Formation of Bromochlorodibenzo-p-dioxins and Furans from the High

Sep 13, 2005 - Response to Comment on “Molecular Mechanism of Dioxin Formation from Chlorophenol based on Electron Paramagnetic Resonance Spectrosco...
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Environ. Sci. Technol. 2005, 39, 7940-7948

Formation of Bromochlorodibenzo-p-dioxins and Furans from the High-Temperature Pyrolysis of a 2-Chlorophenol/ 2-Bromophenol Mixture CATHERINE S. EVANS AND BARRY DELLINGER* Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803

The homogeneous, gas-phase pyrolytic thermal degradation of a 50:50 mixture of 2-bromophenol and 2-chlorophenol was studied in a 1 cm i.d., fused silica flow reactor at a total concentration of 88 ppm, reaction time of 2.0 s, and temperatures from 300 to 1000 °C. Observed products included (in decreasing yield) naphthalene, dibenzo-pdioxin (DD), phenol, dibenzofuran (DF), bromobenzene, chloronaphthalene, 4-bromo-6-chlorodibenzofuran (4-B,6CDF), bromonaphthalene, benzene, 4,6-dichlorodibenzofuran (4,6-DCDF), chlorobenzene, 4-monobromodibenzofuran (4MBDF), 4-monochlorodibenzofuran (4-MCDF), 1-monobromodibenzo-p-dioxin (1-MBDD), 2-chloro,4-bromophenol, 2,4-dibromophenol, and 2-bromo-4-chlorophenol. Unlike the case for the pyrolysis of pure 2-chlorophenol, 4,6-DCDF was observed, but the analogous 4,6-DBDF remained undetected similar to the individual results with 2-MBP. This indicates that the presence of bromine increases the concentration of chlorine atoms available for the formation of 4,6-DCDF. Due to bromine atoms acting as better leaving groups than chlorine atoms, the yield of DD was increased over that observed for the pyrolysis of 2-chlorophenol. The addition of bromine to a chlorinated hydrocarbon system results in an increase in the total yield of PCDD/Fs as well as PBDD/Fs and mixed PBCDD/Fs due to the ease of bromine elimination reactions as well as an increase of the chlorine atom concentration.

Introduction A majority of all brominated hydrocarbons today are used as brominated flame retardants (1). These brominated flame retardants are found in a variety of household items such as computers, TVs, furniture, and other electronic equipment (2-3). Recently, high concentrations of brominated flame retardants were detected in indoor dust and air (4-5). In addition, brominated flame retardants, i.e., polybrominated diphenyl ethers (PBDEs), were observed in human blood and breast milk, and in fish and other wildlife (6-9). The growing presence of these PBDEs is a cause for concern because of their toxicity and their potential to form polybrominated dibenzo-p-dioxins and furans (PBDD/Fs) (1012). PBDD/Fs and polybromochlorodibenzo-p-dioxins and furans (PBCDD/Fs) are considered to have toxicities similar * Corresponding author phone: (225) 578-6759; fax: (225) 5784936; e-mail: [email protected]. 7940

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to those of polychlorinated dibenzo-p-dioxins and furans (PCDD/ Fs) (13-15). Little is known concerning how these brominated hydrocarbons are introduced into mammals whether by food or air (10); but it is generally considered that they are emitted into the atmosphere mainly from electronic waste recycling and other combustion sources (16-18). Chlorinated phenols are key intermediates in essentially all proposed pathways of formation of PCDD/F (19-21). Similarly brominated phenols have been identified as precursors to both PBDEs and PBDD/ Fs (22-25). Some studies have shown that brominated phenols may form more PBDD/Fs than the analogous chlorinated phenols form PCDD/Fs (22, 25). Brominated flame retardants are designed to inhibit complete combustion, so that by their very nature there is a propensity for more products of incomplete combustion to be formed. In fact, it has been proposed that the presence of bromine may enhance PCDD/F formation by facilitating addition of chlorine (12). In addition to formation of PBDD/ Fs and PCDD/Fs, there is also the formation of PCBDD/Fs (18, 26-27). In this paper, we report the results of a controlled study of the high-temperature reactions of an equal part mixture of 2-bromophenol and 2-chlorophenol, model compounds for the combustion of chlorinated and brominated hydrocarbons and precursors for the formation of PCDD/Fs, PBDD/ Fs, and PBCDD/Fs. The thermal degradation was studied under pyrolytic conditions for a reaction time of 2.0 s over the temperature range 300-1000 °C. These conditions are representative of conditions in the post-flame region of incinerators and other types of combustion systems in which byproducts are formed (28). Our results complement and expand upon our previously published work on the individual pyrolysis of 2-chlorophenol (2-MCP) and 2-bromophenol (2-MBP) (29-30). Reaction pathways to PBDD/F, PCDD/F, and PBCD/F products are proposed that are consistent with the experimental data. Pathways to other observed products are also analyzed and discussed in relation to formation of PBDD/Fs, PCDD/Fs, PBCD/Fs, and other toxic air pollutants.

