Mechanisms of dioxin formation from the high-temperature oxidation of 2-chlorophenol

Article abstract of DOI:10.1021/es049355z

The homogeneous, gas-phase oxidative thermal degradation of 2-chlorophenol was studied in a 1 cm i.d., fused silica flow reactor at a concentration of 88 ppm, reaction time of 2.0 s, over a temperature range of 300 to 1000 °C. Observed products in order of yield were as follows: 4,6-dichlorodibenzofuran (4,6-DCDF) > dibenzo-p-dioxin (DD) > 1-monochlorodibenzo-p-dioxin (1-MCDD), 4-chlorodibenzofuran (4-MCDF), dibenzofuran (DF), naphthalene, chloronaphthalene, 2,4-dichlorophenol, 2,6-dichlorophenol, phenol, chlorobenzene, and benzene. In contrast to pyrolysis, 4,6-DCDF is the major product rather than DD, and 1-MCDD and naphthalene are formed at temperatures as low as 400 °C. Under oxidative conditions, ?OH and Cl? are the major carriers, which favors 4,6-DCDF formation over DD or 1-MCDD through abstraction of H? through diketo- and ether- intermediates. It is proposed that below 500 °C, unimolecular tautomerization/HCl elimination and CO elimination/isomerization reactions result in the formation of 1-MCDD and naphthalene, respectively.

Full text of DOI:10.1021/es049355z

Environ. Sci. Technol. 2005, 39, 122-127
products (13-16). In our recently published study of the
Mechanisms of Dioxin Formation
from the High-Temperature
Oxidation of 2-Chlorophenol
high-temperature pyrolysis of 2-chlorophenol, we reported
that formation of PCDDs (dibenzo-p-dioxin (DD) and
1-monochlorodibenzo-p-dioxin (1-MCDD)) was favored over
the formation of PCDFs (4,6-dichlorodibenzofuran (4,6-
DCDF)) (17). These studies suggest that molecular oxygen
and oxygen containing radicals, i.e., hydroxyl radical, are
playing a large role in the product distribution and PCDD/F
ratio.
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 *
Department of Chemistry, Louisiana State University,
Baton Rouge, Louisiana 70803
In this article, we report the thermal degradation of
2-chlorophenol under oxidative conditions for a reaction time
of 2.0 s over the temperature range of 300 to 1000 °C and
compare these results to the results of our previous study of
the pyrolysis of 2-chlorophenol under otherwise similar
conditions (17). Reaction pathways to PCDD/F products are
proposed that are consistent with the experimental data.
Pathways to non-PCDD/F products are also analyzed, and
the intermediates in these pathways are discussed in their
relation to formation of PCDD/F and other pollutants. The
effect of molecular oxygen on the PCDD to PCDF ratio is
explained based on these reaction pathways.
The homogeneous, gas-phase oxidative thermal degradation
of 2-chlorophenol was studied in a 1 cm i.d., fused
silica flow reactor at a concentration of 88 ppm, reaction
time of 2.0 s, over a temperature range of 300 to 1000
°C. Observed products in order of yield were as follows: 4,6-
dichlorodibenzofuran (4,6-DCDF) > dibenzo-p-dioxin (DD)
> 1-monochlorodibenzo-p-dioxin (1-MCDD), 4-chloro-
dibenzofuran (4-MCDF), dibenzofuran (DF), naphthalene,
chloronaphthalene, 2,4-dichlorophenol, 2,6-dichlorophenol,
phenol, chlorobenzene, and benzene. In contrast to
pyrolysis, 4,6-DCDF is the major product rather than DD,
and 1-MCDD and naphthalene are formed at temperatures
as low as 400 °C. Under oxidative conditions, •OH and
Cl• are the major carriers, which favors 4,6-DCDF formation
over DD or 1-MCDD through abstraction of H• through
diketo- and ether- intermediates. It is proposed that below
500 °C, unimolecular tautomerization/HCl elimination and
CO elimination/isomerization reactions result in the formation
of 1-MCDD and naphthalene, respectively.
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 (18). 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. Previous work found that an inside diameter
of 1 cm was acceptable in reducing the wall effects (19). 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.
