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Surface catalyzed halogenation-dehalogenation reactions of aromatic bromine compounds adsorbed on fly ash
Chemosphere, Voi.22, No.12, Printed in Greaa Britain
pp 1121-1129,
1991
0045-6535/91 $3.00 + 0.00 Pergamon Press plc
SURFACE CATALYZED HALOGENATION-DEHALOGENATION REACTIONS OF AROMATIC BROMINE COMPOUNDS ADSORBED ON FLY ASH B. Zier, D. Lenoir, E. S. Lahaniatis, A. Kettrup GSF-Forschungszentrum ftir Umwelt und Gesundheit GmbH Institut f'tir 0kologische Chemie Ingolst~dter LandstraBe 1, W-8042 Neuherberg, Germany
ABSTRACT Brominated aromatic compounds like brominated benzenes, diphenylethers and dibenzodioxins adsorbed on the
surface of fly ash from a municipal waste incininerator give mixed brominated/chlorinated und completely chlorinated aromatic compounds. These consecutive halogenation-dehalogenation reactions proceed by a nucleophilic mechanism, which is favoured by a high concentration of chloride on the fly ash. Results of kinetic and stereoselective behavior of these reactions will be discussed. The relevance of these results for PCDD/PCDF formation from bromine precursors in municipal waste incinerators will be discussed.
INTRODUCTION In 1977 chlorinated hydrocarbons I as well as chlorinated dibenzodioxins (PCDD) and dibenzofurans (PCDF) 2 were found in the fly ash of municipal waste incinerators (MWI). Later mixed brominated/chlorinated dibenzodioxins (PBCDD), -furans (PBCDF) 3 and polybrominated dibenzodioxins (PBDD), -furans (PBDF) 4 have been identified as side products in MWI emissions. In contrast to PCDD/PCDF there is less known about the toxicity of brominated and mixed halogenated dioxins and furans. Toxicity of PBDD/PBDF has been found to be similar to their chlorine analogues 5-7. Many investigations about formation and decomposition thermal processes of PCDD/PCDF were performed 8-10. Models for elucidation of mechanisms have been developed in the last few years 11,12. Recently, the importance of surface catalyzed pathways for formation and destruction of PCDD/PCDF has been realized 13,14. It could be shown that PCDD adsorbed on fly ash are chlorinated by treatment with HCI at temperatures between 50 and 300 *C 15. Many other aromatic compounds can be chlorinated in this way. An electrophilic mechanism via formation of surface bound Fe(III)-chloride has been suggested 16. In 1987 it was reported that the reaction of chlorophenols like pentachlorophenol and trichlorophenol adsorbed on fly ash yielded
1121
1122
PCDD (Ullman ether synthesis) 13,17. The yields of this reaction depend on copper(I)- species possibly present on fly ash 18. Formation of biphenyls from brominated benzenes is also caused by active, copper containing surfaces (Ullman I reaction) 19. In this paper a possible formation mechanism of PBCDD/F, PCDD/F and other mixed halogenated or chlorinated compounds from brominated aromatic compounds on fly ash surfaces will be presented. It has been shown that these halogenation-dehalogenation reactions occur by a nucleophilic addition-eliminationmechanism. This work gives an explanation for the presence of PBCDD/PBCDF in fly ash and the increase of PCDD/F output of a MWI by adding brominated compounds to the MWI input20. The role of PBDD/PBDF as precursors of PCDD/PCDF is a new interesting contribution to dioxin discussion.
EXPERIMENTAL MATERIALS Fly ash samples were obtained from a municipal waste incinerator in Munich (Munich South, F.R.G.). A chlorine content of 4.9 % and a bromine content of 0.065 % has been determined by X-ray fluorescence spectroscopy. After homogenisation the fly ash sample was soxhlet-extracted with acetone: hexane (1:1) for 48 hours. The fly ash was then dried at 100 °C for 24 hours in a stream of dry nitrogen and stored in a glass flask. 1,2,4,5-tetrabromobenzene,
hexabromobenzene, and decabromodiphenyl ether (Aldrich), 2,3,7-tribromo-para-
dibenzodioxin and 1,2,3,4-tetrabromo-para-dibenzodioxin (Promochem) were all p.A. quality. Solvents were all suitable to residue analysis.