Experimental Section All experiments were performed using a high-temperature flow reactor system referred to in the archival literature as the system for thermal diagnostic studies (STDS). The detailed design has been published elsewhere (31). In short, the STDS consists of a high-temperature, 1 cm i.d., fused silica flow reactor equipped with an in-line Varian Saturn 2000 GC/MS. The flow reactor is housed inside a furnace located inside a Varian GC where the temperatures surrounding the reactor are controlled. Pressure inside the reactor is also maintained at 1.00 ( 0.15 atm. Gas-phase products are cryogenically trapped at the head of the GC column in preparation for chemical analysis. A total of 1.16 × 10-6 mol of liquid 2-MCP and 2-MBP was injected into a helium gas stream by a syringe pump through a vaporizer maintained at 280 °C. To maintain a 50:50 mixture and a constant reactor gas concentration of 88 ppm (44 ppm of 2-MBP (Aldrich) and 44 ppm of 2-MCP (Aldrich)), the syringe pump flow rate was adjusted appropriately to maintain the constant reactor concentration of 88 ppm as the reactor temperature was varied from run to run. Gasphase samples of 2-MBP and 2-MCP were swept by the helium flow through heated transfer lines (300 °C) into a 35 cm long, 1.0 cm i.d., fused silica tubular flow reactor where the controlled high temperature was maintained from 300 to 1000 °C. The helium flow rate was varied with temperature 10.1021/es0510966 CCC: $30.25

 2005 American Chemical Society Published on Web 09/13/2005

FIGURE 1. “Dioxin” products from the gas-phase pyrolysis of 2-MBP and 2-MCP. [2-MBP]0 + [2-MCP]0 ) 88 ppm in helium. Gas-phase reaction time of 2.0 s. so that the residence time inside the reactor was held at 2.0 s. The unreacted 2-MBP and 2-MCP and the thermal degradation products were then swept through a heated transfer line to another Varian GC where they were cryogenically trapped at the head of a CP-Sil 8 phase capillary column (30 m, 0.25 mm i.d., 0.25 µm film thickness). So that the individual reaction products could be separated, the column was temperature programmed from -60 to 300 °C at 15 °C/ min. Detection and quantification of the products were performed using a Varian Saturn mass spectrometer, which was operated in the full-scan mode (40-650 amu) for the duration of the GC run. The length of each experimental run was approximately 45 min. Product concentrations were calculated on the basis of the calibrations with standards of the products (Aldrich and Cambridge Isotope Lab) and the peak area counts from the chromatogram. The yields of the products were calculated using the following expression:

Y ) {([product])/([2-MBP]0 + [2-MCP]0)} × 100 where [product] is the concentration of the particular product formed (in moles), [2-MBP]0 is the initial concentration of 2-MBP (in moles) injected into the reactor, and 2-[MCP]0 is the initial concentration of 2-MCP (in moles) injected into the reactor. Multiple runs were performed for each temperature to ensure the repeatability of the experiments. Once the experimental procedure was fully developed, the repeatability of the experiments was within 10%. Product concentrations (other than PBDD/Fs) were calculated on the basis of the calibrations with standards of the products (Aldrich and Cambridge Isotope Lab) and the peak area counts from the chromatogram. Standards of PBCDD/Fs and PBDD/Fs with less than four bromines were not available. Concentrations of observed PBDD/Fs are reported on the basis of calibrations for the analogous PCDD/ F. This is a reasonably accurate approach as the peak area counts for various chlorinated and brominated aromatics and PCDD/Fs and PBDD/Fs were compared, and it was found that the difference in calibration factors for brominated aromatic hydrocarbons and chlorinated aromatic hydrocarbons varied less than 10%. Only three chromatographic peaks were observed that were tentatively identified as PBDD/Fs or PBCDD/Fs on the basis of their mass spectra: 1-bromodibenzo-p-dioxin,

