Reactions of 2,4,6-trichlorophenol on model fly ash: Oxidation to CO and CO2, condensation to PCDD/F and conversion into related compounds

Article abstract of DOI:10.1016/S0045-6535(00)00244-7

Thermal treatment of 2,4,6-trichlorophenol on a magnesium silicate-based model fly ash in the temperature range between 250°C and 400°C leads predominantly to carbon monoxide and carbon dioxide. The fraction of 2,4,6-trichlorophenol which is oxidized to CO and CO₂ increases from 3% at 250°C to 75% at 400°C. Further products are polychlorinated benzenes, dibenzo-p-dioxins, dibenzofurans and phenols. The homologue and isomer patterns of the chlorobenzenes suggest chlorination in the ipso-position of the trichlorophenol. The formation of PCDD from 2,4,6-trichlorophenol and 2,3,4,6-tetrachlorophenol on municipal solid waste incinerator fly ashes and model fly ash were compared and the reaction order calculated.

Full text of DOI:10.1016/S0045-6535(00)00244-7

Chemosphere 42 (2001) 697±702
Reactions of 2,4,6-trichlorophenol on model ¯y ash: oxidation
to CO and CO , condensation to PCDD/F and conversion into
related compounds
2
a,
K. Hell , L. Stieglitz , E.R. Altwicker , R. Addink , R. Will
a
b
b
a
*
a
Forschungszentrum Karlsruhe, ITC-CPV, Postfach 3640, D-76021 Karlsruhe, Germany
b
Rensselaer Polytechnic Institute, 110 8th Street, Troy, New York 12180-3590, USA
Abstract
Thermal treatment of 2,4,6-trichlorophenol on a magnesium silicate-based model ¯y ash in the temperature range
between 250°C and 400°C leads predominantly to carbon monoxide and carbon dioxide. The fraction of 2,4,6-tri-
chlorophenol which is oxidized to CO and CO increases from 3% at 250°C to 75% at 400°C. Further products are
2
polychlorinated benzenes, dibenzo-p-dioxins, dibenzofurans and phenols. The homologue and isomer patterns of the
chlorobenzenes suggest chlorination in the ipso-position of the trichlorophenol. The formation of PCDD from 2,4,6-
trichlorophenol and 2,3,4,6-tetrachlorophenol on municipal solid waste incinerator ¯y ashes and model ¯y ash were
compared and the reaction order calculated. Ó 2001 Elsevier Science Ltd. All rights reserved.
Keywords: Heterogeneous reaction; 2,3,4,6-tetrachlorophenol; Fly ash; Reaction order
1
. Introduction
cursors are formed in MSWI de-novo from particulate
carbon. Among the possible precursors for the forma-
tion of PCDD/F, it seems that PCPh have the highest
potential to generate PCDD (Altwicker and Milligan,
1993; Addink et al., 1995; Ghorishi and Altwicker,
1996), while chlorinated biphenyls (Scholz et al., 1997)
as well as biphenyl (Wilhelm et al., 1999) are the most
e€ective precursors in the PCDF formation processes.
Similar to the oxidative degradation of particulate
carbon on ¯y ash (Milligan and Altwicker, 1993; Stie-
glitz et al., 1993), phenol and monochlorophenols are
Polychlorinated dibenzodioxins and dibenzofurans
PCDD/F) and several classes of other chlorinated aro-
(
matic compounds are formed in thermal industrial
processes like municipal solid waste incineration
(
(
(
MSWI; Stieglitz et al., 1989a,b), metal ore sintering
Buekens et al., 1998), smelting of secondary aluminum
Lahl et al., 1995), etc. Besides carbon as a source for the
generation of PCDD/F ± the so-called de-novo-synthesis
Stieglitz et al., 1989a,b, 1997; Stieglitz, 1998) ± chlori-
(
nated phenols (PCPh) play an important role as pre-
cursors (Dickson and Karasek, 1987; Milligan and
Altwicker, 1996) or intermediates (Stieglitz et al.,
mainly oxidized to CO and CO
observed a steep increase in the conversion of mono-
chlorophenols to CO and CO with rising temperature,
from 5% at 350°C to about 70% at 450°C. In contrast,
Milligan (1994) reported no increase in the CO and CO
concentrations versus the de-novo baseline, when
,3,4,6-Cl Ph was ¯owed over ¯y ash, while observing
2
. Born et al. (1993)
2
1989a,b; Blaha and Hagenmaier, 1995; Hell et al., 1997)
in the formation of PCDD. PCPh as well as other pre-
2
2
4
*
nearly complete conversion of the chlorophenol; possi-
bly due to di€erences in ¯y ash activity and a higher
degree of chlorination inhibiting the oxidation. As 2,4,
Corresponding author. Tel.: +49-7247-823-395; fax: +49-
247-822-244.
