Mechanisms of Dioxin Formation from the High-Temperature Pyrolysis of 2-Bromophenol

Article abstract of DOI:10.1021/es034387s

Brominated hydrocarbons are the most commonly used flame retardants. Materials containing brominated hydrocarbons are frequently disposed in municipal and hazardous waste incinerators as well as being subjected to thermal reaction in accidental fires. This results in the potential for formation of brominated dioxins and other hazardous combustion byproducts. In contrastto chlorinated hydrocarbons, the reactions of brominated hydrocarbons have been studied only minimally. As a model brominated hydrocarbon that may form brominated dioxins, we studied the homogeneous, gas-phase pyrolytic thermal degradation of 2-bromophenol in a 1-cm i.d., fused-silica flow reactor at a concentration of 90 ppm, with a reaction time of 2.0 s, and over a temperature range of 300 to 1000 °C. Observed products included dibenzo-p-dioxin (DD), 1-monobromodibenzo-p-dioxin (I-MBDD), 4-monobromodibenzofuran (4-MBDF), dibenzofuran (DF), naphthalene, bromonaphthalene, 2,4- and 2,6-dibromophenol, phenol, bromobenzene, and benzene. These results are compared and contrasted with previous results reported for 2-chlorophenol. At temperatures lower than 700 °C, formation of 2-bromophenoxyl radical, which decomposes through CO elimination to form a bromocyclopentadienyl radical, forms naphthalene and 2-bromonaphthalene through radical recombination/rearrangement reactions. However, unlike the results for 2-chlorophenol, where naphthalene is the major product, DD becomes the major product for the pyrolysis of 2-bromophenol. The formation of DD and 1-MBDD are attributed to radical-radical reactions involving 2-bromophenoxyl radical with the carbon- (bromine) centered radical and the carbon- (hydrogen) centered radical mesomers of 2-bromophenoxyl radical, respectively. The potential product, 4,6-dibromodibenzofuran (4,6-DBDF) for which the analogous product, 4,6-dichlorodibenzofuran (4,6 DCDF), was observed in the oxidation of 2-chlorophenol, was not detected. This is attributed to the pyrolytic conditions of our experiments (e.g., shorter reaction times and higher temperatures) that favor reaction intermediates that form DD and 1-MBDD.

Full text of DOI:10.1021/es034387s

Environ. Sci. Technol. 2003, 37, 5574-5580
retardants that will increase the quantity of brominated
Mechanisms of Dioxin Formation
from the High-Temperature Pyrolysis
of 2-Bromophenol
material reaching thermal waste treatment facilities (4).
Materials containing brominated flame retardants and other
brominated materials are also subject to accidental fires (7).
Because the presence of bromine inhibits combustion, the
conditions in these accidental fires will be sufficient to in-
hibit complete combustion, thus increasing the likeli-
hood of formation of products of incomplete combustion
C A T H E R I N E S . E V A N S A N D
B A R R Y D E L L I N G E R *
(
7). This results in increased concern over potential emissions
Louisiana State University, Department of Chemistry,
Baton Rouge, Louisiana 70803
of polybrominated dibenzo-p-dioxins and dibenzofurans
(
PBDD/ Fs).
Chlorinated phenols are key intermediates in essentially
all proposed pathways of formation of polychlorinated
dibenzo-p-dioxins and dibenzofurans (PCDD/ Fs) (8-12).
Similarly, brominated phenols and brominated flame re-
tardants, e.g. polybrominated biphenyl ethers (PBDEs), have
been identified as precursors to PBDD/ Fs (1, 4, 13-15). More
importantly, some studies have shown that brominated phe-
nols may form more PBDD/ Fs than the analogous chlorinated
phenols form PCDD/ Fs (13, 16). With this knowledge, it is
interesting to note that the toxicity of the PBDD/ Fs has been
shown to be similar to the analogous PCDD/ Fs (17, 18).
Previous work has also shown that the addition of bromine
during combustion increases the production of PBDD/ Fs,
PCDD/ Fs, and mixtures of brominated and chlorinated
dibenzo-p-dioxins and dibenzofurans (4, 19). Some research
suggests that bromination is 10 times more efficient than
chlorination (13). This has been attributed to the predominant
Brominated hydrocarbons are the most commonly used
flame retardants. Materials containing brominated
hydrocarbons are frequently disposed in municipal and
hazardous waste incinerators as well as being subjected
to thermal reaction in accidental fires. This results in the
potential for formation of brominated dioxins and other
hazardous combustion byproducts. In contrast to chlorinated
hydrocarbons, the reactions of brominated hydrocarbons
have been studied only minimally. As a model brominated
hydrocarbon that may form brominated dioxins, we
studied the homogeneous, gas-phase pyrolytic thermal
degradation of 2-bromophenol in a 1-cm i.d., fused-silica
flow reactor at a concentration of 90 ppm, with a reaction
time of 2.0 s, and over a temperature range of 300 to
2
form of bromine being Br versus HCl for chlorine (4). The
1
000 °C. Observed products included dibenzo-p-dioxin (DD),
addition of bromine may not only increase bromination but
also increase chlorination, with the bromine serving as a
good leaving group for displacement by chlorine (4, 19-20).