4-bromo-6-chlorodibenzofuran, and 4-bromodibenzofuran. The mass spectral library match qualities for each of these species were 264, 282, and 248, respectively. These identifications are consistent with products on the basis of reaction pathways from previous research on formation of PCDD/F from 2-MCP and the formation of PBDD/F from 2-MBP (2930). In the 2-MCP study, the PCDD/F standards were available to confirm the identifications on the basis of GC retention time and mass spectral pattern (29). Although standards were not available to confirm the identifications with PBDD/F standards, we are confident in the assignments based on the combination of mechanistically anticipated product formation; comparison of GC retention times, mass spectral response, and mass spectral patterns of chlorinated and brominated hydrocarbons; and previous studies of the formation of PCDD/F from 2-MCP (29). The heats of reaction, ∆Hrxn’s, for key steps in product formation pathways were calculated using AM1, semiempirical molecular orbital formalism. The calculations were performed using the MOPAC computation program that is contained within the ChemBats3D Pro computer application (32). Without experimental benchmarks, the calculated ∆Hrxn cannot be considered to be completely accurate. They are shown to assess the likelihood of potential parallel pathways. Pseudoequilibrium calculations were performed to estimate the concentrations of small reactive radicals and molecules. The Chemkin Equil code was used to calculate the concentrations of these species over a range of reaction temperature from 570 to 1270 K (33). The initial inputs for the calculations were the same as the experimental runs in that the initial concentrations of 2-MBP and 2-MCP were held at 44 ppm each. Over this temperature range, carbon dioxide and water were the expected major product species. Our focus was the reactive species involved in halogenation and PXDD formation including H•, Cl2, Br2, Cl•, and Br•.

Results The temperature dependence of the gas-phase pyrolytic thermal degradation of a 50:50 mixture of 2-MCP and 2-MBP and the yield of PCDD/F and PBDD/F products are presented in Figure 1 and Table 1. Figures 1 and 2 are represented on a semilogarithmic scale in which the percent yields of products (or percent VOL. 39, NO. 20, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Percent Yield of Products of Gas-Phase Pyrolysis of 2-Bromophenol and 2-Chlorophenola temperature (°C) product

350

400

450

500

550

600

650

700

750

800

850

900

2-chlorophenol 2-bromophe nol dibenzo-p-dioxin 1-bromodibenzo-p-dioxin dibenzofuran 4-chlorodibenzofuran 4-bromodibenzofuran bromochlorodibenzofuran 4,6-dichlo rodibenzofuran phenol 2-chloro-4-bromophenol 2-bromo-4-chlorophenol 2,4-dibromophenol naphthalene 1-chloronaphthalene 1-bromonaphthalene acenaphthalene benzene chlorobenzene bromobenzene phenylethyne diphenylethyne benzofuran biphenylene

44.7 53.2 bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl

42.5 53.2 0.11 bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl

43.1 53.9 0.15 bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl

33.5 52.9 0.28 bdl bdl bdl bdl bdl bdl 0.07 0.01 0.004 0.005 0.001 bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl

29.4 38.9 0.42 bdl bdl bdl bdl bdl bdl 0.05 0.005 bdl 0.005 0.001 bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl

28.5 37.8 0.66 bdl bdl 0.002 0.01 0.02 bdl 0.11 0.009 bdl 0.005 0.005 0.007 0.003 bdl bdl 0.002 0.001 bdl bdl bdl bdl

17.6 19.3 0.67 0.002 0.02 0.003 0.02 0.02 0.03 0.15 0.006 0.003 bdl 0.02 0.02 0.006 bdl bdl 0.004 0.008 bdl bdl bdl bdl

19.5 18.0 1.16 0.005 0.06 0.02 0.06 0.16 0.09 0.81 0.009 0.03 bdl 0.41 0.07 0.04 bdl 0.003 0.04 0.04 bdl 0.04 bdl bdl

7.08 9.29 0.89 0.02 0.13 0.03 0.03 0.09 0.08 1.10 bdl bdl bdl 1.68 0.11 0.09 bdl 0.02 0.07 0.09 bdl 0.04 bdl 0.005

0.79 0.71 0.94 bdl 0.23 0.02 0.03 0.03 bdl 0.16 bdl bdl bdl 4.54 0.17 0.15 0.07 0.06 0.07 0.17 0.008 0.07 0.05 bdl

0.26 0.32 0.23 bdl 0.18 0.001 0.003 bdl bdl 0.009 bdl bdl bdl 4.23 0.04 0.03 0.05 0.14 0.03 0.05 0.02 0.008 0.06 bdl

0.12 0.07 bdl bdl 0.19 bdl bdl bdl bdl bdl bdl bdl bdl 1.14 0.002 0.005 0.03 0.07 bdl bdl bdl bdl bdl bdl

a Percent yield ) {([product])/([2-MBP] + [2-MCP] )} × 100. bdl ) below detection limit. The remaining products are presented in Figure 2 and 0 0 Table 1.