REACTION CONDITIONS Experiments were carried out in a reaction apparatus developed in our laboratory (Figure 1). In a typical experiment 1 mg 1,2,4,5-tetrabromobenzene in 100 Ixl dichloromethane was added to 0.5 g of fly ash. The solvent was removed at 40 °C in a gentle stream of nitrogen. This fly ash and 3 g of "not occupied" fly ash was then loosely packed in a borosilicate glass tube, 5 mm i.d. The section of the tube with the fly ash was then placed in an oven, at a temperature of 300 *C and connected with an impinger containing 60 ml of hexane/dichloromethane (1:1). Flow rate of dry nitrogen was 10 ml/min. After reaction time of 1 hour fly ash and glass tube were extracted with dichloromethane/benzene (9:1). The solution was combined with the solvent of the impinger, dried with 3 g sodium sulfate and concentrated to 4 ml. This solution was used for GC-MS analysis.
GC-MS ANALYSIS The GC-MS analysis was performed in SIM (single ion monitoring) and SCAN mode with the 5970B system from Hewlett-Packard using a quartz capillary column (10 m * 0.25 mm i.d. SP2330, 0.25 I/m, Supelco). GC-MS methods using an internal standard for calibration of the mass spectrometer were dependent on concentration of brominated compounds used. Analysis parameters: 1 ml/min Helium carder gas flow, splitless injection at 300 °C, interface temperature 250 °C, temperature program: 50 °C 1 min, 10 */min to 247 °C.
1123
Quarz Wool
/',,,
jo.o'
(388 Flow
~~////////////////////.)~
~--~
Implnger
1mgBr-o~mpound 1-10~gFlyAsh adsorbed on 0,5 g Fly Ash Figurel Schematic drawing of the apparatus.The fly ash contained in the glass tube is heated for lh at 300 °C in a flow of dry air.
RESULTS AND DISCUSSION A series of brominated, aromatic compounds has been adsorbed on fly ash and treated for 1 h with air at 300 °C. Extraction and analysis yield mixed brominated/chlorinated as well as completely chlorinated products (Table 1). Under these conditions large quantities of mixed brominated/chlorinated and completely chlorinated compounds are formed. 1,2,4,5-tetrabromobenzene (Br4Bz) yields 1,2,4,5-telrachlorobenzene (C14Bz) besides three mixed halogenated intermediates, which are 1,2,4-tribromo-5-chlorobenzene (Br3C1Bz), dibromo-dichloro-benzene (Br2CI2Bz) and 1-bromo-2,4,5-trichlorobenzene (BrC13Bz), respectively.
Table 1 Formation of mixed brominated/chlorinated and completely chlorinated products by reaction of brominated compounds on 3.5 g of fly ash at 300 *C in a stream of dry air; substitution pattern of the products are the same as of the educts.
Brominated Compound
Yield Of Completely Chlorinated Compound
(%)
Yield Of Mixed Brom./Chlor. Compounds
Recovery
64.4
(%)
(%)
1,2,4,5-Tetrabromobenzene
13.3
45.8
Hexabromobenzene
27.9
56.9
87.5
2,3,7 -Tribromodibenzodioxin
21.5
54.8
1,2,3,4-Tetrabromodibenzodioxin
36.1
23.9 5.1
Decabromodiphenyl ether
85.1
6.4
41.9 91.5
1124
MECHANISM O F THE REACTION These reactions proceed by a consecutive series of aromatic substitutions, showing stereoselective behavior. Bromine is substituted by chlorine of the surface on the fly ash. An issue of interest is the mechanism of bromination of these aromatic substitutions. If there would be an electrophilic mechanism, first hydrogen must be exchanged by chlorine in telIabromobenzene. A probable mechanistic alternative is chlorination by free radical processes. It is not likely since aromatic substitution by chlorine atoms is usually quite inefficient. Reaction of 1,2,4,5-tetrabromobenzene to 1,2,4,5-tetrachlorobenzene shows only substitution in ipso positions. Therefore these reactions can only proceed via nucleophilic aromatic substitution by the addition-elimination mechanism the so-called SNAr mechanism (Figure 2). The chlorine content of used fly ash is about 5 %. Halogenafion-dehalogenation reactions with additive chemism are equilibrium reactions. It is usually possible to shift these in the desired direction by use of an excess of the halide ion21. Reaction of 1 mg 1,2,4,5-tetrabromobenzene on 3.5 g of fly ash takes place by an calculated 100 fold excess of chloride to bromide. Therefore the equilibrium is totally shifted towards formation of 1,2,4,5tetrachlorobenzene.