E-mail address: katharina.hell@itc-cpv.fzk.de (K. Hell).
7
0
045-6535/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 4 5 - 6 5 3 5 ( 0 0 ) 0 0 2 4 4 - 7

6
98
K. Hell et al. / Chemosphere 42 (2001) 697±702
6
-Cl
3
Ph can be the dominant congener among the Cl
3
Ph
>90%) and among the total sum of PCPh (>40%) in the
Cl
ble 1). The high concentration of the Cl
almost exclusive formation of 1,2,3,5-Cl
4
Bz), followed by 1,3,5-Cl
3
Bz, Cl
5
Bz and Cl
Bz and the
Bz isomer
6
Bz (Ta-
(
4
emissions of a MSWI (Aittola and V a nni, 1992; Jay and
Stieglitz, 1995), we investigated the reactions of 2,4,6-
4
suggest chlorination in ipso-position: nucleophilic ad-
dition of a chlorine and elimination of the hydroxyl
group.
Cl
tion of CO, CO
order for the condensation of 2,4,6-Cl
Cl Ph, respectively, to PCDD on di€erent ¯y ashes was
then calculated.
3
Ph on a model ¯y ash with emphasis on the forma-
, PCBz and PCDD/F. The reaction
Ph and 2,3,4,6-
2
3
4
3.2. Formation of PCDD/F
At 250°C, the PCDD concentration increases with
prolonged reaction time, while at 300°C, a concentration
maximum is obtained after 15 min. Even higher tem-
perature leads to high PCDD concentrations for very
short reaction times (1 min) followed by a steep de-
crease. This ®nding can be explained as follows: with
increasing temperature the destruction rate grows more
rapidly than the formation rate so that a concentration
maximum of PCDD results at 300°C (Fig. 1). Similar
results were obtained for PCDF with the di€erence that
a concentration maximum is observed at 350°C and the
concentrations are much smaller than those of PCDD.
2
. Material and methods
The experiments were conducted in a tubular glass
reactor (1.5 cm diameter ´ 50 cm length) which was
mounted vertically in an oven. The model ¯y ash (83.7%
Florisil, 14.7% KCl, 1.1% CuCl
Cl Ph) was placed on a glass frit in the middle of the
2
2
á 2H O and 0.5% 2,4,6-
3
glass tube and ¯ushed with a gas stream of 20% oxygen
in helium (50 ml/min). Florisil itself is composed of 85%
2
SiO and 15% MgO. The volatile organic compounds
were collected in a toluene impinger. Subsequently, the
gas phase was passed through a mass spectrometer for
3
The highest conversion of 2,4,6-Cl Ph to PCDD/F is
1%. In a recent study, Sakurai et al. (1996) reported a
yield of 1±2% of PCDD between 250°C and 300°C,
while PCDF were only formed in the presence of HCl in
the gas phase.
2
the online measurement of CO and CO . Quanti®cation
was performed by means of an external standard
method, using a calibration gas mixture (Messer Gries-
heim). The extract of the model ¯y ash and the solution
of the toluene impinger were cleaned-up together. The
PCPh, PCBz and PCDD/F were measured by HRGC/
3.3. Oxidation to CO and CO
2
1
3
LRMS with a 30 m DB-5 column and quanti®ed by
internal standards.
