this paper, we present a systematic theoretical study, which aims at providing new insight into the gas-phase formation route of PCDD/Fs from CP precursors via the substituted Ph· and the α-ketocarbene. The calculated results may improve our understanding for the formation mechanism of PCDD/Fs from CP precursors and be informative to environmental scientists.

The calculated reaction enthalpies at 298.15 K at the BB1K/ 6-311+G(3df,2p)//BB1K/6-311G(d,p) level are 20.35 and 39.84 kcal/mol, which agree well with the experimental values of 20.50 and 39.17 kcal/mol derived from the experimental standard enthalpies of formation.31,32 These facts indicate the acceptable accuracy and reliability of the level of theory we used. Formations of Chlorinated Phenyl Radicals and the αKetocarbene. As described in the introduction section, R1− R6 shown in Scheme 1 are potential intermediates for the formation of PCDD/Fs from 2-CP. They may arise via the reactions of 2-CP with various atmospheric free radicals, such as HOx, NOx, and XOx (X = Cl, Br, and I), produced from atmospheric photochemistry. As an example, we here consider their formation from the reaction of 2-CP with the hydroxyl radical OH, the most important atmospheric radical in terms of its reactivity.33 The possible pathways and the calculated energy profiles are depicted in Figure 1a and b. As shown in Figure 1a, the hydroxyl group H atom in 2-CP is labeled as H1, and the four H atoms at the benzene ring are numbered as H2−H5 in order. R1−R5 can be formed by OH

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MODELS AND COMPUTATIONAL DETAILS In this work, 2-CP was selected as the model compound of CPs. It has two conformers, labeled as syn and anti depending on the hydroxyl H atom close or far away from the orthosubstituted Cl atom, respectively (Scheme 1). The syn conformer is calculated to be about 3 kcal/mol more stable than the anti one owing to the intramolecular hydrogen 8490

dx.doi.org/10.1021/es400632j | Environ. Sci. Technol. 2013, 47, 8489−8498


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Figure 1. Schematic representation (a) and the calculated relative energy profiles (b) of the reaction of 2-CP with the OH radical to form R1−R6.

abstracting H1−H5 atoms via transition states TS1−TS5, respectively. Our calculations indicate that the formations of R1 and R2 are related to intermediates IM1 and IM2, respectively, while R3−R5 are formed via the direct H abstraction reactions. IM1 and IM2 are two adduct complexes of 2-CP with the attacking OH. In IM1 the attacking OH forms a hydrogen bond with H1 atom, whereas in IM2 the attacking OH forms a six-membered ring with 2-CP via two intermolecular hydrogen bonds. These two complexes are calculated to be slightly more stable than the separate reactants (1.8 kcal/mol for IM1 and 2.0 kcal/mol for IM2). IM1 is converted to R1 via transition state TS1 with a barrier of 5.3 kcal/mol. The relative energies of TS1 and R1 with respect to the initial reactants are 3.5 and −27.2 kcal/mol, respectively, which are in good agreement with the values of 3.0 and −27.1 kcal/mol reported by Altarawneh et al.10 IM2 evolves into R2 via TS2 with a barrier of 7.7 kcal/mol. This barrier value is also in line with the reference value (7.02 kcal/ mol) involved in the analogous reaction of 4-CP with OH reported by Han et al.34 Similarly, the barriers involved in the reactions forming R3−R5 are found to be 5.7, 5.9, and 6.8 kcal/ mol, respectively. Clearly, the formations of all these radicals (R1−R5) involve very similar and small barriers. Among these radicals, R1 (27.2 kcal/mol below the reactants), the most extensively considered responsible intermediate for the formations of PCDD/Fs, is much more stable than R2−R5 (2.0−3.7 kcal/mol below the reactants), implying that R2−R5