BF
B r ~
Br
Br + CIBr
Br Br ~
Br
Br ~ Br
Br
Br Br Br
Br
Br
- Br- 1 Br
Br~]/CI Br Figure 2. Scheme for nucleophilic aromatic substitution for the reaction of 1,2,4,5-tetrabromobenzene with surface bond chloride by the SNAr mechanism
t125
KINETIK OF THE REACTION To obtain kinetic parameters of the reactions the following model is developed: The compounds adsorbed on fly ash move in direction of the gas stream and react with surface bond chloride, see figure 1. When the molecules leave the fly ash section, reaction is broken off. The amount unoccupied fly ash in the glass tube can be considered as a relative reaction time. Table 2 shows the results for the reaction of 1,2,4,5tetrabromobenzene dependent on the amount of fly ash used. For the disappearence of the educt, 1,2,4,5-tetrabromobenzene, a good first order fit could be obtained (see Fig. 3):
Br4Bz (A)
,
kl
•
products;
k_ 1 where K 1 = kl/k_ 1 and k 1 >> k_ 1
Table 2 Relative yields (%) of 1,2,4,5 substituted benzene products after reaction of I mg 1,2,4,5tetrabromobenzene with 1 - 10 g of fly ash at 300 °C
Amount Fly Ash (g)
0
1
3
5
10
Products 100
42.0
8.2
0.9
n.d.
Br3CI-Benzene
0
29.9
13.7
3.3
n.d.
Br2C12-Benzene
0
16.0
23.3
7.3
n.d.
BrC13-Benzene
0
9.2
34.2
17.9
n.d.
C14-Benzene
0
2.9
20.6
70.6
100
100
86.0
64.4
69.7
Br4-Benzene
Absolute Recovery
71.7
n.d.: not detected; detection limit: 0.04 ppm (lxg/g) Graphic solution by exponential regression of this equation results in a straight line, with a slope of -K 1. A correlation coefficient of 0.9973 shows that experimental values can very well fitted to the first order model. Although mechanism contains a bimolecular elementary step, reaction is pseudomonomolecular, because of the high excess of inorganic chloride present on fly ash. Therefore the reaction of 1,2,4,5-telxabromobenzene to 1,2,4,5-tetrachlorobenzene is a pseudomonomolecular consecutive four-stage reaction: k1 Br4Bz(A)
,
k2 •
Br3C1Bz03)
k_ 1
,
k3 "
k_2
Br2C12Bz(C)
.,
k4 •
k_3
BrC13Bz(D)
.~
• C14Bz(E) k. 4
where K n = kn/k_n Fitting experimental data by exponential regression (Figure 3) relative rate constants could be obtained. Values for K 1, K 2, K 3 and K 4 are nearly equal, agreeing with results from reactions in liquid, where bromine and chlorine have nearly similar polarity22.
1126
Oonoentratlon 0 / 0 o E
o,s\ m
0,6
&
1A m C
0,4
'
oZ
/
~m
D
m
Ii
5/',,.
0
I
4
12
8
Time
Figure 3
Concentration curves fitted by exponential regression for the reaction of 1,2,4,5-tetrabromobenzene (A) with surface bond chloride; a consecutive four-stage reaction with three intermediates (B, C, D) and the product E Assuming that other brominated compounds used follow first order kinetics too, relative rate constants (krel) and half lives (tl/2) can be calculated (Table 3).