FIGURE 2. “Nondioxin” products from the gas-phase pyrolysis of 2-MBP and 2-MCP. [2-MBP]0 + [2-MCP]0 ) 88 ppm in helium. Gas-phase reaction time of 2.0 s. yield of unconverted 2-MCP and 2-MBP) are presented on a logarithmic scale versus temperature. Figure 3 depicts the comparison of PCDD/Fs and PBDD/Fs detected for the pyrolysis of pure 2-MCP and 2-MBP with the 50:50 mixture. The thermal degradation of 2-MBP and 2-MCP tracked each other, gradually increasing from 350 to 700 °C, where the rate drastically accelerated, achieving 99% destruction at 850 °C. Significant yields of dibenzo-p-dioxin (DD) were observed between 350 and 900°C with a maximum yield of 1.16% at 700 °C. This yield is 4 times greater than that for the pyrolysis of 2-MCP and 5 times less in yield than the pyrolysis of 2-MBP (Figure 3) (29-30). The remainder of the PCDD/F and 7942

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PBDD/F products was observed between 600 and 900 °C. The only other PBDD product observed is 1-bromodibenzop-dioxin (1-MBDD) with a maximum of 0.02% at 750 °C, which was also observed for the pyrolysis of 2-MBP with a maximum yield of 0.03% (30). 1-Monochlorodibenzo-pdioxin (1-MCDD), which was observed at a maximum yield of 0.04% at 700 °C for the pyrolysis of 2-MCP, was not observed in this study (29). The observed PCDF and PBDF products were 4,6-dichlorodibenzofuran (4,6-DCDF) (maximum yield of 0.08% at 700 °C), 4-chlorodibenzofuran (4-MCDF) (maximum yield of 0.03% at 750 °C), and 4-bromodibenzofuran (4-MBDF) (maximum yield of 0.06% at 700 °C). 4-MBDF was detected in yields similar to that observed for the pyrolysis of pure 2-MBP with a maximum yield of 0.05% at 600 °C.

FIGURE 3. Comparison of yields of PBDD/Fs and PCDD/Fs from the pyrolysis of pure 2-MBP and 2-MCP (29-30) with the 50:50 Mixture of 2-MCP and 2-MBP. 4,6-Dibromodibenzofuran (4,6-DBDF) remained undetected similar to the previous pyrolysis results with pure 2-MBP (30); however, unlike the pyrolysis of 2-MCP, 4,6-DCDF was observed (29). DF was detected between 650 and 900 °C with a maximum yield of 0.23% at 800 °C. 4-Bromo-6-chlorodibenzofuran (4-B, 6-CDF) was the only mixed bromo-chloro DD/F product observed with a maximum yield of 0.16% at 700 °C Above 950 °C, no brominated or chlorinated aromatics were detected. Many non-PCDD/F and non-PBDD/F products were also detected (cf. Figure 2 and Table 1). Phenol and naphthalene were the only products observed with a maximum yield above 1% with naphthalene having the highest percent yield of 4.54% at 800 °C. Its maximum yield is at least 2 times higher than the maximum yields observed for both pyrolyses of pure 2-MCP and 2-MBP (Figure 3) (29-30). Phenol was observed between 450 and 900 °C with a maximum yield of 1.10% at 750 °C. The chlorinated products included chlorobenzene (maximum yield of 0.07% at 750 °C) and chloronaphthalene (maximum yield of 0.17% at 800 °C). The analogous brominated products, bromobenzene and bromonaphthalene, were observed from 600 to 900 °C, with the maximum yields of 0.17% and 0.15% at 800 °C, respectively. The other observed brominated product was 2,4-dibromophenol with a maximum yield of 0.006% at 550 °C. Products that contained both bromine and chlorine were 2-chloro-4-bromophenol and 2-bromo-4-chlorophenol over a temperature range 500-700 °C with maximum yields of 0.01% and 0.03%, respectively. The remaining products, which are decomposition and molecular growth products of 2-MCP and 2-MBP between 700 and 900 °C, were acenaphthalene, benzene, phenylethyne, diphenylethyne, benzofuran, and biphenylene.

Discussion Formation of “dioxin” products (DD, 1-MBDD, 1-MCDD, 4,6-DCDF, 4-B, 6-DF, 4-MBDF, 4-MCDF, and DF) indicates that stable phenoxyl radicals are formed. The increase in yield of PCDFs over that observed for pure 2-MCP in this mixture suggests the assistance of bromine in their formation. The formation of the aromatics (phenol, halogenated benzenes, and benzene) over a wide range of temperatures

indicates that both substitutions (halogen and hydroxyl radical displacement by hydrogen atoms) are occurring. Bromination is also evident with the formation of 2,4dibromophenol and 2-chloro-4-bromophenol. Chlorination is also present to a lesser extent with the formation of 2-bromo-4-chlorophenol. Low-temperature formation of naphthalene, chloronaphthalene, and bromonaphthalene indicates a recombination mechanism of two phenoxyl radicals that is in direct competition with the formation of PBDD/Fs and PCDD/Fs. Formation of phenylethyne, diphenylethyne, benzofuran, and biphenylene at high temperatures indicates fragmentation of the aromatic ring and molecular growth pathways that may involve smaller, even- or oddnumbered radicals. 2-MBP and 2-MCP Decomposition. Decompositions of 2-MCP and 2-MBP are both initiated by the loss of the phenoxyl hydrogen by unimolecular, bimolecular, or possibly other low energy pathways (including heterogeneous reactions). The activation energies for the unimolecular decomposition of the phenoxyl-hydrogen bond for 2-MCP and 2-MBP are expected to be very similar.