Introduction
It has recently been recognized that the contribution of gas-
phase pathways to the formation of polychlorinated dibenzo-
p-dioxins and dibenzofurans (PCDD/F) have probably been
underestimated (1, 2). Chlorinated phenols are known to
form PCDD/F via gas-phase pathways through reactions of
phenoxyl radicals (3-7). In fact, it was the overestimation of
the rate of consumption of phenoxyl radical through reaction
with molecular oxygen that led to the presumption that gas-
phase pathways were too slow to contribute significantly to
PCDD/F emissions from full-scale combustors (1, 2, 8).
In the original mechanistic and reaction kinetic models,
it was presumed that chlorinated phenols form only PCDDs
through radical-molecule reactions involving phenoxyl radi-
cals and phenols (8-10). Subsequently, it has been suggested
that chlorinated phenols also form PCDFs as well as PCDDs
through radical-radical reactions of phenoxyls (8, 11).
The first experimental studies of the high-temperature,
gas-phase reactions of 2,4,6-trichlorophenol (T ) 300-800
°C, reaction time ) 2.0 s) reported the formation of PCDDs
without PCDF (12). Subsequent studies of the thermal
oxidation of 2,4,6-trichlorophenol in the presence of hexane
under similar reaction conditions resulted in the formation
of PCDFs in addition to PCDDs. Other studies of the oxidation
of chlorophenols (i.e. 2-chlorophenol, 3-chlorophenol, 4-chlo-
rophenol, 2,4-dichlorophenol and pentachlorophenol) under
“slow combustion conditions” (i.e. T ) 300-600 °C, reaction
times between 10 s and 10 min) reported PCDFs as the major
To maintain a constant concentration of 88 ppm,
2-monochlorophenol (2-MCP) (Aldrich) was injected into a
20% O2 in helium gas stream by a syringe pump through a
vaporizer maintained at 280 °C. Gas-phase samples of 2-MCP
then were swept by the 20% O2 in 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 temperature was
parametrically varied between 300 and 1000 °C. The 20% O2
in helium flow rate was varied with temperature so that the
residence time inside the reactor was held at 2.0 s. The
unreacted 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). 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, which was operated in the full-scan mode (40
to 650 amu) for the duration of the GC run. The length of
each experimental run was approximately 45 min.
Product concentrations were calculated based on 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 expression
* Corresponding author phone: (225)578-6759; fax: (225)578-3458;
e-mail: barryd@lsu.edu.
Y ) {([Product])/[2-MCP]0}*100
9
122 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 1, 2005
10.1021/es049355z CCC: $30.25
2005 American Chemical Society
Published on Web 11/24/2004

detected PCDD/F products. Initially at 400 °C, 1-MCDD was
the only PCDD/F product observed, achieving a maximum
yield of 0.92% at 600 °C. DD and 4,6-DCDF were detected
between 500 and 900 °C with maximum yields of 1.32 and
9.4%, respectively. The other PCDD/F products, 4-MCDF
and DF, were not observed until 650 °C where they achieved
maximum yields of 0.69 and 0.20% respectively. Above 900
°C no chlorinated aromatic products were detected.
Nondioxin products were also detected from the oxidation
of 2-MCP. (cf. Figure 2 and Table 1). Chlorinated products
included chlorobenzene (maximum yield of 0.03% at 650
°C), 1-chloronaphthalene (maximum yield of 0.10% at 650
°C), 2,4-dichlorophenol, and 2,6-dichlorophenol (maximum
yields of 0.28 and 0.59, respectively at 600 °C). Non-
chlorinated products included naphthalene, benzene, and
phenol. Naphthalene is the only non-PCDD/F product
observed at 400 °C and achieves a maximum yield of 0.22%
at 700 °C. However the percent yield of naphthalene remains
reasonably constant between 400 °C and 900 °C. Benzene
and phenol were observed between 600 and 700 °C and
achieved maximum yields at 650 °C.
FIGURE 1. “Dioxin” products from the gas-phase oxidation of 2-MCP.
