Surface-Mediated Formation of Polybrominated Dibenzo-p-dioxins

May 20, 2005 - We studied the surface-mediated pyrolytic thermal degradation of 2-bromophenol, a model brominated hydrocarbon that may form brominated...
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Environ. Sci. Technol. 2005, 39, 4857-4863

Surface-Mediated Formation of Polybrominated Dibenzo-p-dioxins and Dibenzofurans from the High-Temperature Pyrolysis of 2-Bromophenol on a CuO/Silica Surface CATHERINE S. EVANS AND BARRY DELLINGER* Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803

We studied the surface-mediated pyrolytic thermal degradation of 2-bromophenol, a model brominated hydrocarbon that may form brominated dioxins in combustion and thermal processes, on silica-supported copper oxide in a 1 mm i.d., fused silica flow reactor at a constant concentration of 88 ppm over a temperature range of 250550 °C. Observed products included dibenzo-p-dioxin (DD), 1-monobromodibenzo-p-dioxin (1-MBDD), dibromodibenzo-p-dioxin (DBDD), tribromodibenzo-p-dioxin (TrBDD), 4-monobromodibenzofuran (4-MBDF), dibenzofuran (DF), 2,4,6-tribromophenol, 2,4- and 2,6-dibromophenol, and polybrominated benzenes. These results are compared and contrasted with previous work on surface-catalyzed pyrolysis of 2-chlorophenol. Polybrominated dibenzofurans (PBDFs) are formed by the Langmuir-Hinshelwood mechanism, and the formation of polybrominated dibenzop-dioxins (PBDDs) is through an Eley-Rideal mechanism. Yields of PBDDs are at least 16× greater for 2-bromophenol than for analogous PCDDs from 2-chlorophenol. Higher yields of polybrominated phenols and polybrominated benzenes are also observed. This can be attributed to the relative ease of bromination over chlorination and the higher concentration of bromine atoms in the 2-bromophenol system versus chlorine atoms for the 2-chlorophenol system.

Introduction Since the 1990s, there has been an increase in concern over the risk of brominated flame retardants and their presence in a wide variety of materials and organisms (1-4). Materials containing brominated chemicals are frequently disposed of in thermal treatment facilities. Previous studies have shown that combustion of brominated flame retardants as well as brominated phenols can form polybrominated dibenzo-pdioxins and dibenzofurans (PBDD/Fs) (5-9). Since the presence of bromine inhibits combustion, the likelihood of forming products of incomplete combustion will increase, resulting in a greater potential for emission of PBDD/Fs. Brominated flame retardants are added to materials that are subject to possible burning where they may be subject to * Corresponding author phone: (225) 578-6759; fax: (225) 5783458; e-mail: [email protected]. 10.1021/es048057z CCC: $30.25 Published on Web 05/20/2005

 2005 American Chemical Society

combustion. Since the flame retardants mitigate rapid combustion, these materials may be subject to lower temperature thermal degradation that actually increases the likelihood of formation of products of incomplete combustion or degradation (10, 11). Previous research has shown that brominated phenols can form higher yields of PBDD/Fs than the analogous chlorinated phenols form polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs) (12-14). It is important to note that the toxicity of the PBDD/Fs has been shown to be similar to that of the analogous PCDD/Fs (15, 16). Furthermore, it has been noted that the addition of bromine to a combustion source can increase the emissions of PCDD/Fs as well as mixtures of brominated and chlorinated dibenzo-p-dioxins and dibenzofurans (PBCDD/Fs) (2, 17). While gas-phase reactions of bromophenols to form PBDD/Fs have been studied, the surface-mediated reactions to PBDD/F have not been carefully addressed (12-14). It has been stated that gas-phase formation pathways are only responsible for about 30% of the total dioxin emissions (18, 19), with surface-mediated reactions being responsible for the remaining 70%. The significant difference in these reactions is that while gas-phase reactions typically occur at temperature higher than 600 °C in the combustion zone, surface-mediated reactions can occur at temperatures between 250 and 600 °C. There are two notable surface processes, de novo formation from reaction of a halogen (chlorine or bromine) and oxygen with carbon in fly ash (20-22) and transition-metal-mediated reactions of precursors (23-26). Recent work on the surface-mediated pyrolysis of the model dioxin precursor, 2-chlorophenol, led to proposed mechanisms for surface-mediated formation of PCDD/F (24, 25). In this work the model surface used was 5% copper(II) oxide on a silica surface. By using a simplified surface as a model for fly ash, the effects of other unknown species present in fly ash were avoided. It has also been noted that copper and iron ions are probably the most active metals in PCDD/F formation (27). Instead of pure copper oxide, the mixture with silica allows the study of small surface domains for welldispersed CuO species that emulate the conditions for combustion-generated fly ash. In this paper, we report the results of the pyrolytic thermal degradation of 2-bromophenol as a model precursor for surface-mediated combustion reactions to form PBDD/Fs. The surface-mediated reactions were studied over a temperature range from 250 to 550 °C, which is typical of postcombustion temperatures where surface-mediated reactions are know to occur (20-26). Regardless of nominal, average oxygen concentrations in postcombustion zones, a range of oxygen concentrations exists due to less than perfect fuel-air mixing. Pyrolysis represents one extreme. A subsequent paper will address oxidation reactions. The simplified model surface of 5% copper (II) oxide on silica was also used in order to facilitate comparison with previous results from similar experiments with 2-chlorophenol (24). Reaction pathways to PBDD/F products as well as the intermediates involved are analyzed and discussed.