In this paper, we report the results of the study of 2-bro-
mophenol as a model compound for the combustion of
brominated hydrocarbons and a precursor for the formation
of PBDD/ Fs. The thermal degradation of 2-bromophenol
was studied under pyrolytic conditions for a reaction time
of 2.0 s over the temperature range of 300 to 1000 °C. These
conditions are appropriate to formation in the post-flame
zone of combustion systems. Our results complement and
expand upon previously published studies of chlorinated
and brominated phenols under slow-combustion conditions
at lower temperatures and reaction times on the order of 10
to 100 seconds and 1 h as well as our previously published
work on the pyrolysis of 2-chlorophenol (2-MCP) (21-24).
Reaction pathways to PBDD/ F products are proposed that
are consistent with the experimental data. Pathways to non-
PBDD/ F products are also analyzed, and the intermediates
in these pathways are discussed in relation to formation of
PBDD/ F and other pollutants.
1-monobromodibenzo-p-dioxin (1-MBDD), 4-monobromo-
dibenzofuran (4-MBDF), dibenzofuran (DF), naphthalene,
bromonaphthalene, 2,4- and 2,6-dibromophenol, phenol,
bromobenzene, and benzene. These results are compared
and contrasted with previous results reported for
2-chlorophenol. At temperatures lower than 700 °C,
formation of 2-bromophenoxyl radical, which decomposes
through CO elimination to form a bromocyclopentadienyl
radical, forms naphthalene and 2-bromonaphthalene through
radical recombination/rearrangement reactions. However,
unlike the results for 2-chlorophenol, where naphthalene
is the major product, DD becomes the major product for the
pyrolysis of 2-bromophenol. The formation of DD and
1
-MBDD are attributed to radical-radical reactions involving
2-bromophenoxyl radical with the carbon- (bromine)
centered radical and the carbon- (hydrogen) centered
radical mesomers of 2-bromophenoxyl radical, respectively.
The potential product, 4,6-dibromodibenzofuran (4,6-
DBDF) for which the analogous product, 4,6-dichlorodiben-
zofuran (4,6 DCDF), was observed in the oxidation of
Experimental Section
2-chlorophenol, was not detected. This is attributed to the
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 (25). 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.
pyrolytic conditions of our experiments (e.g., shorter
reaction times and higher temperatures) that favor reaction
intermediates that form DD and 1-MBDD.
Introduction
Emissions of brominated hydrocarbons have been reported
from various combustion sources (1-6). There has been a
notable increase in everyday use of brominated flame
To maintain a constant concentration of 90 ppm, 2-mono-
bromophenol (2-MBP)(Aldrich) was injected into a helium
*
Corresponding author phone: (225) 578-6759; fax: (225) 578-
4
936; e-mail: barryd@lsu.edu.
5
5 7 4
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 24, 2003
10.1021/es034387s CCC: $25.00

2003 Am erican Chem ical Society
Published on Web 10/22/2003

gas stream by a syringe pump through a vaporizer maintained
at 280 °C. Gas-phase samples of 2-MBP then 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
3
00 to 1000 °C. The helium flow rate was varied with
temperature so that the residence time inside the reactor
was held at 2.0 s. The unreacted 2-MBP and the thermal
degradation products were then swept through a heated
transfer line to another Varian GC where they were cryo-
genically trapped at the head of a CP-Sil 8 phase capillary
column (30 m, 0.25 mm i.d., 0.25 µm film thickness). To
separate the individual reaction products, the column was
temperature programmed from -60 to 300 °C at 15 °C/ min.
Detection and quantification of the products were obtained
using a Varian Saturn mass spectrometer that was operated
in the full-scan mode (40 to 650 amu) for the duration of the
GC run.
FIGURE 1. “Dioxin” products from the gas-phase pyrolysis of 2-MBP.
[2-MBP] ) 88 ppm in helium. Gas-phase reaction time of 2.0 s.
0
Product concentrations (other than PBDD/ Fs) were
calculated based on the 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. Con-
centrations of observed PBDD/ Fs are reported based on
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/ F based on their mass
spectra: 1-bromodibenzo-p-dioxin, 4,6-dibromodibenzo-
furan, and 4-bromodibenzofuran. The mass spectral library
match qualities for these species were 264, 326, and, 248,
respectively. These products are the same three PBDD/ Fs
that were anticipated on the basis of predicted pathways
from previous research on formation of PCDD/ F from the
analogous 2-chlorophenol (24). In the 2-chlorophenol study,
the PCDD/ F standards were available to confirm the iden-
tifications on the basis of 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 mecha-
nistically 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-chlo-
rophenol (24).
Without experimental benchmarks, the calculated ∆Hrxn
cannot be considered to be completely accurate. They are
shown to assess the likelihood of potential parallel pathways.