may be more active and short-lived, and thus should be considered as potential intermediates during the formation of PCDD/Fs. Concerning the formation of the α-ketocarbene R6, we calculated two possible pathways derived from R1 and R2, respectively. As shown in Figure 1a. A second OH attacks R1 or R2 to result in IM3 or IM4, which are converted into R6 via transition states TS6 and TS7 with the energy barriers of 7.3 and 4.9 kcal/mol, respectively. Notably, we find that R6 with its triplet ground state lies 31.6 kcal/mol below the initial reactants. Thus it is even more stable than R1, the most wellknown responsible intermediate for the formation of PCDD/ Fs. To our surprise, such a stable chlorinated α-ketocarbene (phenoxyl diradicals) has scarcely been considered to contribute to the formation of PCDD/Fs in the literature. However, its parent structure, 2-oxocyclohexa-3,5-dienylidene, was observed experimentally with a long lifetime in the order of microseconds,25 and proposed as an intermediate forming dibenzofuran and dibenzo-p-dioxin from the pyrolytic thermal degradation of catechol.26 Therefore, we believe that R6 is also an important intermediate during the formation of PCDD/Fs. To form a PCDD/F molecule from R1−R6, the subsequent reactions must occur at the ortho site of the hydroxyl group of 2-CP. Thus only R1, R2, and R6 are potential intermediates for forming PCDD/Fs, while R3−R5 are not direct precursors for the formation of PCDD/Fs although they have potential to form other POPs such as chlorinated naphthalenes, etc.23,24 In 8491



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Figure 2. Schematic representation (a) and the calculated relative energy profiles (b) for the formation of 1-MCDD via radical/radical coupling reactions.

Scheme 2. Resonance Structures for R1 and R6

coupling reactions involving R1 have been studied by Zhang et al.22 in detail. For the sake of comparison, we here calculated the energetically most favorable pathway again. Formation of 1-MCDD. Possible pathways for the formation of 1-MCDD via the radical combination reactions of R1, R2, and R6 are shown schematically in Figure 2a, and the calculated PES profiles are given in Figure 2b. As can be seen from Figure 2a, four potential pathways, denoted as R1+R1, R1+R2-a, R1+R2-b, and R1+R6, are proposed for 1MCDD formation.

the following sections, by considering all possible combinations among R1, R2, and R6, we discuss the formation of 1-MCDD, 1,6-DCDD, 4,6-DCDF, and 4-DCDF, which are among the experimentally observed PCDD/Fs in the oxidation and pyrolysis of 2-CP.4,5,35 It is noted that Zhang et al.22 and Altarawneh et al.10 have theoretically studied the formation of these PCDD/Fs from R1 in detail, and provided valuable insight into the relevant molecular mechanism. Our present work aims at studying new pathways for their formation. Proposed new pathways are based on the self/cross-coupling reactions of R1, R2, and R6. Among these reactions the self8492



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Figure 3. Schematic representation (a) and the calculated relative energy profiles (b) for the formation of 1,6-DCDD via radical/radical coupling reactions.

In the R1+R2 pathway, the initial step is the coupling between the oxygen-centered radical mesomer of R1 with R2, resulting in intermediate IM9. This process is calculated to be also barrierless and strongly exothermic by 74.5 kcal/mol. This value is comparable to the exothermicity (70.7 kcal/mol) calculated by Okamoto et al.9 for the homologous reaction of chlorinated phenyl 3,4-dichloro-2-hydroxyphenyl with 2,4,5trichlorinated phenoxy. From IM9, the reaction evolves into two branches: R1+R2-a and R1+R2-b. The R1+R2-a route goes through an intramolecular elimination of HCl, leading to the formation of 1-MCDD directly. However, this process incurs a high energy barrier up to 63.8 kcal/mol. Alternatively, in the case of R1+R2-b route, the hydroxyl group H atom in IM9 can be abstracted by a OH radical to yield intermediate IM7. The barrier of this process is found to be only 4.5 kcal/ mol. Obviously, the R1+R2-b route is much more favorable than R1+R2-a. Thus the R1+R2 route crosses into the R1+R1 route via transition state TS13 to form the final product, 1MCDD.