[2-MCP]o ) 88 ppm in helium. Gas-phase reaction time of 2.0 s.
where [Product] is the concentration of the particular product
formed (in moles) and [2-MCP]0 is the initial concentration
of 2-MCP (in moles) injected into the reactor. (Please note
that in our previous publication on the pyrolysis of 2-MCP
(17) the concentration of the chlorinated products that
contained two phenyl rings was multiplied by two to account
for the fact that the maximum yield for total PCDD/F from
chlorinated phenols can be no greater than 50%.) These
observed oxidation products were dibenzo-p-dioxin, 4,6-
dichlorodibenzofuran, 1-chlorodibenzo-p-dioxin, dibenzo-
furan, naphthalene, chloronaphthalene, and diphenylethyne.
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%.
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.
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 es-
timate the concentrations of reactive species such as •OH,
O•, H•, and Cl•. The Chemkin Equil code was used to calculate
the concentrations of these species over a range of reaction
temperature from 570 to 1270 K. The initial inputs for the
calculations were the same as the experimental runs in that
the initial 2-MCP concentration was held at 88 ppm and the
initial O2 concentration was held at 20%. Over this temper-
ature range, the major products species resulting in the
calculation were CO2, H2O, HCl, and Cl2. Other species
included were •OH, O•, H•, H2, HO2, H2O2, CO, and Cl•.
Discussion
The formation of dioxin products (DD, 1-MCDD, and 4,6-
DCDF) indicates that stable phenoxyl radicals are formed in
significant yields through loss of the hydroxyl-hydrogen. The
formation of aromatics (phenol, chlorobenzene, and ben-
zene) indicates that simple substitution reactions are oc-
curring. The formation of 2,4-dichlorophenol and 2,6-
dichlorophenol indicate evidence of chlorination of 2-MCP.
The formation of larger aromatic molecules at low temper-
atures (naphthalene and chloronaphthalene) is the result of
reactions involving the release of CO from the phenoxyl
radicals that will then recombine to form naphthalenes.
2-MCP Decomposition. The addition of oxidative de-
struction pathways with the addition of molecular oxygen
results in the rapid decomposition of 2-MCP beginning at
650 °C rather than 750 °C, the temperature observed under
pyrolytic conditions (17). The decomposition of 2-MCP can
in principle be 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 with a reported rate coefficient for phenol of k1-
(430-930 °C) ) 3.2 × 1015exp(-86 500/RT) s-1 (8, 20). The
bimolecular reaction with O2 via reaction 1a is endothermic
by 28 kcal/mol and is viable only as minor initiation reaction.
C6H4ClOH f C6H4ClO• + H•
∆Hrxn ) 78 kcal/mol (reaction 1)
C6H4ClOH + O2 f C6H4ClO• + HO2•
∆Hrxn ) 28 kcal/mol (reaction 1a)
Results
Bimolecular propagation reactions under oxidative con-
ditions include attack by H•, Cl•, •OH, and O•. In our previous
paper on the thermal degradation of 2-MCP under pyrolytic
conditions, the ∆Hrxn for reactions with H• and Cl• were
discussed (17). It was determined that the most favorable
reactions for generating chlorophenoxyl radicals were the
abstraction of hydrogen by H• or Cl•. With the addition of
O2, one can easily generate •OH and O• that can also abstract
hydrogen via highly exothermic reactions (reactions 2
and 3).
The temperature dependence of the oxidative thermal
degradation of 2-MCP 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 Table 1 and Figure
2. Figures 1 and 2 are presented as the percent yields of
products (or percent of unconverted 2-MCP) on a logarithmic
scale versus temperature. The thermal degradation of 2-MCP
initially increased gradually from 400 to 650 °C, where the
rate drastically accelerated, and achieving 99% destruction
at 850 °C.