C6H4ClOH f C6H4ClO• + H• ∆Hrxn ) 78 kcal/mol (1a) C6H4BrOH f C6H4BrO• + H•

∆Hrxn ) 79 kcal/mol (1b)

The calculated heats of reaction are consistent with the activation energy for the reported rate coefficient for the decomposition of phenol, kphenol (430-930 °C) ) 3.2 × 1015exp(-86,500 cal/RT) s-1 (34-35). For the mixture, both 2-MCP and 2-MBP decompose at a rate intermediate between their rates as pure compounds for which the rate of decomposition of 2-MBP is slightly faster and that of 2-MCP is slightly slower (29-30). In our previous work, we reported that formation of chlorinated and/or brominated phenoxyl radical is dominated by reactions 1a and 1b, respectively, with contributions from abstraction of VOL. 39, NO. 20, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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the phenoxyl hydrogen by H• (reaction 2a,b) and Cl• (reaction 3a) or Br• (reaction 3b) (29-30).

C6H4ClOH + H• f C6H4ClO• + H2 ∆Hrxn ) -25 kcal/mol (2a) C6H4BrOH + H• f C6H4BrO• + H2 ∆Hrxn ) -23 kcal/mol (2b) C6H4ClOH + Cl• f C6H4ClO• + HCl ∆Hrxn ) -25 kcal/mol (3a) •

•

C6H4BrOH + Br f C6H4BrO + HBr ∆Hrxn ) -8 kcal/mol (3b) Since the strengths of the phenoxyl-hydrogen bonds in 2-MCP and 2-MBP are similar to that in nonhalogenated phenol (∆Hrxn (phenoxyl‚‚‚hydrogen) ) 78 kcal/mol), the rate coefficients for reactions 2a and 2b can be based on analogous reactions with phenol as k2a,b (730-880 °C) )1.15 × 1014exp(-12400/RT) cm3/mol/s (36). Reaction 3a is also estimated to be similar to that for reaction of chlorine atoms with nonhalogenated phenol, k3a (25 °C) ) 1.43 × 1014 cm3/mol/s (36). In a previously studied mixed CHC/BHC system, BrCl was formed along with HCl, HBr, Br2, and Cl2 (12). Equilibrium thermodynamic modeling of the mole fraction of these compounds formed in combustion processes suggests that BrCl is formed in higher yields than Cl2 (12). It was also calculated that, in the presence of oxygen, Br2 is the dominant molecular bromine species whereas HCl is the dominant chlorine species (12). However, Br2 readily dissociates at postcombustion temperatures, making Br• the dominant brominated species overall. Our pseudoequilibrium calculations show that, for the pyrolysis of the mixture of 2-MCP and 2-MBP, Cl• appears at 700 °C compared to the pyrolysis of pure 2-MCP where Cl• is not present (cf. Table 2). This increase is due to Br• reacting with Cl2 with a rate coefficient of k4(293-333 K) ) 4.5 × 1012exp(-6.9 kcal/RT) cm3/mol/s (36) to form more Cl• by reaction 4.

Br• + Cl2 f BrCl + Cl•

(4)

We also attribute a slight increase in the rate of 2-MCP decomposition to the increase in Cl• and the resulting increase in the rate of hydrogen abstraction via reaction 3a. The decrease in the rate of 2-MBP decomposition is attributed to competition for reaction with the reactive radical pool with 2-MCP. Because of the low activation energies for reaction of 2-MBP and 2-MCP, the purely gas-phase reactions described above can account for all the decomposition of 2-MBP and 2-MCP above 750 °C. We have previously shown that, above 725 °C, the purely gas-phase reactions of 2,4,6-trichlorophenol (2,4,6-TCP) can account for its decomposition behavior as well as observed dioxin products (37). However the initiation of the decomposition of 2-MBP and 2-MCP (and 2,4,6-TCP) at temperatures as low as 400 °C requires us to consider other possible low-energy initiation pathways. Heterogeneous wall reactions may result in a wall collision assisted reaction analogous to reactions 1a and 1b. Trace impurities may also result in low-temperature decomposition. The potential contribution of these reactions have been previously analyzed for the oxidation of 2,4,6-trichlorophenol and have been shown not to significantly impact subsequent propagation reactions (38). Formation of Phenol, Chlorobenzene, Bromobenzene, Benzene, Phenylethyne, and Diphenylethyne. With the early 7944

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TABLE 2. Pseudoequilibrium Calculations of HCl, Cl, HBr, and Br for Pyrolysis of Pure 2-MCP (29) 50:50 Mixture of 2-MCP and 2-MBP and Pure 2-MBP (30)a 2-MCP mol fraction (% Cl) 2-MCP Cl2 HCl Cl 2-MBP Br2