The R1+R1 pathway involves the self-coupling reaction of the chlorinated phenoxy radicals. As shown in Scheme 2, R1 is a delocalized radical and has four resonance structures. Thus the oxygen-centered radical mesomer can couple with the ortho carbon−hydrogen centered one, resulting in a keto-ether structure, IM5. This process is calculated to be barrierless and exothermic by 21.1 kcal/mol as shown in Figure 2b. Then an adduct IM6 was formed with the addition of OH to IM5. The intermediate IM6 evolves into the chlorinated diphenyl ether IM7 via transition-state TS9 with a low-energy barrier of 2.0 kcal/mol. Next, the ring closure of IM7 followed with the intra-annular elimination of Cl lead to the formation of 1MCDD. The ring-closure step proceeds via transition-state TS10 with an energy barrier of 27.5 kcal/mol, which is the ratecontrolling step for the formation of 1-MCDD. The Cl elimination step involves a trivial energy barrier of only 0.6 kcal/mol. In the study of Zhang et al.,22 they located the ring closure and the intra-annular elimination of Cl as a one-step reaction with a transition state of 27.41 kcal/mol. 8493



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Figure 4. Schematic representation (a) and the calculated relative energy profiles (b) for the formation of 4,6-DCDF via radical/radical coupling reactions.

From the energy profiles shown in Figure 2b, we find that both the R1+R2-b and the R1+R6 pathways are located to lie below the R1+R1 pathway, the well-accepted one of the formation of PCDD/Fs. Moreover, the R1+R6 route involves the least steps for the formation of 1-MCDD. Therefore, we conjecture that the self-coupling reaction of R1 and its crosscoupling reactions with R2 and R6 all contribute to the formation of 1-MCDD. In other words, apart from R1, both chlorinated phenyl radical and the chlorinated α-ketocarbene also play important roles in the formation of 1-MCDD. Formation of 1,6-DCDD. Figure 3a shows possible pathways for the formation of 1,6-DCDD, including the R1+R1, R1+R2-a, R1+R2-b, R1+R6 and R6+R6 pathways. The corresponding energy profiles are depicted in Figure 3b. It is noted that the R1+R1, R1+R2-a, R1+R2-b, and R1+R6 pathways for the formation of 1,6-DCDD shown in Figure 3a are very similar to those for formation of 1-MCDD shown in Figure 2a. Except for the ring-closed intermediate IM14, the Helimination transition state TS16, and the ring-closing transition state TS17, each species shown in Figure 3 can

The R1+R6 route relates the coupling reaction of the chlorinated phenoxy radical with the chlorinated α-ketocarbene, which has a triplet ground state with the delocalized character. As shown in Scheme 2, the α-ketocarbene has a diradical character with two electrons located on the oxygenand carbon-centers, respectively. Our calculations show that the weight of the diradical form is larger than that of the ketocarbene form (24.90% vs 10.05%), implying the diradical form has larger contribution to the overall structure. Thus the oxygen-center resonance structure of R1 can couple to the carbon center of the diradical form of R6, directly leading to the formation of IM7, the common and necessary intermediate for the formation of 1-MCDD along all three routes shown in Figure 2, releasing an energy of 77.2 kcal mol−1. It should be noted that IM7 is calculated to the overall minimum along the energy surface forming 1-MCDD. From the energy profiles shown in Figure 2, we find that the R1+R6 route is both the thermodynamically and kinetically most favorable one for the formation of 1-MCDD. 8494



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Figure 5. Schematic representation (a) and the calculated relative energy profiles (b) for the formation of 4-MCDF via radical/radical coupling reactions.