Table 3
Relative rate constants (krel) and half lives (t 1/7) for the examined aromatic bromine compounds calculated by the concentrations of educts afte~ reaction with 3.5 g fly ash
Compound 1,2,4,5 -Tetrabromobenzene Hexabromobenzene 2,3,7-Tribromodioxin 1,2,3,4-Tetrabromodioxin Decabromodiphenyl ether
Rel. Concenlration
(%) 8.2
3.1 17.2 1.7 <0.1
kre 1 1
1.3 0.6 1.5 >2.5
tl/2 1
0.8 1.6 0.7 <0.4
q
0
0
&
0
0
=:r"
0
p~
0 M
N 0
0
db
N
o
~2
~7
t~
g~
0
v
§,
if21
c~
o~----
o
c~
o
E
N 0 r~
9
o
q
o
0
~a
~° 0
r,
~
0
o
0
o.
4
o
0
o
0
o
0
o.
0
4
0
o
0
o
__.
o
o
.
1128
STEREOCHEMISTRY OF THE REACTION The stereoselective behaviour of halogenation-dehalogenation mechanism can be studied by reaction of 1,2,3,4tetrabromodioxin (1,2,3,4-Br4DD) with surface bond chloride. Possible reaction pathways are shown in figure 4. Theoretically two 1,2,3,4 substituted Br3C1DD, four Br2C12DD and two BrC13DD isomers can be formed. The elution profile (Figure 5) obtained from the reaction on fly ash shows that lateral positions (2,3,7,8) and peri positions (1,4,6,9) have different rate constants; the two Br3Cl-isomers appear in a ratio of approximately 1:1.5. Further reaction should result in four different Br2Cl2-isomers. The elution profile having only three seperated signals shows that these isomers could not be resolved chromatographically. The isomer ratio of the two BrC13 isomers is 1:1.1.
CONCLUSION Investigations show, that brominated aromatic compounds adsorbed on chloride containing fly ash are chlorinated in ipso positions by a nucleophilic addition-elimination mechanism at temperatures at 300 oC. Therefore, PBDD/PBDF formed during combustion of e.g. polybrominated diphenylethers, still widely used as flame retardants, are chlorinated on fly ash. Results of our eperiments are an important contribution to PCDD/PCDF formation on fly ash of electrostatic filters of MWI. PBDD/PBDF are precursors of PCDD/PCDF. The annual production in Europe of about 4500 tonnes of brominated diphenylethers for flame retarded plastics shows the relevance of these compounds for PCDD/PCDF emissions. Experimental data, showing that PCDD/PCDF output of MWI is distinctively increased by brominated diphenyl ethers could be deduced. Chloride and bromide contents of fly ash have an influence on PCDD/PCDF and PBDD/PBDF ratio. Because of the natural CI/Br ratio of 250:1 chlorinated analogues are farly more formed. Nucleophilic aromatic substitution seems to have dominant importance under fly ash catalyzed reactions. Further investigations should clear whether other functional groups like hydroxy, nitro or fluoride react in the same way.
REFERENCES 1.
E.S. Lahaniatis, H. Parlar, F. Korte, Chemosphere 6, 11, 1977.
2.
K. Olie, P.L. Vermeulen, O. Hutzinger, Chemosphere 6, 455, 1977.
3. 4.
K.-H. Schwind, J. Hosseinpour, H. Thoma, Chemosphere 17, 1875, 1988. M. Wiiken, A. Beyer, J. Jager, Organohalogen Compounds Vol. 2, Dioxin 90, ed. O. Hutzinger and H.
5.
Fiedler, Bayreuth, Ecoinforma Press, 377, 1990. M. Neupert, H. Weiss, B. Stock, J. Thies, Chemosphere 19, 115, 1989.
6.
E. L~ser, I. Ivens, Chemosphere 19, 759, 1989.
7.
J.J. Diliberto, L.B. Kedderis, L.S. Birnbaum, Organohalogen Compounds Vol. 1, Dioxin 90, ed. O. Hutzinger and H. Fiedler, Bayreuth, Ecoinforma Press, 309, 1990.
8.
K.J. Thorn6-Kozmiensky, Mtillverbrennung und Umwelt, Bd.l-3, EF-Verlag f'ttr Energie und Umwelttechnik GmbH, Berlin, 1984-1989.
9.