8.6 × 10-5 (97%) ND 1.9 × 10-6 (2.2%) ND

HBr Br

2-MCP Cl2 HCl Cl 2-MBP Br2 HBr Br

8.5 × 10-5 (96.5%) ND 1.99 × 10-6 (2.2%) ND

2-MCP/2-MBP mol fraction (% Cl or Br)

2-MBP mol fraction (% Br)

500 °C 0 ND 4.39 × 10-5 (99.7%) ND 0 2.50 × 10-12 (1.1 10-5%) 4.39 × 10-5 (99.7%) 2.36 × 10-11 (5.4 × 10-5%)

8.6 × 10-5 (97%) ND 1.9 × 10-6 (2.2%) ND

700 °C 0 ND 4.3 × 10-5 (99.7%) 1.37 × 10-12 (3.1 × 10-6%) 0 3.3 × 10-11 (1.0 × 10-4%) 4.3 × 10-5 (99.7%) 3.3 × 10-9 (0.007%)

8.5 × 10-5 (96.5%) 9.09 × 10-12 (2.0 × 10-5%) 1.9 × 10-6 (2.2%) 1.2 × 10-9 (0.001%)

a Calculations are in mole fractions, and numbers in parentheses represent the percent yield of chlorine or bromine for each species.

onset of phenol formation at temperatures as low as 500 °C, the dominant source of phenol in this study is probably the exothermic displacement of bromine by H• from either 2-MBP. The displacement of chlorine from 2-MCP is 12 kcal/ mol less exothermic than the displacement of bromine from 2-MBP and therefore less favorable (reaction 4 a,b).

C6H4XOH + H• f C6H5OH + X ∆Hrxn(X)Br) ) -29 kcal/mol (4a) ∆Hrxn(X)Cl) ) -17 kcal/mol (4b) Chlorobenzene and bromobenzene are formed at much lower yields and higher temperatures than phenol. This is due to the more endothermic displacement of the hydroxyl group by H• for both 2-MCP and 2-MBP (reactions 5 a,b).

C6H4XOH + H• f C6H5X + •OH ∆Hrxn(X)Br) ) 2 kcal/mol (5a) ∆Hrxn(X)Cl) ) 0 kcal/mol (5b) Chlorobenzene and bromobenzene are formed over the same temperature range as for the pyrolysis of pure 2-MCP and 2-MBP (29-30). However, bromobenzene has a maximum yield that is 2.5 times higher than chlorobenzene suggesting that the presence of bromine facilitates displacement of hydroxyl radical. Considering the relatively high temperatures (725-875 °C) for formation of benzene, chlorobenzene, and bromobenzene and the formation of phenylethyne and diphenylethyne in the same temperature range, it is evident that 2-MBP and 2-MCP fragment into primarily C2 radicals (vinyl and ethynyl) and C2 molecules (ethylene and acetylene). Molecular growth pathways resulting in formation of benzene and substituted benzene involving C2 units are well documented in the literature (39-41).

SCHEME 1. Postulated Pathways for the Formation of DD, 1-MBDD

Formation of 2,4-Dibromophenol, 2-Chloro-4-bromophenol, and 2-Bromo-4-chlorophenol. 2,4-Dibromophenol and 2-chloro-4-bromophenol are formed from the bromination of 2-MBP. 2-Bromo-4-chlorophenol is formed from the chlorination of 2-MCP. Since displacement of H• by Br• or Cl• is endothermic, the direct reaction of Br• or Cl• with 2-MBP or 2-MCP is unlikely. The formations of dibromophenol and 2-chloro-4-bromophenol are instead due to recombination of 2-bromophenoxyl radicals and Br• at the resonance-stabilized para site (∆Hrxn ) -29 kcal/mol) (cf. Scheme 1 in ref 42). Subsequent tautomerization from the keto to enol form of the recombination product results in the formation of the respective 2,4-dibromophenol and 2-bromo-4-chlorophenol (∆Hrxn )-17 kcal/mol). Bromination of 2-MBP was also evident for the pyrolysis of pure 2-MBP (28); however, chlorination was not observed for the pyrolysis of 2-MCP (29). This is attributed to chlorine being predominantly tied up as stable HCl, whereas HBr is much less stable and not a sink for bromine atoms (12). With the combination of 2-MCP and 2-MBP, there is evidence of chlorination with the formation of 2-bromo-4-chlorophenol. We believe that this is largely appreciable to the presence of bromine that results in formation of BrCl that decomposes to release Cl• (12). In addition, reaction 4 also leads to an increase in the concentration of Cl• (36). Also, our theoretical pseudoequilibrium calculations show an increase in Cl• concentration with the presence of bromine (2-MBP) in the system (cf. Table 2). 2,6-Dibromophenol and 2-bromo-6chlorophenol were not detected, indicating a preference for halogenation at the para site. Formation of Naphthalene, Bromonaphthalene, Chloronaphthalene, and Acenaphthalene. We have previously suggested that the low-temperature formation (500 °C) of naphthalene does not require the complete fragmentation of 2-MBP or 2-MCP and molecular growth pathways involving these fragments (29-30). Instead, low-temperature formation of naphthalene is due to the chloro- and bromo-phenoxyl radicals eliminating CO to form cyclopentadienyl radicals that recombine to form naphthalene by subsequent rearrangement and elimination of hydrogen atoms (43-44). The reported ∆Hrxn for the overall hydrocarbon reaction is 9.23 kcal/mol (43). We believe that the brominated and chlorinated cyclopentadienyl radicals can also recombine to form naphthalene by the elimination of two Cl• or two Br•, and this reaction would occur at a faster rate because of the lower carbon-bromine and carbon-chlorine bond strengths and thermochemistry (45). Chloronaphthalene and bromonaphthalene can be formed in a similar manner with the elimination of one H• and either Cl• or Br•. However, these products are observed at much lower yields than naphthalene due to the more favorable elimination of bromine and chlorine than hydrogen atoms.