find its counterpart in Figure 2. Intermediates IM11−IM13, IM15, and IM16, and transition states TS14, TS15, and TS18 geometrically correspond to intermediates IM5−IM7, IM9, and IM10, and transition states TS9, TS10, and TS13 in Figure 2, respectively. These counterparts are mutual conformers and can transform each other via the rotation of the benzene ring around the C−O bond. The calculated relative energy profiles (Figure 3b) for the formation of 1,6-DCDD are also analogues of those shown in Figure 2b. In this case, the conclusions summarized above from Figure 2 also apply to the formation of 1,6-DCDD. Thus we do not discuss these pathways again. According to Figure 3b, from an energetic point of view, the R1+R2-b pathway is overwhelmingly superior to the R1+R2-a pathway which involves a hydrogen molecule elimination process with a barrier as high as 106.5 kcal/mol−1. Similar to the formation of 1-MCDD, the three available pathways resulting in 1,6-DCDD (R1+R1, R1+R2-b, and R1+R6) involve the most stable intermediate IM13, which lies below the reactants by 135.7 kcal/mol. Once formed, IM13 carries out the ring closure via TS15 with a barrier of 24.5 kcal/mol, resulting in IM14, which is then converted into 1,6-DCDD via the intramolecular Helimination reaction involving TS18 with a barrier of 34.5 kcal/ mol. These barrier values are also in reasonable agreement with those (24.53 and 29.51 kcal/mol) reported by Zhang et al.22

Besides the four paths mentioned above, there is an additional R6+R6 pathway for the formation of 1,6-DCDD, which involves the self-coupling reaction of R6. This process directly results in the formation of 1,6-DCDD with a large energy release of 51.2 kcal/mol. Therefore, the R6+R6 pathway is expected to compete against the three available pathways discussed above. Formation of 4,6-DCDF. The proposed reaction schemes for the formation of 4,6-DCDF are illustrated in Figure 4a, and the relevant relative energy profiles are shown in Figure 4b. As shown in Figure 4a, five pathways, R1+R1, R1+R2, R2+R2-a, R2+R2-b, and R2+R6 were postulated to explain the formation of 4,6-DCDF. We can see that four paths (R1+R1, R1+R2, R2+R2-a and R2+R6) converged to a common intermediate, IM20, which is the prestructure of 4,6-DCDF and lies below the reactants by 149.0 kcal/mol. From the energy profiles shown in Figure 4b, it is found that before IM20 all the four pathways involve small barriers, implying that the formation of the prestructure of 4,6-DCDF would occur readily. Subsequently, through the ring-closure process IM20 evolves into IM21, which then carries out the intramolecular OH elimination step, leading to the final product 4,6-DCDF. The barriers involved in these two successive processes are calculated to be 31.3 and 19.0 kcal/mol, which are larger than those involved in the processes before IM20. Thus, the 8495



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Figure 6. Calculated Gibbs free energy profiles for the formation of 1-MCDD at four different temperatures.

In addition, to show the relative energy change of systems with temperature, we calculated the Gibbs free energy profiles of systems at different temperatures, including room temperature (298 K) and several higher temperatures (600, 800, and 1000 K) relevant to real situations in waste incineration processes. Figure 6, as a representative example, shows the calculated free energy profiles for the formation of 1-MCDD at four different temperatures. The corresponding of those for 1,6DCDD, 4,6-DCDF, and 4-MCDF are given in the Supporting Information (Figures S1−S3) for simplicity. From Figure 6, it is found that the trend of free energy profiles in higher temperatures is consistent with that at room temperature (298 K), although the relative thermodynamic stabilities of respective species change in some extent with temperature. Thus, we believe that the present results are instructive in understanding the formation mechanism of PCDD/Fs in real situations in waste incineration processes. In summary, we have studied theoretically in detail the gasphase formation of four typical PCDD/Fs from 2-CP. It is found that 2-chlorophenoxy radical (R1), the chlorinated phenyl radicals (R2−R5), and the chlorinated α-ketocarbene (R6) can be easily formed via the reaction of 2-CP with OH radical. R6 is found to be a more stable intermediate than R1, while R2−R5 are relatively less favorable in energy. Further, we studied the self- and cross-coupling reactions of R1, R2, and R6 to form 1-MCDD, 1,6-DCDD, 4,6-DCDF, and 4-MCDF. The formation pathways of PCDD/Fs proposed in the present work are proved to be both thermodynamically and kinetically favorable. The most important conclusion drawn out from the present work is that, besides the chlorophenoxy radical, the chlorinated phenyl radicals and the α-ketocarbene also greatly contribute to the formation of PCDD/Fs, which improves our understanding for the formation mechanism of PCDD/Fs from chlorophenol precursors.