The anticipated PCDD/F products, dibenzo-p-dioxin
(DD), 1-chlorodibenzo-p-dioxin (1-MCDD), and 4,6-dichlo-
rodibenzofuran (4,6-DCDF), were all observed for the oxida-
tion of 2-MCP (cf. Figure 1 and Table 1). 4-Chlorodibenzo-
furan (4-MCDF) and dibenzofuran (DF) were additionally
C6H4ClOH + •OH f C6H4ClO• + H2O
∆Hrxn ) -42 kcal/mol (reaction 2)
C6H4ClOH + O• f C6H4ClO• + •OH
∆Hrxn ) -25 kcal/mol (reaction 3)
9
VOL. 39, NO. 1, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 123

TABLE 1. Percent Yield of Products of Gas-Phase Oxidation of 2-Chlorophenol
temperature (°C)
product
400
98
450
500
550
600
650
700
750
800
850
900
2-chlorophenol
82.1
62.7
57.2
0.23
0.29
51.6
1.32
0.92
31.8
0.49
9.34
0.09
0.16
5.74
0.05
0.09
3.99
3.52
0.36
dibenzo-p-dioxin
1-chlorodibenzo-p-dioxin
dibenzofuran
0.12
0.14
0.16
0.44
0.20
0.69
3.62
0.18
0.10
0.81
0.21
0.39
0.04
0.03
0.05
4-chlorodibenzofuran
4,6-dichlorodibenzofuran
naphthalene
1-chloronaphthalene
phenol
0.27
1.23
0.22
0.13
0.69
0.11
0.34
0.16
9.43
0.16
0.24
0.08
0.18
0.08
0.07
0.07
0.14
0.13
0.21
0.28
0.59
2,4-dichlorophenol
2,6-dichlorophenol
benzene
0.07
0.21
0.01
0.02
0.05
0.02
chlorobenzene
0.004
0.005
0.006
a Percent yield ) {([Product])/[2-MCP]0}*100.
SCHEME 1. Reaction Mechanism for the Formation of
2,4-Dichlorophenol and 2,6-Dichlorophenol from the
2-Chlorophenoxyl Radical
FIGURE 2. “Nondioxin” products from the gas-phase oxidation of
2-MCP. [2-MCP]o ) 88 ppm in helium. Gas-phase reaction time of
2.0 s.
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 de-
composition. 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 (2).
Formation of Phenol, Chlorobenzene, Benzene, 2,4-
Dichlorophenol, and 2,6-Dichlorophenol. The formation
of phenol is likely due to the exothermic displacement of Cl•
by H• (∆Hrxn ) -17 kcal/mol). Chlorobenzene and benzene
are formed with much lower yields and at slightly higher
temperatures than phenol. These lower yields are due to the
more endothermic displacement of •OH from 2-MCP by
hydrogen to form chlorobenzene (∆Hrxn ) 0 kcal/mol) and
the displacement of •OH from phenol by hydrogen to form
benzene (∆Hrxn ) 4 kcal/mol). The temperature ranges of
formation of all these products are lower than for previous
results of 2-MCP under pyrolytic conditions (17). This is due
to the early onset of reaction of 2-MCP under oxidative
conditions and the oxidation of the products at higher
temperatures.
2,4-Dichlorophenol and 2,6-dichlorophenol are produced
from the chlorination of the 2-MCP. Since displacement of
H•by Cl•is endothermic, the direct reaction of Cl•with 2-MCP
is unlikely. The formation of dichlorophenol is instead due
to recombination of 2-chlorophenoxyl radicals and Cl•.
Scheme 1 depicts the formation of 2,4-dichlorophenol and
2,6-dichlorophenol by Cl• attack at the resonance-stabilized,
ortho- or para- carbon sites of the chlorophenoxyl radicals
(∆Hrxn ) -43 kcal/mol). Subsequent tautomerization results
Rate coefficients based on analogous reactions with
phenol for reactions 2 and 3 are k2 (1000-1150 K) ) 6.0 ×
1012 cm3/mol/s and k3 (340-870 K) ) 1.28 × 1013exp(-2900/
RT) cm3/mol/s (21). Based on pseudoequilibrium calculations
of the concentrations of reactive radicals and their rate
coefficients for reaction with phenol, the rate of reaction 2
is 190× faster than reaction 3 and a factor of 30 faster than
reaction 1. Thus, reaction 2 is one of the dominant sources
of chlorophenoxyl radical under oxidative conditions.
Our pseudoequilibrium calculations also indicate that with
the addition of oxygen, there is an increase in the concen-
tration of Cl•. This can be explained by reaction 4 in which
•OH abstracts hydrogen from HCl to form Cl•.