Results
The temperature dependence of the thermal degradation of
2-MBP and the yield of “dioxin” products are presented in
Figure 1 and Table 1 for a reaction time of 2.0 s. The nondioxin
products are presented in Figure 2 and Table 1. Figures 1
and 2 are given on a semilogarithmic scale in which the
percent yields of products (or percent of unconverted 2-MBP)
are on a logarithmic scale versus temperature.
The thermal degradation of 2-MBP gradually increased
from 300 to 650 °C, where the rate drastically accelerated,
achieving 99% destruction at 850 °C.
Dibenzo-p-dioxin (DD), dibenzofuran (DF), 4-monobro-
modibenzofuran (4-MBDF), and 1-bromodibenzo-p-dioxin
(1-MBDD) were observed between 500 and 900 °C. The
anticipated product, 4,6-dibromodibenzofuran (4,6-DBDF)
was not observed. Unlike DD and 4-MBDF, which were
detectable at temperatures as low as 500 °C, significant yields
of 1-MBDD were not observed until 700 °C with a maximum
yield of 0.03%. The major product, DD, was observed between
500 and 900 °C with a maximum yield of 6% at 600 °C. The
other dioxin products, DF and 4-MBDF, were detected at
much lower yields with maximums of 0.17% and 0.05% at
600 °C, respectively. No brominated aromatics were detected
above 950 °C.
Nondioxin products were also detected from the pyrolysis
of 2-MBP (cf. Figure 2 and Table 1). Brominated products
included bromobenzene (maximum yield of 0.04% at 750
°C), bromonaphthalene (maximum yield of 0.19% at 650 °C),
The yields of the products were calculated using the
following expression:
2
,4-dibromophenol, and 2,6-dibromophenol (maximum
Y ) {[Product]/ [2-MBP] } × 100
yields of 0.06% and 0.05% at 650 °C, respectively). Other
products included naphthalene, benzene, phenol, acenaph-
thalene, phenylethyne, and diphenylethyne.
0
where [Product] is the concentration of the particular product
Initially naphthalene and phenol were observed at 500 °C
followed by 2,4-dibromophenol, 2,6-dibromophenol, and
bromonaphthalene at 600 °C. However phenol and bro-
monaphthalene achieved their highest yields of 0.36% and
0
formed (in moles) and [2-MBP] is the initial concentration
of 2-MBP (in moles) injected into the reactor. (Please note
that in our previous publication on the pyrolysis of 2-chlo-
rophenol (24) the concentration of the chlorinated products
that contained two phenyl rings was multiplied by two. These
products were dibenzo-p-dioxin, 4,6-dichlorodibenzofuran,
0
.19% at 600 and 650 °C, respectively, whereas naphthalene
achieved a maximum yield of 1.83% at 950 °C. All other
products were not detected until 750 °C and reached their
maximum yields at 900 °C.
1
-chlorodibenzo-p-dioxin, dibenzofuran, naphthalene, chlo-
ronaphthalene, and diphenylethyne.) Once the experimental
procedure was fully developed, the repeatability of the
experiments was within 10%.
Discussion
The heats of reaction, ∆Hrxn, for key steps in product
formation pathways were calculated using AM1, semiem-
pirical molecular orbital formalism. The calculations were
performed using the MOPAC computation program that is
contained within the ChemBats3D Pro computer application.
The formation of dioxin products (DD, 1-MBDD, 4-MBDF,
and DF) indicates that stable phenoxyl radicals are formed
in significant yields through loss of the hydroxyl-hydrogen.
The formation of aromatics (phenol, bromobenzene, and
benzene) indicates that simple substitution reactions are
VOL. 37, NO. 24, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
5 5 7 5

a
TABLE 1. Percent Yield of Products of Gas-Phase Pyrolysis of 2-Bromophenol
temperature (°C)
650
30.1
product
300
400
450
500
550
45.7
600
700
17.4
750
5.8
0.64
0.02
0.05
800
850
900
950
2
-brom ophenol
100 61.3 56.3 52.8
29.9
5.96
1.8
0.19 0.05
0.01
0.39
0.19 0.06
0.02
dibenzo-p-dioxin
-brom odibenzo-p-dioxin
dibenzofuran
-brom odibenzofuran
naphthalene
brom onaphthalene
acenaphthalene
phenol
0.52
1.78
3.97
0.003
0.15
0.02
0.07
0.19
1.42
0.03
0.08
1
0.17
0.05
0.04
0.03
0.05 0.05
0.04 0.02
4
0.03
0.03
0.02
0.04
0.36
0.13
0.71
0.08
1.15 1.35
0.07 0.04
1.44 1.83
0.02 0.01
0.02 0.03
0.002 0.01 0.02
0.17
0.07
0.004
0.36
0.04
0.04
0.24
0.06
0.05
0.24
0.004
2
2
,4-dibrom ophenol
,6-dibrom ophenol
.003 0.001
0.02
benzene
0.05 0.07
0.03 0.006
0.02 0.03
0.15 0.19
0.09 0.10
brom obenzene
phenylethyne
diphenylethyne
0.02
0.03
0.03
0.04
0.01
0.04
0.05 0.03
0.10
a
Percent yield ) {([Product])/[2-MBP]
0
} × 100.