H. Vogg, M. Metzger, L. Stieglitz, Waste Management Research 5, 285, 1987.
10. G.A. Eiceman, R.E. Clement, F.W. Karasek, Anal. Chem. 51, 2343, 1979. 11. D.I. Townsend, Chemosphere 12, 637, 1983.
1129
12. W.M. Shaub, W. Tsang, in Human and Environmental Risks of Chlorinated Dioxins and Related Compounds, ed. R.E. Tucker, A.L. Young, A.P. Grey, Plenum, New York, 1983. 13. L.C. Dickson, F.W. Karasek, J. Chromatogr. 127, 389, 1987. 14. L. Stieglitz, G. Zwick, J. Beck, W. Roth, H. Vogg, Chemosphere 18, 1219, 1989. 15. H.O. Rghei, G.A. Eiceman, Chemosphere 11, 569, 1982. 16. R.V.Hoffman, G.A. Eiceman, Y.-T. Long, M.C.Collins, M.-Q. Lu, Environ. Sci. Technol. 24, 1635, 1990. 17. F.W. Karasek, L.C. Dickson, Science 237, 754, 1987. 18. 19. 20. 21. 22.
L.C. Dickson, D. Lenoir, O. Hutzinger, Chemosphere 19, 277, 1989. T. Lippert, A. Wokaun, D. Lenoir, Ber. Bunsenges. Phys. Chem. 94, 1465, 1990. U. Lahl, M. Wilken, J. B6ske, A. Wiebe, Miill und Abfall 2, 83, 1991. J. Sauer, R. Huisgen, Angew. Chem. 72, 294, 1960. E. Berliner, L.C. Monarck, J. Am. Chem. Soc. 74, 1574, 1952.
(Received in Germany 16 April 1991; accepted 29 May 1991)
pp 1121-1129,
1991
0045-6535/91 $3.00 + 0.00 Pergamon Press plc
SURFACE CATALYZED HALOGENATION-DEHALOGENATION REACTIONS OF AROMATIC BROMINE COMPOUNDS ADSORBED ON FLY ASH B. Zier, D. Lenoir, E. S. Lahaniatis, A. Kettrup GSF-Forschungszentrum ftir Umwelt und Gesundheit GmbH Institut f'tir 0kologische Chemie Ingolst~dter LandstraBe 1, W-8042 Neuherberg, Germany
ABSTRACT Brominated aromatic compounds like brominated benzenes, diphenylethers and dibenzodioxins adsorbed on the
surface of fly ash from a municipal waste incininerator give mixed brominated/chlorinated und completely chlorinated aromatic compounds. These consecutive halogenation-dehalogenation reactions proceed by a nucleophilic mechanism, which is favoured by a high concentration of chloride on the fly ash. Results of kinetic and stereoselective behavior of these reactions will be discussed. The relevance of these results for PCDD/PCDF formation from bromine precursors in municipal waste incinerators will be discussed.
INTRODUCTION In 1977 chlorinated hydrocarbons I as well as chlorinated dibenzodioxins (PCDD) and dibenzofurans (PCDF) 2 were found in the fly ash of municipal waste incinerators (MWI). Later mixed brominated/chlorinated dibenzodioxins (PBCDD), -furans (PBCDF) 3 and polybrominated dibenzodioxins (PBDD), -furans (PBDF) 4 have been identified as side products in MWI emissions. In contrast to PCDD/PCDF there is less known about the toxicity of brominated and mixed halogenated dioxins and furans. Toxicity of PBDD/PBDF has been found to be similar to their chlorine analogues 5-7. Many investigations about formation and decomposition thermal processes of PCDD/PCDF were performed 8-10. Models for elucidation of mechanisms have been developed in the last few years 11,12. Recently, the importance of surface catalyzed pathways for formation and destruction of PCDD/PCDF has been realized 13,14. It could be shown that PCDD adsorbed on fly ash are chlorinated by treatment with HCI at temperatures between 50 and 300 *C 15. Many other aromatic compounds can be chlorinated in this way. An electrophilic mechanism via formation of surface bound Fe(III)-chloride has been suggested 16. In 1987 it was reported that the reaction of chlorophenols like pentachlorophenol and trichlorophenol adsorbed on fly ash yielded
1121
1122
PCDD (Ullman ether synthesis) 13,17. The yields of this reaction depend on copper(I)- species possibly present on fly ash 18. Formation of biphenyls from brominated benzenes is also caused by active, copper containing surfaces (Ullman I reaction) 19. In this paper a possible formation mechanism of PBCDD/F, PCDD/F and other mixed halogenated or chlorinated compounds from brominated aromatic compounds on fly ash surfaces will be presented. It has been shown that these halogenation-dehalogenation reactions occur by a nucleophilic addition-eliminationmechanism. This work gives an explanation for the presence of PBCDD/PBCDF in fly ash and the increase of PCDD/F output of a MWI by adding brominated compounds to the MWI input20. The role of PBDD/PBDF as precursors of PCDD/PCDF is a new interesting contribution to dioxin discussion.