•OH + HCl f H2O + Cl•
(reaction 4)
Reaction 4 has a rate coefficient of k4 (300-700 K) ) 1.77
× 1012exp(-0.89 kcal/RT) cm3/mol/s (21). With the increase
in the concentration of Cl•, the abstraction of hydrogen by
Cl• with a rate coefficient for phenol of k (25 °C) ) 1.43 ×
1014 cm3/mol/s continues to be the dominant source for the
formation of chlorophenoxyl radicals (21). In fact based on
our pseudoequilibrium calculations at 700 °C, the rate for
the abstraction of hydrogen by Cl• is 5000 times faster than
the rate for reaction 2.
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
PCDD/F products (1). However the initiation of the decom-
position of 2,4,6-TCP and 2-MCP at temperatures as low as
400 °C requires us to consider other possible low energy
in the formation of the respective dichlorophenols (∆Hrxn
-20 kcal/mol) (22). Under pyrolytic conditions, HCl is a
largely unreactive sink for chlorine. Under oxidative condi-
)
9
124 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 1, 2005

SCHEME 2. Postulated Pathways for the Formation of DD, 1-MCDD, and 4,6-DCDF
tions, •OH and HO2• can react exothermically with HCl to
produce a relative abundance of Cl• (23, 24). Consequently,
chlorination reactions are enhanced under oxidative condi-
tions (25-27).
The yields of naphthalene and chloronaphthalene are
significantly lower under oxidative than pyrolytic conditions,
most probably due to the more rapid rate of oxidation of the
cyclopentadienyl radical (17). The oxidation rate of naph-
thalene is also increased. In addition, the yield of PCDD/F
is expected to increase quadratically with the increase in
chlorinated phenoxyl radical concentration. PCDD/F forma-
tion competes with the formation of naphthalene by elimi-
nation of CO to cyclopentadienyl radical (vide infra), and the
concentration of naphthalene never becomes a major product
as it did under pyrolytic conditions.
Formation of Dibenzo-p-dioxin, 1-Chlorodibenzo-p-
dioxin, 4,6-Dichlorodibenzofuran, 4-Chlorodibenzofuran,
and Dibenzofuran. Scheme 2 summarizes previously identi-
fied reaction pathways to DD, 1-MCDD, and 4,6-DCDF from
the reaction of the different mesomers of 2-chlorophenoxyl
radical.
Formation of Naphthalene and Chloronaphthalene.
Formation of polycyclics such as naphthalene and chlo-
ronaphthalene has been traditionally ascribed to molecular
growth pathways involving largely C2 fragments (28-30).
However, the low-temperature onset of formation of naph-
thalene (400 °C) suggests a pathway that does not require
the complete fragmentation of 2-MCP. The recombination
of two cyclopentadienyl radicals has been previously shown
to be a favorable pathway for formation of naphthalene where
the reported ∆Hrxn for this overall reaction is 9.23 kcal/mol
(31, 32). In our previous work on the pyrolysis of 2-MCP, we
presented a reasonable pathway for the formation of
naphthalene from the 2-chlorophenoxyl radical through
elimination of CO to form a cyclopentadienyl radical with a
calculated ∆Hrxn of 20 kcal/mol and an assigned rate
coefficient of k6 (730-1300 °C) ) 1 × 1011.4exp(-22100 (
450/T) s-1 (17, 20). Another possible low-temperature route
to the formation of naphthalene is described in Scheme 2 as
a competitive pathway to the formation of 4,6-DCDF (33).
Pathway IA in Scheme 2 depicts the mechanism for DD
formation; the oxygen-centered radical mesomer recombines
with the carbon (chlorine substituted) centered radical
mesomer to form a keto-ether. Following the abstraction of
chlorine by H•, Cl•, or •OH, DD is formed by intra-annular
elimination of Cl•. Another possible pathway for the formation
9
VOL. 39, NO. 1, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 125

SCHEME 3. Proposed Pathways for the Formation of 4-MCDF
of DD is through a radical-molecule reaction, IB, shown in
parentheses below the radical-radical pathway in Scheme
2. This reaction depicts the oxygen centered radical mesomer
reacting with 2-MCP via Cl• displacement to form a chlo-
rohydroxy diphenyl ether (HDE) followed by the abstraction
of H• by •OH. Finally, DD is formed by intra-annular
displacement of Cl•. It has been previously suggested that
this radical-molecule reaction is too slow to account for the
observed yields of the DD (1, 2, 11).