Bimolecular propagation reactions under pyrolytic con-
•
•
ditions include attack by H and Br . On the basis of AM1
semiempirical molecular orbital calculations, thermody-
•
namically feasible reactions of H include: phenoxyl hydrogen
abstraction (reaction 2), bromine abstraction (reaction 3),
bromine displacement (reaction 4), and hydroxyl displace-
ment (reaction 5). Aromatic hydrogen abstraction is ther-
modynamically unfavorable. The AM1 calculated ∆Hrxn values
shown are not precise and are given here for comparison
purposes only.
•
•
C H BrOH + H f C H BrO + H
6
4
6
4
2
∆Hrxn ) -23 kcal/ mol (Reaction 2)
•
•
C H BrOH + H f C H OH + HBr
6
4
6
4
∆
Hrxn ) -11 kcal/ mol (Reaction 3)
FIGURE 2. “Nondioxin” products from the gas-phase pyrolysis of
-MBP. [2-MBP] ) 88 ppm in helium. Gas-phase reaction time of
.0 s.
•
•
2
2
0
C H BrOH + H f C H OH + Br
6
4
6
5
∆
Hrxn ) -29 kcal/ mol (Reaction 4)
occurring. The formation of 2,4- and 2,6-dibromophenol is
evidence of bromination of 2-MBP. The formation of larger
aromatic molecules (naphthalene, acenaphthalene, and
bromonaphthalene) is the result of molecular growth path-
ways that may involve smaller, even- or odd-numbered
radicals. The formation of phenylethyne and diphenylethyne
indicates fragmentation of the aromatic ring and reactions
of even-numbered radicals.
•
•
C6H4BrOH + H f C6H5Br + OH
∆Hrxn ) 2 kcal/ mol (Reaction 5)
Bromine atoms can also abstract phenoxyl hydrogens (cf.
reaction 6). In contrast, the abstraction of an aromatic
hydrogen by bromine (reaction 7) for comparison to the
analogous chlorine reaction, vide infra) (24), abstraction of
bromine by bromine, displacement of hydrogen by bromine,
and displacement of hydroxyl by bromine are highly en-
dothermic and unfavorable.
2
-MBP Decom position. The decomposition of 2-MBP is
initiated by loss of the phenoxyl hydrogen by unimolecular,
bimolecular, or possibly other low energy pathways (including
heterogeneous reactions). Unimolecular decomposition of
the oxygen-hydrogen bond (reaction 1) is rapid. Our AM1
semiempirical molecular orbital calculations yield a heat of
reaction of 79 kcal/ mol.
•
•
C H BrOH + Br f C H BrO + HBr
6
4
6
4
∆
Hrxn ) -8 kcal/ mol (Reaction 6)
•
•
C H BrOH + Br f C H BrOH + HBr
6
4
6
3
•
•
C H BrOH f C H BrO + H
6
4
6
4
∆Hrxn ) 19 kcal/ mol (Reaction 7)
∆
Hrxn )
Rate coefficients based on analogous reactions with
7
9 kcal/ mol (Reaction 1)
phenol for reactions 2, 3, 4, and 6 are k (730-880 °C) ) 1.15
× 10 exp(-12 400/ RT) cm / mol/ s (30), k (800-1010 °C) )
1.0 × 10 exp(-11 300/ RT) cm / mol/ s (31), k (800-1010 °C)
(25 °C)
2
1
4
3
This is consistent with the activation energy for the
reported rate coefficient for decomposition phenol, kphenol
3
1
3
3
4
1
5
-1
13
3
(
430-930 °C) ) 3.2 × 10 exp(-86 500/ RT) s (26, 27), and
) 1.5 × 10 exp(-7500/ RT) cm / mol/ s (31), and k
6
1
4
3
the suggestion that the activation energy is 5 kcal lower for
the reaction of 2,4,6-trichlorophenol than phenol (28). Studies
have also shown that the O-H bond in 2-MBP is 0.10 kcal/
mol weaker than the O-H bondin 2-MCP, and therefore H
elimination is expected to be faster for 2-MBP than for 2-MCP
)1.43 ×10 cm / mol/ s (30), respectively. No previous kinetic
data was found for reaction 5. However, because of the very
low endothermicity of the reaction, it cannot be ruled out
as a decomposition pathway.
On the basis of the experimentally determined product
formation, the dominant initiation pathways appear to be
(
29).