Experimental Section The catalytic material was prepared by the method of incipient wetness. A water solution of copper(II) nitrate (Aldrich) with a concentration chosen to obtain a 5% copper in silica system was used as the active-phase precursor. Silica powder (Aldrich, 500 m2/g) was introduced into the equivaVOL. 39, NO. 13, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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lent volume of precursor solution for incipient wetness to occur. The solution was well mixed and dried at 120 °C for 24 h. All experiments were performed on a well-characterized flow reactor system previously referred to as the system for thermal diagnostic studies (STDS) (28). For the surfacecatalyzed reactions, 1 mg of the catalytic material was placed between quartz wool plugs in a 1 mm i.d. fused silica reactor in the STDS. All transfer lines were maintained at a constant temperature of 250 °C. Prior to each flow-reactor experiment, the catalytic material was oxidized in situ at 500 °C for 1 h with a 20% O2/He flow rate of 5.5 cm3/min to convert the copper nitrate to CuO. For each experimental run, 2-bromophenol (2-MBP) (Aldrich) was introduced into a helium stream by a syringe pump through a vaporizer maintained at 200 °C. The rate of injection was selected to maintain a constant concentration of 88 ppm for temperatures ranging from 250 to 550 °C. The unreacted 2-MBP and the thermal oxidation 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 (30m, 0.25 mm i.d., 0.25 µm film thickness). To separate the individual reaction products, the column was temperatureprogrammed from -60 to 300 °C at 15 °C/min. Detection and quantification of the products were obtained on a Varian Saturn mass spectrometer, which was operated in full-scale mode (10-650 amu) for the duration of the GC run. The yields of the products were calculated by use of the expression:

FIGURE 1. PBDD/F products from surface-catalyzed thermal degradation of 2-bromophenol under pyrolytic conditions. Gas-phase reaction time was 2.0 s.

Y ) {[product]/[2-MBP]0} × 100 where [product] is the concentration of the particular product formed (in moles) and [2-MBP]0 is the initial concentration of 2-MBP (in moles) injected into the reactor. Product concentrations (other than PBDD/Fs) were calculated on the basis of calibrations with standards of the products (Aldrich and Cambridge Isotope Lab) and the peak area counts from the chromatogram. Standards for 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 four chromatographic peaks were observed that were tentatively identified as PBDD/F on the basis of their mass spectra: 1-bromodibenzo-p-dioxin, dibromodibenzo-pdioxin, tribromodibenzo-p-dioxin, and 4-bromodibenzofuran. The mass spectral library match qualities for each of these species were 264, 343, 422, and 248, respectively. These products are four PBDD/Fs that were anticipated on the basis of the predicted pathways from previous research on formation of PCDD/F from the analogous 2-chlorophenol (25, 29). In the 2-chlorophenol study, the PCDD/F standards were available to confirm the identifications based on GC retention time and mass spectral pattern. 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-chlorophenol (24).