EXPERIMENTAL MATERIALS Fly ash samples were obtained from a municipal waste incinerator in Munich (Munich South, F.R.G.). A chlorine content of 4.9 % and a bromine content of 0.065 % has been determined by X-ray fluorescence spectroscopy. After homogenisation the fly ash sample was soxhlet-extracted with acetone: hexane (1:1) for 48 hours. The fly ash was then dried at 100 °C for 24 hours in a stream of dry nitrogen and stored in a glass flask. 1,2,4,5-tetrabromobenzene,
hexabromobenzene, and decabromodiphenyl ether (Aldrich), 2,3,7-tribromo-para-
dibenzodioxin and 1,2,3,4-tetrabromo-para-dibenzodioxin (Promochem) were all p.A. quality. Solvents were all suitable to residue analysis.
REACTION CONDITIONS Experiments were carried out in a reaction apparatus developed in our laboratory (Figure 1). In a typical experiment 1 mg 1,2,4,5-tetrabromobenzene in 100 Ixl dichloromethane was added to 0.5 g of fly ash. The solvent was removed at 40 °C in a gentle stream of nitrogen. This fly ash and 3 g of "not occupied" fly ash was then loosely packed in a borosilicate glass tube, 5 mm i.d. The section of the tube with the fly ash was then placed in an oven, at a temperature of 300 *C and connected with an impinger containing 60 ml of hexane/dichloromethane (1:1). Flow rate of dry nitrogen was 10 ml/min. After reaction time of 1 hour fly ash and glass tube were extracted with dichloromethane/benzene (9:1). The solution was combined with the solvent of the impinger, dried with 3 g sodium sulfate and concentrated to 4 ml. This solution was used for GC-MS analysis.
GC-MS ANALYSIS The GC-MS analysis was performed in SIM (single ion monitoring) and SCAN mode with the 5970B system from Hewlett-Packard using a quartz capillary column (10 m * 0.25 mm i.d. SP2330, 0.25 I/m, Supelco). GC-MS methods using an internal standard for calibration of the mass spectrometer were dependent on concentration of brominated compounds used. Analysis parameters: 1 ml/min Helium carder gas flow, splitless injection at 300 °C, interface temperature 250 °C, temperature program: 50 °C 1 min, 10 */min to 247 °C.
1123
Quarz Wool
/',,,
jo.o'
(388 Flow
~~////////////////////.)~
~--~
Implnger
1mgBr-o~mpound 1-10~gFlyAsh adsorbed on 0,5 g Fly Ash Figurel Schematic drawing of the apparatus.The fly ash contained in the glass tube is heated for lh at 300 °C in a flow of dry air.
RESULTS AND DISCUSSION A series of brominated, aromatic compounds has been adsorbed on fly ash and treated for 1 h with air at 300 °C. Extraction and analysis yield mixed brominated/chlorinated as well as completely chlorinated products (Table 1). Under these conditions large quantities of mixed brominated/chlorinated and completely chlorinated compounds are formed. 1,2,4,5-tetrabromobenzene (Br4Bz) yields 1,2,4,5-telrachlorobenzene (C14Bz) besides three mixed halogenated intermediates, which are 1,2,4-tribromo-5-chlorobenzene (Br3C1Bz), dibromo-dichloro-benzene (Br2CI2Bz) and 1-bromo-2,4,5-trichlorobenzene (BrC13Bz), respectively.