5
5 7 6
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 24, 2003

formation of 2-bromophenoxyl radical via reaction 1 (with
some contribution from reactions 2 and 6) and formation of
phenol via reaction 4. Reaction 4 is significant for 2-MBP
and is faster than the analogous chlorine displacement in
thalene, bromonaphthalene, and acenaphthalene are tra-
ditionally ascribed to molecular growth pathways involving
largely C2 fragments (32-35). However, the low-temperature
onset of formation of naphthalene (475 °C) suggests a
pathway that does not require the complete fragmentation
of 2-MBP and that is instead initiated at low temperatures.
Recently, resonantly stabilized cyclopentadienyl radicals
have been recognized as potential precursors in the formation
of naphthalene (36, 37). We have already discussed how
2-MBP initially decomposes to 2-bromophenoxyl radical. It
is well documented that the thermal decomposition of a
phenoxyl radical expels CO to form a cyclopentadienyl radical
2
-MCP because of the weaker carbon-bromine (80.5 kcal/
mol) versus carbon-chlorine (95.7 kcal/ mol) bond (20). The
rxn for reaction 3 (-11 kcal/ mol) is similar to that for the
∆
H
analogous reaction of 2-MCP (-14 kcal/ mol). Reaction 7,
hydrogen abstraction by a halogen atom, was found to be
thermoneutral for 2-MCP and significantly endothermic and
unfavorable for 2-MBP. Aromatic hydrogen atom abstractions
by bromine atoms are not likely to play a role in the
combustion chemistry of brominated hydrocarbons, as the
resulting HBr is not highly stable.
with a reported rate coefficient of k
8
(730-1300 °C) ) 1 ×
1
1.4
-1
10 exp(-22 100(450/ T) s and a ∆H of 20 kcal/ mol (27,
f
Because of the low activation energies for reaction of
-MBP, the purely gas-phase reactions described above can
38). We believe that a similar elimination of CO from
2-bromophenoxyl radical forms bromocyclopentadienyl
radical (reaction 8) as follows:
2
account for all the decomposition of 2-MBP above 750 °C.
However, the initiation of 2-MBP decomposition at tem-
peratures 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 reaction 1. 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 been shown to not
significantly impact subsequent propagation reactions (32).
Form ation of Phenol, Brom obenzene, Benzene, Dibro-
m ophenol, Phenylethyne, and Diphenylethyne. On the basis
of the early onset of phenol formation at temperatures as
low as 500 °C, the dominant source of phenol in our study
•
•
C H BrO f c-C H Br + CO (Reaction 8)
6
4
5
4
Recombination of two 2-bromocyclopentadienyl radicals,
•
•
followed by rearrangement and H or Br elimination can
•
result in formation of naphthalene if two Br are eliminated,
•
•
formation of bromonaphthalene if one Br and one H are
eliminated, and formation of dibromonaphthalene if two H•
are eliminated. The reported ∆Hrxn for the overall hydro-
carbon reaction is 9.23 kcal/ mol (36). The dominance of
naphthalene, lower concentration of bromonaphthalene, and
•
absence of dibromonaphthalene suggest that Br elimination
is favored as would be expected considering the lower
carbon-bromine bond strength and thermochemistry (20).
This result also suggests that brominated hydrocarbons may
undergo this type of molecular growth reaction more readily
than hydrocarbons due to the more favorable elimination of
bromine than hydrogen atoms. The formation of naphthalene
was similar for both 2-MBP and our previous studies with
•
is probably the exothermic displacement of bromine by H
(
reaction 4). Higher yields of phenol are observed and at
lower temperatures for 2-MBP than 2-MCP (24). This is a
result of the lower activation energy for bromine displacement
than chorine displacement.
Formation of bromobenzene occurs at much lower yields
and higher temperatures than formation of phenol. This is
consistent with the more endothermic displacements of the
2
-MCP (24). However, chloronaphthalene is formed in greater
yields than bromonaphthalene (24). This is consistent with
the elimination of bromine being much faster than that of
chlorine as suggested by the weaker carbon-bromine versus
carbon-chorine bond (20).
Acenaphthalene is formed in lower yields than naph-
thalene or bromonaphthalene at higher temperatures when
benzene, phenylethyne, and diphenylethyne are also formed.
This suggests that acenaphthalene is formed by traditional
C2 molecular growth pathways where 2-MBP is completely
fragmented.
Form ation of Dibenzo-p-dioxin, 1-Brom odibenzo-p-
dioxin, 4-Brom odibenzofuran, and Dibenzofuran. As ex-
pected, DD and 1-MBDD were formed. The anticipated
product, 4,6-DBDF, was not observed. However, 4-MBDF
and unbrominated DF were formed.
•
hydroxyl group by H to form bromobenzene (reaction 5)
•
than the displacement of Br to form phenol. Benzene is
formed in low yields and relatively high temperatures due
to the endothermic displacement of the hydroxyl group by
H from phenol (∆Hrxn ) 4 kcal/ mol).
It is also notable that bromobenzene is initially produced
at 600 °C while benzene is not formed until 750 °C.