Results The temperature dependence of the surface-mediated pyrolytic thermal degradation of 2-MBP and the yield of PBDD/F 4858

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FIGURE 2. Non-PBDD/F products from surface-catalyzed thermal degradation of 2-bromophenol under pyrolytic conditions. [2-MBP]0 ) 88 ppm in helium. Gas-phase reaction time was 2.0 s. products are presented in Figure 1 and Table 1. The remaining detected products are presented in Figure 2 and Table 1. Figures 1 and 2 are presented on a semilogarithmic scale in which the percent yields of products (or percent yield of unconverted 2-MBP) are on a logarithmic scale versus temperature. The thermal degradation for 2-MBP increased rapidly from 250 to 550 °C, achieving 99% destruction at the low temperature of 550 °C. Dibenzo-p-dioxin (DD), 1-monobromodibenzo-p-dioxin (1-MBDD), dibromodibenzo-p-dioxin (DBDD), tribromodibenzo-p-dioxin (TrBDD), 4-monobromodibenzofuran (4MBDF), and dibenzofuran (DF) were the PBDD/F products observed. DD, 1-MBDD, and DBDD were all observed between 250 and 500 °C with maximum yields of 1.90%, 1.66%, and 2.98% at 350 °C, respectively. 4-MBDF (maximum yield of 0.004 at 400 °C) and DF (maximum yield of 0.04 at 450 °C) were observed over a narrower temperature range of 350-450 °C. TrBDD was also detected at 400 °C at very low yields. No brominated aromatics were observed above 600 °C. Non-PBDD/F products were also detected from the surface-catalyzed pyrolysis of 2-MBP (Figure 2 and Table 1). The phenol products, 2,4-dibromophenol, 2,6-dibromophenol, and 2,4,6-tribromophenol, were detected between 250 and 500 °C. 2,6-Dibromophenol achieved a maximum yield of 4.94% at 300 °C, whereas 2,4-dibromophenol achieved its maximum yield of 2.99% at 350 °C. 2,4,6-Tribromophenol reached a maximum yield of 3.79 at 350 °C. Bromobenzene (maximum yield of 0.25% at 400 °C) and 1,2-dibromobenzene (maximum yield of 0.34% at 350 °C) were detected between 250 and 475 °C. 1,2,4-Tribromobenzene (maximum yield of

TABLE 1. Percent Yielda of Products of Surface-Catalyzed Gas-Phase Pyrolysis of 2-Bromophenol pyrolysis temp (°C)

2-bromophenol dibenzo-p-dioxin 1-bromodibenzo-p-dioxin dibromodibenzo-p-dioxin dibenzofuran 4-bromodibenzofuran 2,4-dibromophenol 2,6-dibromophenol 2,4,6-tribromophenol bromobenzene 1,2-dibromobenzene 1,2,4-tribromobenzene 1,2,3,5-tetrabromobenzene bromoform dibromonaphthalene tribromoethane bromoquinone dibromoquinone a

250

300

350

400

450

475

500

550

68.9 0.27 0.47 0.14

15.5 0.80 1.60 2.62

4.84 1.90 1.66 2.98

0.34 0.32 0.14 0.24

0.02 0.14 0.006 0.003

0.02 0.03

0.002

0.002 2.99 2.96 3.79 0.03 0.34 0.03 0.02 0.002 0.50 0.009 0.19 0.31

0.004 0.29 0.07 0.03 0.25 0.15 0.02 0.02

0.08 0.27 0.09 0.48 0.04 0.002 0.03 0.04 0.05 0.05 0.05 0.01

0.007 0.005 0.01

0.005 0.004

0.003 0.003

0.06 0.44 0.01 0.002

0.001 0.04

1.52 4.94 1.23 0.003 0.13 0.007 0.27 0.003 0.13 0.004 0.13 0.11

0.03

600

0.001

0.03

0.01 0.15

Percent yield ) ([product]/[2-MBP]0) × 100.

0.03% at 350 °C) and 1,2,3,5-tetrabromobenzene (maximum yield of 0.27% at 300 °C) were observed between 300 and 450 °C. Dibromonaphthalene achieved a maximum yield of 0.50% at 350 °C and was observed between 300 and 450 °C. Bromoform, tribromoethane, bromoquinone, and dibromoquinone were the remaining products, all detected between 300 and 400 °C.