ring-closure step is the rate-determining step for the formation of 4,6-DCDF. In addition, it should be noted that the dimerized intermediate IM24, resulting from the self-coupling reaction of R2, may also form 4,6-DCDF via an intramolecular elimination reaction of H2O. This process is denoted as path R2+R2-b. However, this route is found to be energetically very demanding with a significant barrier of 69.6 kcal/mol, which is not competitive with other paths discussed above. Thus we conjecture that all four pathways R1+R1, R1+R2, R2+R2-a, and R2+R6 may contribute to the formation of 4,6-DCDF. In particular, the R2+R6 route involves the direct formation of IM20 without transition states or intermediates, which may be favored over all other pathways. Formation of 4-MCDF. Two possible pathways, R1+R1 and R1+R2, for the formation of 4-MCDF are given in Figure 5. The R1+R1 route contains five elementary steps: the radical coupling, Cl abstraction by an H radical, H shift, ring closure, and intramolecular elimination of OH. While the R1+R2 route involves four basic steps: the radical coupling, Cl abstraction by an H radical, ring closure, and OH elimination. Note that the Cl atom along both the two pathways is abstracted by an H radical, a common radical in the atmosphere, which is proved to be more effective for abstracting the Cl atom at the benzene ring than the OH radical.22 Figure 5b shows the calculated energy profiles. In these two pathways, IM28 and IM31 are identified as the most stable intermediates, which lie below the reactants by 152.8 and 151.9 kcal/mol, respectively. The calculated energy demands for the ring closure processes in the R1+R1 and R1+R2 pathways are 31.0 and 31.2 kcal/mol, respectively, which are regarded as the rate-determining steps for the formation of 4-MCDF. It is evident from Figure 5 that both these two pathways contribute to the formation of 4MCDF. From the calculated results discussed above, it is clear that besides the chlorophenoxy radical (R1), the chlorinated phenyl radical (R2) and the chlorinated α-ketocarbene (R6) are also important precursors of PCDD/Fs. The contributions of these two kinds of intermediates to PCDD/Fs have not fully considered in previous studies. In this respect our present work provides new insight into the formation of PCDD/Fs.

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ASSOCIATED CONTENT

S Supporting Information *

The calculated free energy profiles for the formation of 1,6DCDD, 4,6-DCDF, and 4-MCDF at four different temperatures and all of the optimized geometries in terms of Cartesian 8496