Formation of 1-MCDD, shown as pathway IIA in Scheme
2, 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-MCDD occurs through intra-annular displacement
of Cl•. Pathway IIB depicts an alternate pathway in which
ring closure is by unimolecular elimination of HCl.
Pathways IIIA and IIIB in Scheme 2 depicts the proposed
pathway of 4,6-DCDF formation and the competing pathway
of naphthalene formation. Initially, for both pathways, two
carbon-hydrogen centered radical mesomers react to form
a common diketo-dimer intermediate. The dimer can then
follow pathway IIIA by the abstraction of hydrogen by •OH
or Cl• and undergo tautomerization followed by the dis-
placement of hydroxyl to form 4,6-DCDF. In pathway IIIB,
the initially formed diketo-dimer possibly forms naphthalene
by elimination of two CO molecules followed by a unimo-
lecular isomerization and elimination of two Cl• (33).
for pyrolysis. 4. 1-MCDD is observed at temperatures as low
as 400 °C under oxidative conditions.
The first two observations are readily explained. The
addition of oxygen to the systems results in oxidation of the
decomposition products of 2-MCP before they can form the
observed aromatic molecular growth products. The addition
of oxygen also results in lower temperature formation of
chlorophenoxyl radical where it can react to form PCDD/F
products rather than decompose. The explanations of
observations 3 and 4 are slightly more subtle.
The maximum yields of DD under oxidative and pyrolytic
conditions are 1.32 and 0.28%, respectively. The maximum
yields of 1-MCDD are 0.92 and 0.05%. In contrast the
maximum yield of 4,6-DCDF, which was not detected under
pyrolytic conditions, was 9.43% under oxidative conditions.
Thus DD only increased by a factor of 4.7, 1-MCDD increased
by a factor of 18.4, while 4,6-DCDF increased at least 100×
with the addition of oxygen to the system.
Examination of the major pathways to DD, 1-MCDD, and
4,6-DCDF in Scheme 2 reveals that chlorine is abstracted to
form DD, one hydrogen is abstracted to form 1-MCDD, and
one of two hydrogens is abstracted to form 4,6-DCDF. H• is
the major chain carrier in the 2-MCP system under pyrolytic
conditions, while •OH and Cl• are the major carriers under
oxidative conditions. The ∆Hrxn for abstraction of Cl• by •OH
(pathway IA in Scheme 2) is endothermic by 45 kcal/mol.
Thus, chlorine abstraction by •OH is not expected to be
competitive with the abstraction of chlorine by H• (∆Hrxn
)
We believe that DF is simply a recombination product of
unchlorinated phenoxyl radical formed from decomposition
of phenol (34). The reaction proceeds by mechanisms
analogous to those shown for formation of 4,6-DCDF in
pathways IIIA in Scheme 2.
-64 kcal/mol) and Cl• (∆Hrxn ) -19 kcal/mol). The addition
of oxygen consequently has little effect on the yield of DD
other than the increase in Cl• present that increases the
concentration of chlorophenoxyl radicals. However in path-
ways IIA and IIIA, ∆Hrxn for the abstraction of hydrogen by
•OH is exothermic by 16 kcal/mol. Thus, the addition of
oxygen to the system increases the •OH and Cl• concentra-
tions, which increases the rate of abstraction of hydrogen to
form 1-MCDD and 4,6-DCDF. The increase in yield of 4,6-
DCDF due to an increase in •OH and Cl• concentrations is
exponentially larger than for 1-MCDD because the hydrogen
abstraction rate coefficient is doubled due to the presence
of two sites for hydrogen abstraction in the diketo inter-
mediate involved in 4,6-DCDF formation versus the “single”
keto intermediate in formation of 1-MCDD.
Formation of 1-MCDD and Naphthalene from 400 to
600 °C. Previous studies of the oxidation of 2-MCP over a
wide temperature range and longer reaction times have also
reported 4,6-DCDF as the major product (13-16). Only the
study of Weber and Hagenmaier who studied lower tem-
perature reactions (330-600 °C) reports 1-MCDD as a major
product at a low temperature of 375 °C (15, 16) in addition
to DD, 4,6-DCDF, and 4-MCDF. The lower formation
temperature is likely due to the longer residence times of 10
min of their experiments. Our results also show a low
temperature formation of 1-MCDD as well as naphthalene.