Bromobenzene is formed from 2-MBP at lower temperatures
than chlorobenzene is formed from 2-MCP (24). This is in
part attributable to the earlier onset of decomposition of
•
2
-MBP than 2-MCP; however, bromine (versus chlorine)
substitution may also facilitate OH displacement.
,4-Dibromophenol and 2,6-dibromophenol form from
the bromination of 2-MBP. Previous research suggests that
bromine predominantly exists as Br whereas chlorine is
mainly found as HCl (4). With the excess Br , bromination
2
2
It is notable that DF forms at higher temperatures than
DD and 1-MBDF. It also forms as phenol begins to decom-
pose. Thus, we believe that DF is a recombination of
unbrominated phenoxyl radical formed from decomposition
of phenol (39). Essentially, the phenoxyl radical converts to
the keto-mesomer that then recombines with another keto-
mesomer to form a diketo-dimer. Single proton tautomer-
ization followed by displacement of a hydroxyl radical forms
DF. Alternatively, double tautomerization converts the diketo-
dimer to the dihydroxy biphenyl (DHB)-dimer which can
them eliminate water to form DF. Interestingly, the formation
of DF from 2-MBP occurs at lower temperatures (600 °C)
2
can occur more readily than chlorination. It has also been
noted that bromination is 10 times more efficient that
chlorination (13). Thus, the evidence of bromination for
-MBP is acceptable even though chlorination of 2-MCP was
not evident in our previous study (24).
Considering the relatively high temperatures (725-875
C) for formation of benzene and bromobenzene, and the
formation of phenylethyne and diphenylethyne in the same
temperature range, it is evident that 2-MBP fragments into
primarily C2 radicals (vinyl and ethynyl) and C2 molecules
2
°
•
(
ethylene and acetylene). Molecular growth pathways result-
ing in formation of benzene and substituted benzene
than from 2-MCP (700 °C). The Br concentration in the
•
2-MBP system is higher than the Cl concentration in the
involving C2 units are well documented in the literature (32-
2-MCP system due to the decomposition temperature of Br
2
•
3
5).
being lower than that of HCl (4). The higher Br concentration
leads to faster rates of hydrogen abstraction and DF forma-
tion. The yields of DF for both 2-MCP and 2-MBP are the
Form ation of Naphthalene, Brom onaphthalene, and
Acenaphthalene. Formation of polycyclics such as naph-
VOL. 37, NO. 24, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
5 5 7 7

SCHEME 1. Postulated Pathways for the Formation of DD, 1-MBDD, and 4,6-DBDF
same, lending further evidence that they are formed by the
same reaction pathway from the unhalogenated phenoxy
radical, but with different rates as discussed above.
Comparison of DD and 1-MBDD formation from 2-MBP
with previous results for 2-MCP suggest that they are formed
directly from the brominated phenoxyl radical of 2-MBP (24).
Scheme 1 depicts possible reaction pathways to DD, 1-MBDD,
and 4,6-DBDF from the reaction of the different mesomers
of 2-bromophenoxyl radical. The pathways are analogous to
the pathways suggested for 2-MCP (24).
ers react to form the diketo-dimer. The dimer can then
react via the upper pathway through abstraction of hydro-
•
gen by H , followed by tautomerization and displacement
of the hydroxyl group to form 4,6-DBDF. An additional
possibility is the lower pathway where the diketo-dimer
double tautomerizes and then eliminates H
DBDF.
2
O to from 4,6-
The lack of formation of 4,6-DBDF could be considered
surprising considering previous studies in the literature
that reported dibenzofuran as the preferred product of
reaction of unchlorinated phenoxyl radicals through the
carbon-centered radical mesomer and chlorinated diben-
zofurans as the preferred product from the reaction of various
chlorinated phenols under oxidative conditions (21-22,
40-42). In these studies, the observed formation of diben-
zofurans as the dominant product was justified on the basis
that the carbon-centered radical mesomer, which leads to
the dibenzofuran, was more stable than the oxygen-centered
mesomer (39). However, in our previous study of 2-MCP, we
noted that 4,6-DCDF was not formed (24). A previous study
of the pyrolysis of 2-MBP for a reaction time of 1 h, reported
DD as the main product at 600 °C without report of 4,6-
DBDF as a product (23).
In the uppermost pathway, the oxygen-centered radical
mesomer recombines with the carbon- (bromine) centered
radical mesomer to form a keto-ether. Following the ab-
•
straction of bromine by H , DD is formed by intra-annular
•
elimination of Br .
Another possible pathway for the formation of DD is
through a radical-molecule reaction shown in parentheses
below the radical-radical pathway in Scheme 1. This reaction
depicts the oxygen-centered radical mesomer reacting with
•
2
-MBP via Br displacement to form a bromohydroxy diphenyl
•
ether (HDE) followed by the abstraction of hydrogen by H .
DD is then formed by intra-annular displacement of bromine.
It has been previously suggested that this radical-molecule
reaction is too slow to account for the observed yields of DD
from 2-MCP (32).