Discussion Low-temperature decomposition of 2-MBP by 500 °C indicates that the surface is indeed mediating its reaction. Formation of PBDD/Fs between 250 and 500 °C suggest that reaction intermediates, that is, bromophenoxyl radicals, are stabilized on the CuO/silica surface. The proposed surface mediated mechanisms for the formation of the PBDD/Fs are the Eley-Rideal pathway for PBDDs and the LangmuirHinshelwood pathway for PBDFs (30). The presence of relatively high concentrations of polybrominated phenols and brominated benzenes indicate the ease of surfacemediated bromination. Adsorption and Reaction of 2-MBP on a CuO/Silica Surface. In order for the oxidation of 2-MBP or further reaction to form PBDD/Fs and other brominated aromatics to occur at low temperatures, 2-MBP must attach to the surface. For chlorinated phenols, this has been shown to occur through chemisorption at copper hydroxide (cf. Figure 3) or copper oxide sites through loss of water. This results in a surface-adsorbed bromophenolate chemically bound to a Cu(II) species (cf. I in Figure 3). With the assistance of the copper oxide surface, 95% of 2-MBP is destroyed by 350 °C. The surface oxygen from the copper oxide surface assists in the oxidation of 2-MBP according to the Mars-van Krevelen mechanism (31), whereby the surface oxygen atoms are consumed by the reagents, leaving oxygen vacancies. Above 500 °C, the Cu(II)O surface assists in the destruction of all PBDD/Fs formed as well as the other products formed at the lower temperatures. It has been well documented that 2-chlorophenol (2-MCP) can chemisorb to a copper oxide surface (24, 32). Once 2-MCP is chemisorbed to the Cu(II)O surface, it donates an electron to the copper that is reduced to Cu(I) (32). The chlorophenolate structure becomes a chlorophenoxylate radical that is resonance-stabilized between the ortho and para aromatic carbons and the oxygen-centered radical mesomers. Electron paramagnetic resonance (EPR) studies where 2-MCP is absorbed to a copper oxide surface suggests a carbon-

FIGURE 3. Mechanism of adsorption of 2-MBP to Cu(II)O/silica surface. The bromophenolate (intermediate I) donates an electron to Cu(II), which reduces to Cu(I), forming a surface-associated bromophenoxyl radical that can exist as an oxygen-centered or carbon-centered mesomer (resonance structure intermediates II). centered radical (25). The results of these studies suggested that the carbon-centered radical becomes the dominant resonance structure. Figure 3 depicts the similar chemisorption of 2-MBP. Since 2-chlorophenol and 2-bromophenol are very similar in structure, it is expected for 2-bromophenol to behave similarly to form a carbon-centered radical. Formation of 2,4-Dibromophenol, 2,6-Dibromophenol, and 2,4,6-Tribromophenol. Significant quantities of polybrominated phenols are observed at temperatures as low as 250 °C. These products are likely formed from the bromination of 2-MBP after they are chemisorbed to the surface. As shown in Figure 4, Br• can attack the carbon-centered bromophenoxyl radical at the ortho or para sites and then form 2,4-dibromophenol and 2,6-dibromophenol via tautomerization. Additional bromination requires abstraction of a hydrogen and addition attack of bromine. 2,6-Dibromophenol has the highest yield of the polybrominated phenols, which is attributable to the relative ease VOL. 39, NO. 13, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Proposed mechanisms for formation of 2,4-dibromophenol, 2,6-dibromophenol, and 2,4,6-tribromophenol.

FIGURE 5. Proposed mechanisms for formation of brominated benzenes. of tautomerization to the nearby ortho carbon. It has previously been suggested that the ortho position for the carbon-centered phenoxyl radical is the most favorable site for reaction (25, 33). Formation of Bromobenzene, 1,2-Dibromobenzene, 1,2,4-Tribromobenzene, and 1,2,3,5-Tetrabromobenzene. The formation of the polybrominated benzenes is depicted in Figure 5. Once 2-MBP is attached to the surface, bromination of the phenoxyl radical proceeds by a mechanism 4860

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similar to that responsible for formation of polybrominated phenols. The polybrominated benzenes observed in the gas phase can then be formed by the displacement by hydrogen or bromine. The yields of brominated benzenes are lower than for brominated phenols as result of displacement via attack at the phenyl-oxygen bond being slower than at the weaker phenoxyl-surface bond. Formation of Bromoquinone and Dibromoquinone. Figure 6 depicts pathways for formation of 3-bromo-3,5-

FIGURE 6. Proposed mechanisms for bromoquinone and dibromoquinone formation.