Scheme 3 depicts reasonable pathways for the formation
of 4-MCDF, which was observed under oxidative but not
pyrolytic conditions. Two pathways are depicted: (1) the
recombination of the carbon (hydrogen)-centered radical
mesomer with the carbon (chlorine)-centered radical me-
somer and (2) the recombination of the carbon (hydrogen)-
centered radical mesomer with an unchlorinated carbon-
centered mesomer to form a diketo-dimer. For the first
pathway, H• abstracts chlorine and, in the second, •OH
abstracts another hydrogen. Both pathways then undergo
tautomerization followed by displacement of hydroxyl to form
4-MCDF. Based on comparisons to simple alkane reactions
for the abstraction of chlorine by H• and the abstraction of
hydrogen by •OH, the lower pathway is a much more
favorable route for 4-MCDF formation under oxidative
conditions, while the upper pathway of Cl• abstraction by H•
is more favorable under pyrolytic conditions (35-37).
Oxidation versus Pyrolysis of 2-MCP. Four major dif-
ferences were observed in the results of oxidation and
pyrolysis of 2-MCP: 1. A decrease in non-PCDD/F products
under oxidative conditions. 2. An increase in total PCDD/F
product yield. 3. 4,6-DCDF is the major product under
oxidation, while DD and 1-MCDD are the major products
The observation of naphthalene and 1-MCDD as the only
organic products below 500 °C suggests (1) that a somewhat
9
126 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 1, 2005

(5) Altwicker, E. R.; Milligan, M. S. Chemosphere 1993, 27, 301.
(6) Addink, R.; Olie, K. Environ. Sci. Technol. 1995, 29, 1425.
(7) Milligan, M. S.; Altwicker, E. R. Environ. Sci. Technol. 1996, 30,
225.
(8) Shaub, W. M.; Tsang, W. Environ. Sci. Technol. 1983, 17, 721.
(9) Bruce, K.; Beach, L. O.; Gullett, B. K. Waste Manage. 1983, 11,
97.
(10) Altwicker, E. R. Sci. Total Environ. 1991, 104, 47.
(11) Louw, R.; Ahonkhai, S. T. Chemosphere 2002, 46, 1273.
(12) Sidhu, S. S.; Masqud, L.; Dellinger, B.; Mascolo, G. Combust. Sci.
Technol. 1995, 100, 11.
(13) Waiter-Protas, I.; Louw, R. Eur. J. Org. Chem. 2001, 3945.
(14) Born, J. G. P.; Louw, R.; Mulder, P. Chemosphere 1989, 19, 401.
(15) Weber, R.; Hagenmaier, H. Chemosphere 1999, 38, 529.
(16) Weber, R.; Hagenmaier, H. Organohalogen Compd. 1997, 31,
480.
(17) Evans, C. S.; Dellinger, B. Environ. Sci. Technol. 2003, 37, 1325.
(18) Rubey, W. A.; Grant, R. A. Rev. Sci. Instrum. 1988, 59, 265.
(19) Taylor, P. H.; Tirey, D. A.; Dellinger, B. Combust. Flame 1996,
106, 1.
(20) Colussi, A.; Zabel, F.; Benson, S. W. Int. J. Chem. Kinet. 1977,
9, 161.
(21) NIST Chemical Kinetics Database 17, Gaithersburg, MD, 1998.
(22) Capponi, M.; Gut, I.; Hellrum, B.; Persy, G.; Wirz, J. Can. J. Chem.
1999, 77, 605.
(23) Soderstrom, G.; Marklund, S. Environ. Sci. Technol. 2002, 36,
1959.
different mechanism of formation may be operative at these
lower temperatures for which the radical pool is not fully
developed, and (2) their formation mechanisms have a
common intermediate or are at least closely related.
After the recombination of chlorinated phenoxyl radicals
to form the diketo intermediate, the formation of naphthalene
shown in pathway IIIB in Scheme 2 is unimolecular (33).