The yield of naphthalene in the mixture of 2-MCP and 2-MBP is 2 times greater than that for either of the pure compounds, while the yields of chloronaphthalene and bromonaphthalene are similar to their yields as pure compounds (29-30). This suggests that both chlorine and bromine facilitate naphthalene formation by an increase in the rate of elimination of Br• and Cl• over that of H• in purely hydrocarbon systems. Acenaphthalene is formed at higher temperatures and lower yields than naphthalene, chloronaphthalene, and bromonaphthalene. This suggests that traditional C2 molecular growth pathways form acenaphthalene when 2-MCP and 2-MBP are completely fragmented. Formation of Dibenzo-p-dioxin, 1-Bromodibenzo-pdioxin, 4-Chlorodibenzofuran, 4-Bromodibenzofuran, 4,6Dichlordibenzofuran, 4-Bromodibenzofuran, 6-Chlorodibenzofuran, and Dibenzofuran. Consistent with our previous results for pure 2-MCP and 2-MBP, DD was observed as the major dioxin product with a yield intermediate between that observed for 2-MCP and 2-MBP (Figure 3). 1-MBDD and DF were also observed in yields similar to that observed in both the 2-MCP and 2-MBP system. Surprisingly, 1-MCDD, which was observed in the previous study of 2-MCP, was not observed. However, 4,6-DCDF (not observed in the 2-MCP system) and 4-B,6-CDF were observed, while the analogous 4,6-DBDF was not observed (also not observed in the 2-MBP system) (29-30). With the exception of DF, formation of PCDD/Fs and PBDD/Fs occurs at low temperatures. The higher temperature formation of DF, which was also observed in the studies of pure 2-MCP and 2-MBP (29-30), suggests it is formed by recombination of nonhalogenated phenoxyl radicals (46) formed by the displacement of chlorine or bromine from 2-MCP or 2-MBP. Scheme 1 depicts the possible reaction pathways for the brominated phenoxyl radical to form DD and 1-MBDD. In the uppermost pathway, the oxygen-centered radical mesomer recombines with the carbon- (bromine-) centered radical mesomer to form a keto-ether. Following the abstraction of bromine by H•, DD is formed by intra-annular elimination of Br•. DD can also be formed from the chlorinated phenoxyl radical in a similar manner. Formation of 1-MBDD, shown as the second pathway in Scheme 1, is initiated by the recombination of the oxygencentered radical mesomer and the carbon- (hydrogen-) centered radical mesomer to form a keto-ether. Following loss of hydrogen to form the phenoxyl diphenyl ether (PDE), ring closure to form 1-MBDD occurs through intra-annular displacement of Br•. The analogous 1-MCDD can, in principle, be formed from the chlorinated phenoxyl radical by this same pathway. However, it was not observed, suggesting another fate for chlorinated phenoxyl radicals. VOL. 39, NO. 20, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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SCHEME 2. Postulated Pathways for the Formation of 4,6-DCDF and 4-B,6-CDF