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the radical/radical cross-condensation of 2,4-dichlorophenoxy with 2chlorophenoxy and 2,4,6-trichlorophenoxy. Environ. Sci. Technol. 2011, 45 (2), 643−650. (14) Olie, K.; Addink, R.; Schoonenboom, M. Metals as catalysts during the formation and decomposition of chlorinated dioxins and furans in incineration processes. J. Air Waste Manage. Assoc. 1998, 48 (2), 101−105. (15) Ismo, H.; Kari, T.; Juhani, R. Formation of aromatic chlorinated compounds catalyzed by copper and iron. Chemosphere 1997, 34 (12), 2649−2662. (16) Konduri, R.; Altwicker, E. Analysis of time scales pertinent to dioxin/furan formation on fly ash surfaces in municipal solid waste incinerators. Chemosphere 1994, 28 (1), 23−45. (17) Sidhu, S. S.; Maqsud, L.; Dellinger, B.; Mascolo, G. The homogeneous, gas-phase formation of chlorinated and brominated dibenzo-p-dioxin from 2,4,6-trichloro- 2,4,6-tribromophenols. Combust. Flame 1995, 100, 11−20. (18) Khachatryan, L.; Burcat, A.; Dellinger, B. An elementary reaction-kinetic model for the gas-phase formation of 1,3,6,8- and 1,3,7,9-tetrachlorinated dibenzo-p-dioxins from 2,4,6-trichlorophenol. Combust. Flame 2003, 132 (3), 406−421. (19) Shaub, W. M.; Tsang, W. Dioxin formation in incinerators. Environ. Sci. Technol. 1983, 17 (12), 721−730. (20) Khachatryan, L.; Asatryan, R.; Dellinger, B. Development of expanded and core kinetic models for the gas phase formation of dioxins from chlorinated phenols. Chemosphere 2003, 52 (4), 695− 708. (21) Tuppurainen, K.; Halonen, I.; Ruokojärvi, P.; Tarhanen, J.; Ruuskanen, J. Formation of PCDDs and PCDFs in municipal waste incineration and its inhibition mechanisms: A review. Chemosphere 1998, 36 (7), 1493−1511. (22) Zhang, Q. Z.; Li, S. Q.; Qu, X. H.; Shi, X. Y.; Wang, W. X. A quantum mechanical study on the formation of PCDD/Fs from 2chlorophenol as precursor. Environ. Sci. Technol. 2008, 42 (19), 7301− 7308. (23) Kaiser, R. I.; Parker, D. S. N.; Zhang, F. PAH formation under single collision conditions: Reaction of phenyl radical and 1,3butadiene to form 1,4-dihydronaphthalene. J. Phys. Chem. A 2012, 116 (17), 4248−4258. (24) 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 (11), 2362−2369. (25) Bonnichon, F.; Richard, C.; Grabner, G. Formation of an αketocarbene by photolysis of aqueous 2-bromophenol. ChemComm 2001, 1, 73−74. (26) Lomnicki, S.; Truong, H.; Dellinger, B. Mechanisms of product formation from the pyrolytic thermal degradation of catechol. Chemosphere 2008, 73 (4), 629−633. (27) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Keith, T.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; V. Zakrzewski, G.; Voth, G.; , G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision B.01; Gaussian, Inc.: Wallingford, CT, 2010. (28) Zhao, Y.; Lynch, B. J.; Truhlar, D. G. Development and assessment of a new hybrid density functional model for thermochemical kinetics. J. Phys. Chem. A 2004, 108 (14), 2715−2719. (29) Nicolaides, A.; Smith, D. M.; Jensen, F.; Radom, L. Phenyl radical, cation, and anion. The triplet-singlet gap and higher excited

coordinate for reactants, products, intermediates, and transition states. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; phone: +86-531-88365833; fax: +86-531-88564464 (D.Z.). E-mail: [email protected]; phone: +86-531-88365017; fax: +86-531-88564464 (J.Z.) Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was jointly supported by National Basic Research Program of China (973 Program, 2013CB934301) and National Natural Science Foundation of China (NSFC 21273131 and 21173126). We acknowledge the reviewers of this work, who have substantively improved its quality and significance.