Table 1 Formation of mixed brominated/chlorinated and completely chlorinated products by reaction of brominated compounds on 3.5 g of fly ash at 300 *C in a stream of dry air; substitution pattern of the products are the same as of the educts.
Brominated Compound
Yield Of Completely Chlorinated Compound
(%)
Yield Of Mixed Brom./Chlor. Compounds
Recovery
64.4
(%)
(%)
1,2,4,5-Tetrabromobenzene
13.3
45.8
Hexabromobenzene
27.9
56.9
87.5
2,3,7 -Tribromodibenzodioxin
21.5
54.8
1,2,3,4-Tetrabromodibenzodioxin
36.1
23.9 5.1
Decabromodiphenyl ether
85.1
6.4
41.9 91.5
1124
MECHANISM O F THE REACTION These reactions proceed by a consecutive series of aromatic substitutions, showing stereoselective behavior. Bromine is substituted by chlorine of the surface on the fly ash. An issue of interest is the mechanism of bromination of these aromatic substitutions. If there would be an electrophilic mechanism, first hydrogen must be exchanged by chlorine in telIabromobenzene. A probable mechanistic alternative is chlorination by free radical processes. It is not likely since aromatic substitution by chlorine atoms is usually quite inefficient. Reaction of 1,2,4,5-tetrabromobenzene to 1,2,4,5-tetrachlorobenzene shows only substitution in ipso positions. Therefore these reactions can only proceed via nucleophilic aromatic substitution by the addition-elimination mechanism the so-called SNAr mechanism (Figure 2). The chlorine content of used fly ash is about 5 %. Halogenafion-dehalogenation reactions with additive chemism are equilibrium reactions. It is usually possible to shift these in the desired direction by use of an excess of the halide ion21. Reaction of 1 mg 1,2,4,5-tetrabromobenzene on 3.5 g of fly ash takes place by an calculated 100 fold excess of chloride to bromide. Therefore the equilibrium is totally shifted towards formation of 1,2,4,5tetrachlorobenzene.
BF
B r ~
Br
Br + CIBr
Br Br ~
Br
Br ~ Br
Br
Br Br Br
Br
Br
- Br- 1 Br
Br~]/CI Br Figure 2. Scheme for nucleophilic aromatic substitution for the reaction of 1,2,4,5-tetrabromobenzene with surface bond chloride by the SNAr mechanism
t125
KINETIK OF THE REACTION To obtain kinetic parameters of the reactions the following model is developed: The compounds adsorbed on fly ash move in direction of the gas stream and react with surface bond chloride, see figure 1. When the molecules leave the fly ash section, reaction is broken off. The amount unoccupied fly ash in the glass tube can be considered as a relative reaction time. Table 2 shows the results for the reaction of 1,2,4,5tetrabromobenzene dependent on the amount of fly ash used. For the disappearence of the educt, 1,2,4,5-tetrabromobenzene, a good first order fit could be obtained (see Fig. 3):
Br4Bz (A)
,
kl
•
products;
k_ 1 where K 1 = kl/k_ 1 and k 1 >> k_ 1
Table 2 Relative yields (%) of 1,2,4,5 substituted benzene products after reaction of I mg 1,2,4,5tetrabromobenzene with 1 - 10 g of fly ash at 300 °C
Amount Fly Ash (g)
0
1
3
5
10
Products 100
42.0
8.2
0.9
n.d.
Br3CI-Benzene
0
29.9
13.7
3.3
n.d.
Br2C12-Benzene
0
16.0
23.3
7.3
n.d.
BrC13-Benzene
0
9.2
34.2
17.9
n.d.