We believe that under pyrolytic conditions, the abstraction
•
of hydrogen by H to form 4,6-DBDF is slower than the
•
Formation of 1-MBDD, shown as the third pathway in
Scheme 1, is initiated by the recombination of the oxygen-
centered 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
abstraction of bromine by H to form DD. This is in contrast
to oxidative conditions where the hydroxyl radical should
rapidly abstract hydrogen and form 4,6-DBDF or 4,6-DCDF
from 2-MBP or 2-MCP, respectively (21-22, 40-42).
On the basis of all past and present results, it appears that
oxidative conditions favor 4,6-DBDF formation, whereas
pyrolysis favors DD formation (21-24, 40-42). It is also
notable that the maximum yield for DD formation from
2-MBP is 20× higher than for 2-MCP. This is reasonable
because the ∆Hrxn for the second displacement of Br to form
•
displacement of Br .
The last two pathways in Scheme 1 depict the possible
pathways to formation of 4,6-DBDF. Initially for both
pathways, two carbon- (hydrogen) centered radical mesom-
5
5 7 8
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 24, 2003

SCHEME 2. Proposed Pathways for Formation of 4-MBDF
DD is exothermic (-12 kcal/ mol) and the ∆Hrxn displacement
of Cl is endothermic (11 kcal/ mol), resulting in formation of
DD being more favorable for 2-MBP.
of the thermochemistry presented in this manuscript 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.
1
-MBDD had slightly lower yields but occurred over the
same temperature range as formation of 1-MCDD from
-MCP. Clearly, 2-MBP and 2-MCP undergo very similar
2
formation mechanisms with the halogen having little effect
on the formation of the product. This suggests that the
radical-radical recombination step is rate limiting, rather
than the ring closure by halogen displacement. The slightly
lower yield of 1-MBDD versus 1-MCDD may be due to
competition with formation of DD. Ab initio molecular orbital
calculations are under way to determine how bromine and
chlorine affect the resonance structure of the halogenated
phenoxyl radical.
Literature Cited
(
(
(
1) Thoma, H.; Rist, S.; Haushulz, G.; Hutzinger, O. Chemosphere
986, 15 (4), 649.
2) Oberg, T.; Warman, K.; Bergstrom, S. Chemosphere 1987, 16,
451.
3) Dumlar, R.; Thoma, H.; Lenoir, D.; Hutzinger, O. Chemosphere
989, 19 (12), 2023.
1
2
1
(4) Soderstrom, G.; Marklund, S. Environ. Sci. Technol. 2002, 36,
1959.
(5) Sakai, S. Organohalogen Compd. 2000, 47, 210.
4
-MBDF was the only product formed for which the
chlorinated analogue was not observed in our study of 2-MCP.
Scheme 2 depicts reasonable pathways for the formation of
(
6) Sakai, S.; Watanbe, J.; Honda, Y.; Takatsuku, H.; Aoki, I.;
Futamatsu, M.; Shiozaki, K.; Chemosphere 2001, 42, 519.
7) Wilken, M.; Schanne, L. Schriften. WAR 1994, 74, 109.
8) Karasek, F. W.; Dickson, L. C. Science 1987, 237, 754.
9) Dickson, L. C.; Lenoir, D.; Hutzinger, O. Environ. Sci. Tech-
nol.1992, 26, 1822.
4
-MBDF. Two pathways are depicted: (1) the carbon-
(
(
(
(
hydrogen) centered radical mesomer can recombine with
the carbon- (bromine) centered radical mesomer; or (2) the
carbon- (hydrogen) centered radical mesomer can recombine
with an unbrominated carbon-centered phenoxyl radical to
(
10) Altwicker, E. R.; Milligan, M. S. Chemosphere 1993, 27, 301.
•
form a diketo-dimer. For the first pathway, H abstracts
(11) Addink, R.; Olie, K. Environ. Sci. Technol. 1995, 29, 1425.
(12) Milligan, M. S.; Altwicker, E. R. Environ. Sci. Technol. 1996, 30,
225.
•
bromine and, in the second, H abstracts another hydrogen.
Both pathways then undergo tautomerization followed by
displacement of hydroxyl to form 4-MBDF. On the basis of
comparisons to simple alkane reactions for the abstraction
(
(
13) Kanters, J.; Louw, R. Chemosphere 1996, 32 (1), 87.
14) Buser, H. R. Environ. Sci. Technol. 1986, 20, 404.
•
•
(15) Dumler, R.; Thoma, H.; Lenoir, D.; Hutzinger, O. Chemosphere
989, 19 (1-6), 305.
16) Sidhu, S. S.; Maqsud, L.; Dellinger, B. Combust. Flame 1995,
00, 11.
of bromine by H and the abstraction of hydrogen by H , the
upper pathway is a much more favorable route for the
1
(
(
4
-MBDF formation (43-45).