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REFERENCES

(1) Schecter, A.; et al. Exposure assessment: Measurement of dioxins and related chemicals in human tissues. In Dioxin and Health; Schecter, A., Ed.; Plenum Press: New York, 1994; p 449. (2) Stockholm Convention on Persistent Organic Pollutants; United Nations: Stockholm, 2001; http://chm.pops.int/Convention/ ConventionText/tabid/2232/Default.aspx. (3) Wu, Y. L.; Lin, L. F.; Hsieh, L. T.; Wang, L. C.; Chang-Chien, G. P. Atmospheric dry deposition of polychlorinated dibenzo-p-dioxins and dibenzofurans in the vicinity of municipal solid waste incinerators. J. Hazard. Mater. 2009, 162 (1), 521−529. (4) Evans, C. S.; Dellinger, B. Mechanisms of dioxin formation from the high-temperature pyrolysis of 2-chlorophenol. Environ. Sci. Technol. 2003, 37 (24), 1325−1330. (5) Evans, C. S.; Dellinger, B. Mechanisms of dioxin formation from the high-temperature oxidation of 2-chlorophenol. Environ. Sci. Technol. 2005, 39 (1), 122−127. (6) Babushok, V. I.; Tsang, W. Gas-phase mechanism for dioxin formation. Chemosphere 2003, 51 (10), 1023−1029. (7) Briois, C.; Visez, N.; Baillet, C.; Sawerysyn, J. P. Experimental study on the thermal oxidation of 2-chlorophenol in air over the temperature range 450−900 °C. Chemosphere 2006, 62 (11), 1806− 1816. (8) Suárez, E.; Suárez, D.; Menéndez, M. I.; López, R.; Sordo, T. L. Formation of trichlorinated dibenzo-p-dioxins from 2,4-dichlorophenol and 2,4,5-trichlorophenolate: A theoretical study. ChemPhysChem 2006, 7 (11), 2331−2338. (9) Okamoto, Y.; Tomonari, M. Formation pathways from 2,4,5trichlorophenol (TCP) to polychlorinated dibenzo-p-dioxins (PCDDs): An ab initio study. J. Phys. Chem. A 1999, 103 (38), 7686−7691. (10) Altarawneh, M.; Dlugogorski, B. Z.; Kennedy, E. M.; Mackie, J. C. Quantum chemical investigation of formation of polychlorodibenzo-p-dioxins and dibenzofurans from oxidation and pyrolysis of 2chlorophenol. J. Phys. Chem. A 2007, 111 (13), 2563−2573. (11) Zhang, Q. Z.; Yu, W. N.; Zhang, R. X.; Zhou, Q.; Gao, R.; Wang, W. X. Quantum chemical and kinetic study on dioxin formation from the 2,4,6-TCP and 2,4-DCP precursors. Environ. Sci. Technol. 2010, 44 (9), 3395−3403. (12) Qu, X. H.; Wang, H.; Zhang, Q. Z.; Shi, X. Y.; Xu, F.; Wang, W. X. Mechanistic and kinetic studies on the homogeneous gas-phase formation of PCDD/Fs from 2,4,5-trichlorophenol. Environ. Sci. Technol. 2009, 43 (11), 4028−4075. (13) Xu, F.; Yu, W. N.; Zhou, Q.; Gao, R.; Sun, X. Y.; Zhang, Q. Z.; Wang, W. X. Mechanism and direct kinetic study of the polychlorinated dibenzo-p-dioxin and dibenzofuran formations from 8497



Article

states of the phenyl cation. J. Am. Chem. Soc. 1997, 119 (34), 8083− 8088. (30) McFerrin, C. A.; Hall, R. W.; Dellinger, B. Ab initio study of the formation and degradation reactions of chlorinated phenols. J. Mol. Struct. (THEOCHEM) 2009, 902, 5−14. (31) Burcat, A.; Rusci, B. Third Millennium Ideal Gas and Condensed Phase Thermochemical Database for Combustion With Updates from Active Thermochemical Tables; ANL-05/20; Argonne National Laboratory: Argonne, IL, 2001; http://www.chem.leeds.ac.uk/ combustion/combustion.html. (32) Zhu, L.; Bozzelli, J. W. Thermochemical properties ΔfH° (298.15 K), S° (298.15 K), and Cp° (T), of 1,4-dioxin, 2,3benzodioxin, furan, 2,3-benzofuran, and twelve monochloro and dichloro dibenzo-p-dioxins and dibenzofurans. J. Phys. Chem. 2003, 32 (4), 1713−1735. (33) Monks, P. S. Gas-phase radical chemistry in the troposphere. Chem. Soc. Rev. 2005, 34 (5), 376−395. (34) Han, Z.; Zhang, D. J.; Sun, Y. M.; Liu, C. B. Reexamination of the reaction of 4-chlorophenol with hydroxyl radical. Chem. Phys. Lett. 2009, 474, 62−66. (35) Sidhu, S.; Edwards, P. Role of phenoxy radicals in PCDD/F formation. Int. J. Chem. Kinet. 2002, 34 (9), 531−541.

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