C
Despite the presence of three chlorine atoms in 2,4,6-
3
Ph the main reaction is the oxidative degradation.
Cl
A further series of experiments was performed with
model ¯y ash and a MSWI ¯y ash from the Environ-
mental Protection Agency (termed EPA ¯y ash hereaf-
2
The quantity oxidized to CO and CO increases from 3%
at 250°C (1 min) to more than 75% at 400°C (60 min)
(Table 2). No CO is formed at 250°C and 300°C (1 min).
Only for reaction temperatures ꢀ300°C (reaction times
ꢀ15 min) small amounts of CO could be detected. A
3
ter) with di€erent 2,4,6-Cl Ph concentrations in the gas
phase. Details of the experimental conditions and data
have been published recently (Hell et al., 1998, 1999).
CO
2
/CO-ratio  5 is observed. This value is slightly
lower than reported in the literature for monochlor-
ophenols (CO
/CO-ratio  6; Born et al., 1993).
In this series of experiments, a model system was
doped with solid 2,4,6-Cl Ph, so that the data are not
2
3
. Results and discussion
3
3.1. Conversion of 2,4,6-trichlorophenol to other chlori-
nated phenols and benzenes
directly comparable with the results obtained on ¯y ash
by Born et al. (1993) and Milligan (1994). On the Con-
trary to what might be expected, the high oxidation rate
shows that the propensity of chlorophenols towards
oxidation is not diminished by the chlorination degree.
From the fact, that PCBz appear to be inert towards
oxidation (their concentration increases with tempera-
ture and time), an oxygen function in the molecule seems
to play a key role in the oxidation process. This suggests
that the catalytically active species might interact with
the adsorbed chlorophenol molecules via their oxygen
function.
3
Thermal treatment of 2,4,6-Cl Ph-doped model ¯y
ash in the temperature range between 250°C and 400°C
and reaction times between 1 and 60 min leads pre-
dominantly to higher chlorinated phenols. The total
conversion of 2,4,6-Cl
into other chlorophenols (main products 2,3,4,6-Cl
and Cl Ph).
3
Ph is >90%; <0.5% is converted
4
Ph
5
The yield of PCBz increases with reaction tempera-
ture and time, from 0.02% (1 lg/g at 250°C, 1 min) to
2
isomer is formed in the highest concentration (>50% of
.2% (120 lg/g at 400°C, 60 min). The 1,2,3,5-Cl
4
Bz
For two experiments, the continuously measured
fractions of CO and CO
2
(volume%) in the gas phase are
the total sum of PCBz and greater than 96% of the
shown in Fig. 2, at 300°C (reaction time 1 min) and at

K. Hell et al. / Chemosphere 42 (2001) 697±702
699
3
time of only 1 min (Fig. 2(a)), CO -evolution is detected
50°C (reaction time 30 min), respectively. For a heating
2
over 25 min. Measurement was stopped only after the
baseline value was reached. It is interesting to note that
for reaction temperatures P 300°C (time P 15 min),
the online CO
mums (Fig. 2b). The main maximum results from oxi-
dation of 2,4,6-Cl Ph. The second maximum was
2
concentration pro®le shows two maxi-
3
observed for experiments in which the PCDD concen-
tration already decreases after short reaction times
(
300°C, time P 15 min and 350±400°C). Thus, the sec-
ond rise of the CO concentration might be related to
the oxidation of the products of the 2,4,6-Cl Ph, e.g.,
2
3
PCDD/F or oligomerization and polymerization prod-
ucts.