1
In summary, we have proposed reasonable mechanisms
17) Mennear, J. H.; Lee, C. C. Environ. Health Perspect. 1994, 102
for the formation of each observed product of the pyrolysis
of 2-MBP. Reaction kinetic models that will be needed to
better understand the competition between molecular growth
pathways to form PAH and the relative yield of bromine-
substituted dibenzo-p-dioxin products and bromine-sub-
stituted dibenzofuran products are under development.
These results, mechanisms, and reaction kinetic codes can
serve as a model for the formation of PBDD and PBDF from
the range of brominated phenols present in full-scale
combustors and help to decipher the origin of PBDD/ F
fingerprints that can be used to identify the specific sources
of PBDD/ F in the environment.
The results of this study clearly suggest an increased
propensity for dioxin and furan formation from brominated
precursors, viz. The maximum yield of DD alone was 20 times
higher for 2-MBP than for 2-MCP (24). The continued used
of brominated flame retardants that may be disposed in
incinerators and energy recovery devices as well as accidental
fires in which brominated chemicals are present, suggests
that additional attention should be paid to PBDD/ F formation
from combustion of brominated chemicals.
(1), 265.
(18) Weber, L. W.; Greim, H. J. Toxicol. Environ. Health 1997, 50 (3),
95.
1
(
(
19) Lemieux, P. M.; Ryan, J. V. Waste Manage. 1998, 18, 361.
20) McMillen, D. F.; Golden, D. M. Annu. Rev. Phys. Chem. 1982,
3
3, 493.
(
(
21) Born, J. G. P.; Louw, R.; Mulder, P. Chemosphere 1989, 19, 401.
22) Yang, Y.; Mulholland, J. A.; Akki, U. In 27th Symposium
(
International) on Combustion, The Combustion Insitute; Wil-
liams & Wilkins Co.: Baltimore, MD, 1998; p 1761.
(23) Borojovich, E.; Aizenshtat, Z. J. Anal. Appl. Pyrolysis 2002, 63,
29.
1
(
(
(
(
24) Evans, C.; Dellinger, B. Environ. Sci. Technol. 2003, 37, 1325.
25) Rubey, W. A.; Grant, R. A. Rev. Sci. Instrum. 1988, 59, 265.
26) Shaub, W. M.; Tsang, W. Environ. Sci. Technol. 1983, 17, 721.
27) Colussi, A.; Zabel, F.; Benson, S. W. Int. J. Chem. Kinet. 1977,
9
, 161.
(
(
28) Louw, R.; Ahonkhai, S. T. Chemosphere 2002, 46, 1273.
29) Carlson, G. L.; Fataley, W. G.; Monocha, A. S.; Bentley, F. F. J.
Phys. Chem. 1972, 76, 1553.
(30) NIST Chemical Kinetics Database 17; Gaithersburg, MD, 1998.
(31) Ritter, E. R.; Bozzelli, J. W. J. Phys. Chem. 1990, 94 (6), 2493.
(
32) Khachtrayan, L.; Asatrayan, R.; Dellinger, B. Chemosphere 2003,
2, 695.
33) Appel, J.; Bockhorn, H.; Frenklach, M. Combust. Flame 2000,
21, 122.
5
(
Acknowledgments
We gratefully acknowledge the assistance of our colleagues,
1
(34) Richter, H.; Howard, J. B. Prog. Energy Combust. Sci. 2000, 26,
Dr. Lavrent Khachatryan and Alexander Burcat, in evaluation
4.
VOL. 37, NO. 24, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
5 5 7 9

(
(
35) Richter, H.; Mazyer, O. A.; Sumathi, R.; Green, W. H.; Howard,
J. A.; Bozzelli, J. W. J. Phys. Chem. A 2001, 105 (9), 1561.
36) Melius, C. F.; Calvin, M.; Marinov, N. M.; Ritz, W. J.; Senkan, S.
M. In 26th Symposium (International) on Combustion, The
Combustion Institute; Williams & Wilkins Co.: Baltimore, MD,
(42) Weber, R.; Hagenmaier, H. Organohalogen Compd. 1997, 31,
480.
(43) Green, J. H. S.; Maccoll, A. J. Chem. Soc. 1955, 2449.
(44) Price, S.; Shaw, R.; Trotman-Dicksenson, A. J. Chem. Soc. 1956,
3855.
1
996; p 685.
(45) Kale, M.; Maccoll, A. J. Chem. Soc. 1957, 5020.
(
(
(
(
(
37) Lu, M.; Mulholland, J. A. Chemosphere 2001, 42, 625.
38) Lin, C. Y.; Lin, M. C. J. Phys. Chem. 1986, 90, 425.
39) Waiter, I.; Born, J. G. P.; Louw, R. Eur. J. Org. Chem. 2000, 921.
40) Weber, R.; Hagenmaier, H. Chemosphere 1999, 38, 529.
41) Mulholland, J. A.; Akki, U.; Yang, Y.; Ryu, J. Y. Chemosphere
Received for review April 24, 2003. Revised manuscript re-
ceived August 15, 2003. Accepted September 17, 2003.
2
001, 42, 719.
ES034387S
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5 8 0
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