SCHEME 3. Postulated Pathways for Formation of 4-MCDF and 4-MBDF

The yield of DD was 5 times less than that for pure 2-MBP and 4 times greater than that for pure 2-MCP (29). It is believed that the formation of DD is due largely to the recombination of brominated phenoxyl radicals because AM1 calculations for the reactions to form DD show the ∆Hrxn for the second displacement of Br to form DD is exothermic (-12 kcal/mol) and the ∆Hrxn displacement of Cl is endothermic (11 kcal/ mol). The 5 times lower yield of DD in the mixture is attributed to the 50% lower concentration of 2-MBP in the mixture. If the rate of formation of DD is approximately second order in the concentration of bromophenoxyl radicals, as it should be based on the recombination mechanism, then the yield of DD should be approximately a factor of 4 lower in the mixture than for the pure 2-MBP. 1-MBDD is observed at much lower yields than DD, and 1-MCDD is not observed at all. These observations are surprising since in our previous study of the pyrolysis of 2-MCP, 1-MCDD was observed in yields slightly higher than the yields of 1-MBDD in the pyrolysis of just 2-MBP (29-30). The maximum yield of 1-MBDD is similar to the yield of 1-MBDD observed in the pyrolysis of 2-MBP (30). These observations suggest that formation of 1-MBDD is not affected by the addition of chlorine as 2-MCP, but the formation of 1-MCDD is affected by the addition of bromine as 2-MBP. Instead, 4,6-DCDF and 4-MCDF were observed, neither of which were detected during the pyrolysis of pure 2-MCP (29). Scheme 2 depicts the possible pathways to the formation of 4,6-DCDF or 4-B,6-CDF. Initially, two carbon- (hydrogen-) centered radical mesomers of the chlorinated phenoxyl (or one chlorinated phenoxyl and one brominated phenoxyl) radical react to form the diketo-dimer. The dimer then reacts through abstraction of hydrogen by H• or Cl•, followed by tautomerization and displacement of the hydroxyl group to form 4,6-DCDF or 4-B,6-CDF. It was previously stated that the presence of bromine increases the Cl• concentration (12). Chlorine atoms facilitate the abstraction of hydrogen atoms in Scheme 3 to form 4,6DCDF and 4-B,6-CDF. However, hydrogen atoms, not chlorine atoms, are involved in the pathways to formation of DD. Thus, the presence of bromine increases the formation 7946

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of 4,6-DCDF and 4-B,6-CDF under pyrolytic conditions by increasing the concentration of chlorine atoms. The 4-B,6-CDF is formed in the highest yields of all the furans, with 4,6-DCDF being formed at lower yields, and 4,6-DBDF not observed at all. The results of our AM1 calculations yield very similar ∆Hrxn values for pathways to 4-B,6-CDF, 4,6-DCDF, and 4,6-DBDF. Brominated phenoxyl radicals predominantly react to form DD instead of 4,6-DBDF. However, a significant fraction also react with chlorinated phenoxyl radicals that can form 4-B,6-CDF due to the increase in reactive chlorine atoms that facilitates ring closure. Both 4-MBDF and 4-MCDF were observed in this study. However, 4-MCDF was not detected from the pyrolysis of 2-MCP, and 4-MBDF was observed for 2-MBP (29-30). Scheme 3 depicts reasonable pathways for the formation of 4-MBDF and 4-MCDF. Two pathways are depicted: (1) the carbon- (hydrogen-) centered radical mesomer recombination with the carbon(halogen-) centered radical mesomer or (2) the carbon (hydrogen)-centered radical mesomer recombination with an unhalogenated carbon-centered phenoxyl radical to form a diketo-dimer. For the first pathway, H• abstracts either chlorine or bromine, and in the second, H• or Cl• abstracts another hydrogen. Both pathways then undergo tautomerization followed by displacement of hydroxyl to form 4-MBDF or 4-MCDF. Again, with the surprising formation of 4-MCDF, it is clear that bromine plays a roll in the formation of the PCDD/Fs by assisting in formation of more Cl•, which facilitates reactions requiring hydrogen abstraction. This study demonstrates some of the effects brominated hydrocarbons have on the overall distribution and formation of PCDD/Fs as well as PBDD/Fs: (1) bromine inhibits complete combustion creating more combustion byproducts that will lead to the formation of PCDD/Fs and PBDD/Fs as well as other polycyclic aromatic hydrocarbons, and (2) bromine increases the concentration of Cl• via reaction 4 that promotes formation of the PCDFs and decreases the PCDD to PCDF ratio. Reaction kinetic models under both oxidative and pyrolytic conditions are needed to better understand the competition between molecular growth pathways to form PAH and the relative yield of brominesubstituted dibenzo-p-dioxin products and bromine-sub-

stituted dibenzofuran products. Since it is well-documented that PCDD/F species form under post-flame, surfacecatalyzed, or mediated conditions (47-50), and we have demonstrated that PBDD/Fs are formed via surface-mediated reactions of 2-bromophenol (51), the surface-mediated reactions of mixed chlorine and bromine systems clearly deserve detailed study.

Acknowledgments We gratefully acknowledge the assistance of our colleagues, Dr.Lavrent Khachatryan and Alexander Burcat, in evaluation of the thermochemistry presented in this paper as well as helpful discussions concerning the mechanisms of dioxin formation.We acknowledge the partial support of this work under EPA cooperative agreement CR 827030 with Dr. Paul Lemieux as well as the Patrick F. Taylor Chair foundation.

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Received for review June 9, 2005. Revised manuscript received July 30, 2005. Accepted August 5, 2005. ES0510966