3.4. Conversion of chlorophenols to PCDD on model ¯y
ash and MSWI ¯y ashes
The rates of formation of PCDD/F from precursors
as well as from carbon are crucial to understand the
concentration levels of PCDD/F in the e‚uents of
MSWI. In earlier studies, the reactivities of several
chlorophenols to PCDD/F over a single ¯y ash were
compared at one temperature (300°C) (Altwicker and
Milligan, 1993; Milligan and Altwicker, 1996). From
that investigation it was concluded that 2,3,4,6-Cl
7
4
Ph is
±8 times more reactive than 2,4,6-Cl Ph. In subsequent
3
studies, ¯y ash dilution of 1:10 (with inert glass beads)
was required to approach di€erential reactor conditions
(
i.e., ‰Cl
x
PhŠꢁ  ‰Cl
x
PhŠ ) which would facilitate ki-
in†
ꢁout†
netic analysis of the results. The EPA ¯y ash used in this
study appears to be much more reactive (a better cata-
lyst) than the model ¯y ash. Even with a 10-fold dilution
of the ¯y ash with inert glass beads di€erential reactor
conditions could not be obtained for 2,4,6-Cl
EPA ¯y ash as well as on model ¯y ash; most of the
,4,6-Cl Ph were converted. Unfortunately, the ¯y ash
Ontario ¯y ash) used in the original study (Milligan and
Altwicker, 1996) with 2,3,4,6-Cl Ph is no longer avail-
able. The PCDD formation rate (Table 3) from 2,4,6-
Cl Ph on EPA ¯y ash (Hell et al., 1998) is similar to that
from 2,3,4,6-Cl Ph in the original study (Milligan and
3
Ph on
2
3
(
4
3
4
Altwicker, 1996) and approximately three times higher
than on model ¯y ash (Table 3). The results suggest that
the nature of the ¯y ash may be crucial in controlling the
absolute and relative reactivities of chlorophenols to
form PCDD/F. Nevertheless, the results can be used to
derive the reaction order for the formation of PCDD
from 2,4,6-Cl
2
3
Ph on EPA and model ¯y ash and from
,3,4,6-Cl Ph on Ontario ¯y ash. The PCDD concen-
4
trations formed on the model ¯y ash and two MSWI ¯y
ashes (10-fold diluted with inert glass beads) as a func-
tion of gaseous chlorophenols in the in¯ow are sum-
marized in Table 3.

7
00
K. Hell et al. / Chemosphere 42 (2001) 697±702
Fig. 1. Formation of PCDD and PCDF (ng/g) on model ¯y ash from 2,4,6-Cl
Table 2
3
Ph as a function of reaction temperature and time.
Fraction of 2,4,6-Cl
Temperature
Time (min)
CO
3 2
Ph (%) which is oxidized to CO and CO
250°C
300°C
350°C
1
400°C
1
15
0
30
0
60
0
1
15
2.6
30
2.9
60
4.6
15
30
60
1
15
30
60
0
2.6
0
4.8
9.5 10.4 15.2 10.6 11.5 11.2 16.6
CO
CO
2
6.1 10.6 11.6
1
5.9 11.0 14.6 27.5 25.5 44.1 56.1 69.5 38.8 51.3 52.6 58.5
4.2 5.0 6.0 5.3 4.6 5.4 4.6 3.7 4.5 4.7 3.5
2
/CO
1
1
1
1
For the calculation of the reaction order, we assume
that the decrease of the chlorophenol concentration,
based on the formation of PCDD, is given by
Taking into account that only a fraction of the chlor-
ophenol is converted into PCDD (6% on EPA ¯y ash,
2.2% on model ¯y ash and 25% on Ontario ¯y ash), the
chlorophenol concentration for this pathway has to be
corrected
d‰PhŠ
n
�
ˆ k‰PhŠ ;
ꢁ1†
dt
‰
PhŠcorr: ˆ f ‰PhŠ:
ꢁ3†
with Ph ˆ 2,4,6-Cl
3 4
Ph or 2,3,4,6-Cl Ph, respectively.
Taking logarithms on both sides leads to
With the assumption that the increase of the PCDD
concentration (formation of PCDD) is linearly corre-
lated to the decrease of the chlorophenol concentration,
the following equation results:
ꢀ
ꢁ
d‰PhŠ
dt
ln
�
ˆ lnk ‡ nln‰PhŠ:
ꢁ2†
Fig. 2. Online volume percentages of CO and CO
1
2 3
formed by oxidation of 2,4,6-Cl Ph on model ¯y ash. The values are normalized to
g model ¯y ash: (a) heating time: 1 min (at 300°C); (b) heating time: 30 min (at 350°C).