C14-Benzene
0
2.9
20.6
70.6
100
100
86.0
64.4
69.7
Br4-Benzene
Absolute Recovery
71.7
n.d.: not detected; detection limit: 0.04 ppm (lxg/g) Graphic solution by exponential regression of this equation results in a straight line, with a slope of -K 1. A correlation coefficient of 0.9973 shows that experimental values can very well fitted to the first order model. Although mechanism contains a bimolecular elementary step, reaction is pseudomonomolecular, because of the high excess of inorganic chloride present on fly ash. Therefore the reaction of 1,2,4,5-telxabromobenzene to 1,2,4,5-tetrachlorobenzene is a pseudomonomolecular consecutive four-stage reaction: k1 Br4Bz(A)
,
k2 •
Br3C1Bz03)
k_ 1
,
k3 "
k_2
Br2C12Bz(C)
.,
k4 •
k_3
BrC13Bz(D)
.~
• C14Bz(E) k. 4
where K n = kn/k_n Fitting experimental data by exponential regression (Figure 3) relative rate constants could be obtained. Values for K 1, K 2, K 3 and K 4 are nearly equal, agreeing with results from reactions in liquid, where bromine and chlorine have nearly similar polarity22.
1126
Oonoentratlon 0 / 0 o E
o,s\ m
0,6
&
1A m C
0,4
'
oZ
/
~m
D
m
Ii
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0
I
4
12
8
Time
Figure 3
Concentration curves fitted by exponential regression for the reaction of 1,2,4,5-tetrabromobenzene (A) with surface bond chloride; a consecutive four-stage reaction with three intermediates (B, C, D) and the product E Assuming that other brominated compounds used follow first order kinetics too, relative rate constants (krel) and half lives (tl/2) can be calculated (Table 3).
Table 3
Relative rate constants (krel) and half lives (t 1/7) for the examined aromatic bromine compounds calculated by the concentrations of educts afte~ reaction with 3.5 g fly ash
Compound 1,2,4,5 -Tetrabromobenzene Hexabromobenzene 2,3,7-Tribromodioxin 1,2,3,4-Tetrabromodioxin Decabromodiphenyl ether
Rel. Concenlration
(%) 8.2
3.1 17.2 1.7 <0.1
kre 1 1
1.3 0.6 1.5 >2.5
tl/2 1
0.8 1.6 0.7 <0.4
q
0
0
&
0
0
=:r"
0
p~
0 M
N 0
0
db
N
o
~2
~7
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g~
0
v
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if21
c~
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o
c~
o
E
N 0 r~
9
o
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o
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~
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4
o
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o
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.
1128
STEREOCHEMISTRY OF THE REACTION The stereoselective behaviour of halogenation-dehalogenation mechanism can be studied by reaction of 1,2,3,4tetrabromodioxin (1,2,3,4-Br4DD) with surface bond chloride. Possible reaction pathways are shown in figure 4. Theoretically two 1,2,3,4 substituted Br3C1DD, four Br2C12DD and two BrC13DD isomers can be formed. The elution profile (Figure 5) obtained from the reaction on fly ash shows that lateral positions (2,3,7,8) and peri positions (1,4,6,9) have different rate constants; the two Br3Cl-isomers appear in a ratio of approximately 1:1.5. Further reaction should result in four different Br2Cl2-isomers. The elution profile having only three seperated signals shows that these isomers could not be resolved chromatographically. The isomer ratio of the two BrC13 isomers is 1:1.1.
CONCLUSION Investigations show, that brominated aromatic compounds adsorbed on chloride containing fly ash are chlorinated in ipso positions by a nucleophilic addition-elimination mechanism at temperatures at 300 oC. Therefore, PBDD/PBDF formed during combustion of e.g. polybrominated diphenylethers, still widely used as flame retardants, are chlorinated on fly ash. Results of our eperiments are an important contribution to PCDD/PCDF formation on fly ash of electrostatic filters of MWI. PBDD/PBDF are precursors of PCDD/PCDF. The annual production in Europe of about 4500 tonnes of brominated diphenylethers for flame retarded plastics shows the relevance of these compounds for PCDD/PCDF emissions. Experimental data, showing that PCDD/PCDF output of MWI is distinctively increased by brominated diphenyl ethers could be deduced. Chloride and bromide contents of fly ash have an influence on PCDD/PCDF and PBDD/PBDF ratio. Because of the natural CI/Br ratio of 250:1 chlorinated analogues are farly more formed. Nucleophilic aromatic substitution seems to have dominant importance under fly ash catalyzed reactions. Further investigations should clear whether other functional groups like hydroxy, nitro or fluoride react in the same way.
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1129
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(Received in Germany 16 April 1991; accepted 29 May 1991)