This can explain the high yields at low temperatures before
the radical pool has developed. The observation of significant
yields of 1-MCDD at low temperature suggests that it may
also be formed by a unimolecular pathway following the
formation of the keto-ether intermediate via radical-radical
recombination. Alternatively to the abstraction of hydrogen
by •OH in pathway IIA in Scheme 2, a simple, intra-ring one
proton tautomerization results in the formation of a hydroxy-
diphenyl ether intermediate that can then form 1-MCDD by
inter-ring elimination of HCl. Above 500 °C, the radical pool
increases rapidly, and bimolecular pathways involving
hydrogen and chlorine abstraction begin to dominate the
formation of 1-MCDD and other PCDD/F products.
In summary, we have proposed reasonable mechanisms
for the formation of each observed product of the oxidation
of 2-MCP. We have also identified mechanistic rationales for
the differences in product distribution and PCDD to PCDF
branching ratios for oxidative versus pyrolytic conditions.
Comparison of oxidation and pyrolysis results have also
identified a possible lower temperature, primarily unimo-
lecular routes to formation of naphthalene and 1-MCDD
that can occur before the radical pool increases significantly
at 600 °C. These results have implications in the mechanism
of molecular growth and particle formation from resonance
stabilized radicals such as cyclopentadienyl and phenoxyl
radicals and their derivatives.
(24) Wilkstrom, E.; Ryan, S.; Touati, A.; Telfer, M.; Gullett, B. K.
Environ. Sci. Technol. 2003, 37, 1108.
(25) Procaccini, C.; Bozzelli, J. W.; Longwell, J. P.; Sarofim, A. F.;
Smith, K. A. Environ. Sci. Technol. 2003, 37, 1684.
(26) Huang, J.; Onal, I.; Senkan, S. M. Environ. Sci. Technol. 1997,
31, 1372.
(27) Satyapal, S.; Werner, J. H.; Cool, T. A. Combust. Sci. Technol.
1995, 106, 229.
(28) Appel, J.; Bockhorn, H.; Frenklach, M. Combust. Flame 2000,
121, 122.
(29) Richter, H.; Howard, J. B. Prog. Energy Combust. Sci. 2000, 26,
4.
Acknowledgments
(30) Richter, H.; Mazyer, O. A.; Sumathi, R.; Green, W. H.; Howard,
J. A.; Bozzelli, J. W. J. Phys. Chem. A 2001, 105(9), 1561.
(31) Melius, C. F.; Calvin, M.; Marinov, N. M.; Ritz, W. J.; Senkan, S.
M. 26th Symp. (Intern.) on Combustion, The Combustion Inst.
1996, 685.
(32) Lu, M.; Muholland, J. A. Chemosphere 2001, 42, 623.
(33) Kim, D. H.; Mulholland, J. A.; Ryu, J.-Y. 26th Symp. (Intern.) on
Combustion, The Combustion Inst. 2004, in press.
(34) Waiter, I.; Born, J. G. P.; Louw, R. Eur. J. Org. Chem. 2000, 921.
(35) Green, J. H. S.; Maccoll, A. J. Chem. Soc. 1955, 2449.
(36) Price, S.; Shaw, R.; Trotman-Dicksenson, A. J. Chem. Soc. 1956,
3855.
We gratefully acknowledge the assistance of our colleagues,
Dr. Lavrent Khachatryan and Alexander Burcat, in evaluation
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 contract 9C-R369-NAEX, EPA grant R828166, and
the Patrick F. Taylor Chair foundation.
Literature Cited
(1) Khachtrayan, L.; Burcat, A.; Dellinger, B. Combust. Flame 2003,
132, 406.
(37) Kale, M.; Maccoll, A. J. Chem. Soc. 1957, 5020.
(2) Khachtrayan, L.; Asatrayan, R.; Dellinger, B. Chemosphere 2003,
52, 695.
Received for review April 28, 2004. Revised manuscript re-
ceived August 7, 2004. Accepted October 8, 2004.
(3) Karasek, F. W.; Dickson, L. C. Science 1987, 237, 754.
(4) Dickson, L. C.; Lenoir, D.; Hutzinger, O. Environ. Sci. Technol.
1992, 26, 1822.
ES049355Z
9
VOL. 39, NO. 1, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 127