K. Hell et al. / Chemosphere 42 (2001) 697±702
701
Table 3
PCDD concentration (lg/g) after thermal treatment of EPA and model ¯y ash (0.2 g ¯y ash and 1.8 g glass beads) as a function of
2,4,6-Cl Ph] (300°C, 15 min),
Ph] (300°C, 30 min), and Ontario ¯y ash (0.1 g ¯y ash and 0.9 g glass beads) as a function of [2,3,4,6-Cl
0% O
[
3
4
1
2
in N
2
, 80 ml/min; x ˆ #of Cl-atoms/phenol
EPA ¯y ash
(
Model ¯y ash
(x ˆ 3)
2.6
Ontario ¯y ash
(x ˆ 4)
150
x ˆ 3)
Cl
x
Ph (ng/ml)
0
18
55
186
400
467
3.9
6.2
166
316
±
410
±
350
700
PCDD (lg/g)
PCDD (lg/g)
PCDD (lg/g)
1.8
±
5.2
±
11.4 40.6
207
±
232
±
±
±
±
±
±
±
±
±
±
±
±
0.2
±
0.9
±
0.6
±
21.4
±
76.0 79.4
±
±
20
±
77
±
110
±
±
±
±
±
ꢀ
ꢁ
d‰PCDDŠ
for Ontario ¯y ash. The formation of PCDD from both
chlorophenols follows approximately a ®rst-order reac-
tion independent on the matrix.
n
ln
ˆ lnꢁkf † ‡ nln‰PhŠ:
ꢁ4†
dt
The calculation of f is based on the yield of PCDD. The
PCDD concentrations were converted in lmol/g s and
those of the chlorophenol concentrations in lmol/g ¯y
ash.
4. Conclusions
The reaction order and the rate coecient can be
determined by plotting ln(d[PCDD]/dt) versus ln[Ph]
In a series of thermal experiments, the formation of
PCDD/F and related compounds from 2,4,6-trichlor-
ophenol as a precursor were studied on model ¯y ash.
Chlorinated benzenes are the major products among the
chlorinated organic compounds derived from the 2,4,6-
trichlorophenol, with lesser quantities of PCDD, PCDF
and higher chlorinated phenols. The PCBz are formed
(
Fig. 3(a)±(c)). Thus, from the slope of the straight line,
the reaction order and from the intercept, the rate co-
ecient for the reaction of 2,4,6-Cl Ph and 2,3,4,6-
Cl Ph, respectively, to PCDD can be obtained. From
3
4
Eq. (4), the following values resulted for the reaction
orders (n): 1.23 for EPA, 1.08 for model and 1.12 for
Ontario ¯y ash, and for the rate coecients (k):
from the 2,4,6-Cl
droxyl group with chlorine. Dechlorination of the 2,4,6-
Cl Ph and isomerization reactions of the chlorophenols
3
Ph by ipso-substitution of the hy-
�
4
� 4
� 5
1
.7 ´ 10 for EPA, 2.4 ´ 10 for model and 3.1 ´ 10
3
formed are negligible. The concentration pro®le of the
PCDD and PCDF as a function of the reaction time
shows a maximum at 300°C and 350°C (15 min), re-
spectively; this could be due to a competition between
condensation reactions of the precursor and dechlori-
nation and oxidation reactions of PCDD/F. The main
3
reaction of the 2,4,6-Cl Ph is oxidation to carbon
monoxide and carbon dioxide. In the concentration
pro®le of carbon dioxide, a time-delayed second maxi-
mum is observed, which might be due to the oxidation of
the PCDD/F formed or oligomerization products.
The condensation reactions of gaseous 2,4,6-tri-
chlorophenol and 2,3,4,6-tetrachlorophenol to PCDD
are approximately ®rst-order on both ¯y ash as well as
the model ¯y ash.
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