FIGURE 7. Proposed Langmuir-Hinshelwood mechanisms for 4,6-DBDF formation. cyclohexadiene-1,2-dione (bromoquinone) and 3,5-dibromo3,5-cyclohexadiene-1,2-dione (dibromoquinone). These pathways are proposed on the basis of previous work for 2-MCP that implies further reduction of Cu(I) (formed after electron transfer from the adsorbed 2-MCP) to Cu(0) and a gas-phase quinone (24, 34). Bromoquinone and dibromoquinone are possible intermediates in pathways to formation of PBDD. Formation of Dibenzo-p-dioxin, 1-Bromodibenzo-pdioxin, Dibromodibenzo-p-dioxin, 4-Bromodibenzofuran, and Dibenzofuran. There are two general schemes for surface-mediated reactions: Langmuir-Hinshelwood and Eley-Rideal (30). It has previously been proposed that formation of PCDDs from chlorophenols occurs via an Eley-Rideal mechanism, in which the reaction occurs between an absorbed species and a gas-phase species, and formation of PCDFs from chlorophenols occurs by the Langmuir-Hinshelwood mechanism, which is the reaction between two absorbed species (30). Because of the similarity in structures of 2-MCP and 2-MBP, these same pathways are likely responsible for formation of PBDD/Fs from the surfacecatalyzed reactions of 2-MBP. Figure 7 depicts the Langmuir-Hinshelwood mechanism for formation of PBDFs. Even though the expected 4,6dibromodibenzofuran (4,6-DBDF) was not detected, unbrominated DF and 4-MBDF were observed at 450 °C (24). Once 2-MBP is absorbed to the surface and forms the carboncentered radical structure (cf. Figure 3), two of these surfacebound carbon-centered phenoxyl radicals undergo recombination and tautomerization to form intermediate 2 in Figure 7. Intermediate 2 then eliminates hydrogen and copper(I) is oxidized back to copper(II) (intermediate 3 in Figure 7). 4,6DBDF is then formed by ring closure through a cyclic transition state that results in the regeneration of copper(II) hydroxide. It is believed that unbrominated phenols follow a similar pathway to form DF. The formation of 4-MBDF occurs by reaction of one brominated phenoxyl radical and an unbrominated phenoxyl radical or the recombination of two bromophenoxyl radicals to form intermediate 2 in Figure 7, which will eliminate bromine instead of hydrogen. It is

likely that the recombination of the two surface-bound bromophenoxyl radicals is the favorable route in that the elimination of bromine is more favorable than that of hydrogen due to the lower bond strength of the carbonbromine bond (35). In fact, 4-MBDF is more likely to form than 4,6-DBDF because bromine plays a role in the formation of 4-MBDF, whereas it does not for the formation of 4,6DBDF. PBDFs were formed in very low yields, indicating that the Langmuir-Hinshelwood pathways are not favorable for 2-MBP. In contrast to the Eley-Rideal mechanism and the formation of PBDDs, bromine has very little effect on the formation of PBDFs. Bromine acts as a better leaving group than chlorine due to the lower bond strength of the carbonbromine bond (35). In addition, the abstraction of bromine by H• to form the PBDDs is more favorable than the abstraction of hydrogen by H•. Also, the formation of PBDFs requires the presence of two surface-bound bromophenolates, whereas the formation of PBDDs requires one surfacebound bromophenolate and a gas-phase bromophenoxyl radical. Under pyrolytic conditions, the surface oxygen acts in the oxidation of 2-MBP and thus leaves vacancies. This lowers the number of available sites and thus lowers the potential for the Langmuir-Hinshelwood mechanism to form PBDFs. Formation of 1-MBDD can occur by two pathways. The first pathway, depicted in Figure 8, represents the first of the two plausible Eley-Rideal mechanisms. The absorbed bromophenoxyl radical can react with another terminal oxygen and (1) eliminate a hydrogen and desorb to form bromoquinone (Figure 6) or (2) tautomerize to surface intermediate 1 depicted in Figure 8. Once intermediate 1 is formed, 2-MBP can react with intermediate 1 at the sites of bromine or hydroxyl substitution. To form 1-MBDD, 2-MBP reacts with intermediate 1 at the hydroxyl site and forms surface-bound intermediate 2 via HBr elimination. Intermediate 2 then forms a non-surface-bound 1-MBDD through a cyclic transition state. VOL. 39, NO. 13, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 8. Proposed Eley-Rideal mechanism for 1-MBDD formation.

FIGURE 9. Proposed Eley-Rideal mechanism for DD and PBDD formation. The remaining PBDDs, DD, DBDD, and TrBDD, as well as 1-MBDD, can be formed by the second plausible EleyRideal pathway depicted in Figure 9. Instead of 2-MBP reacting with intermediate 1 at the hydroxyl-substituted site, it reacts at the bromine-substituted site, eliminating HBr to form intermediate 2 in Figure 9. Intermediate 2 undergoes ring closure via HBr elimination to form a surface-bound DD. DD can then be desorbed or undergo bromination and desorption to form higher brominated PBDDs, DBDD and TBDD. Higher brominated PBDDs can also be formed by reaction of more highly brominated phenols (12). The formation of DBDD exhibits two maxima, 350 and 450 °C. On the basis of the higher yields of dibromophenols and tribromophenols at lower temperatures, they can contribute to the PBDD formation at the lower temperatures, whereas bromination of initially formed DDs may be responsible for the higher temperature PBDDs. Similar observations were made in our previous work on 2-MCP (24); however, the contributions to PCDD formation from higher chlorinated phenols were small due to their low yields. It is well documented that bromination is 10 times more efficient than chlorination (6). This is attributable to the dominant halogen for bromine systems being Br2 versus HCl for chlorine systems (14). In addition, on the basis of our pseudo-equilibrium gas-phase calculations for the pyrolysis of 2-MBP, the concentration of Br• for temperatures between 300 and 1000 °C is at least 100 times greater than the concentration of Cl• for the analogous 2-MCP (13). In fact at 700 °C, Cl• and Cl2 have not even become present, whereas Br• and Br2 achieve concentrations of 1.18 × 10E-14 mol/cm3 and 4.08 × 10E-17 mol/cm3 respectively (13). In the presence of a surface the radical pool of Br• should only increase, since the surface assists in the decomposition of 2-MBP, thereby increasing the radicals present in the system. Br2 readily dissociates to form Br•, whereas HCl (the highest concentration chlorine-containing molecule in the 2-MCP system) is a more stable sink of Cl• (35). Therefore, the higher yields of polybrominated phenols and PBDDs are expected because of the higher concentration of Br2/Br• than Cl2/Cl• for 2-MCP. When the surface-mediated studies of 2-MCP and 2-MBP are compared, the maximum yield of DD for 2-MBP is 16 4862

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times greater than for 2-MCP and the maximum yield of 1-MBDD is 30 times higher than the maximum yield of 1-MCDD (24). A similar result was observed in the gas-phase comparison of 2-MCP and 2-MBP, in the fact that the maximum yield of DD for gas-phase pyrolysis of 2-MBP was 20 times higher than that for 2-MCP (13). Clearly bromine is a major contributing factor to the increase in the formation of DD due to the lower bond strength of the C-Br bond over the C-Cl bond strength, which facilitates formation of the diphenyl ether as well as ring closure by bromine displacement (35). The formation of DD and 1-MBDD competes with the formation of 4,6-DBDF. The rate of formation of dioxins is so fast in the brominated system that it virtually eliminates the formation of furans that was observed in the surfacemediated reactions in chlorine-containing systems.

Acknowledgments We gratefully acknowledge the assistance of our colleagues, Dr. Lavrent Khachatryan and Slawo Lomnicki, for helpful discussions concerning the mechanisms of PBDD/F formation. We acknowledge the partial support of this work under EPA Contract 9C-R369-NAEX, EPA Grant R828166, and the Patrick F. Taylor Chair foundation.

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Received for review December 8, 2004. Revised manuscript received April 8, 2005. Accepted April 19, 2005. ES048057Z

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