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Formation of dibenzodioxins and chlorobenzenes in fly ash catalyzed reactions of monochlorophenols
Chemosphere, Vol.19, Nos.lO/ll, Printed in Great Britain
pp
1629-1633,
1989
0045-6535/89 $3.00 Pergamon Press plc
+
.OO
F O R M A T I O N OF D I B E N Z O D I O X I N S AND C H L O R O B E N Z E N E S IN FLY ASH C A T A L Y Z E D REACTIONS OF MONOCHLOROPHENOLS.
J.G.P.
Born, R. Louw and P. Mulder*.
C e n t e r for C h e m i s t r y and the Environment, Gorlaeus Laboratories, L e i d e n University, P.O. Box 9502, 2300 RA Leiden, The Netherlands.
ABSTRACT Partial a u t o x i d a t i o n of m o n o c h l o r o p h e n o l s to carbon dioxide and carbon m o n o x i d e proceeds at 350 - 400 °C m e d i a t e d by Municipal W_aste l_ncinerator (MWI) fly ash. M o r e o v e r a wide range of (poly)chlorinated benzenes, monobenzofurans and d i b e n z o - p - d i o x i n s is produced, with a c o n s i d e r a b l e f r a c t i o n of the original organic chlorine concentrated into these remaining aromatic rings.
KEYWORDS PCDDs, polychlorobenzenes, chlorination.
polychlorobenzofurans,
fly
ash
catalysis,
oxidation,
INTRODUCTION The f o r m a t i o n of p o l y c h l o r i n a t e d d i b e n z o - p - d i o x i n s (PCDDs) and d i b e n z o f u r a n s (PCDFs) in M W I ' s is a well e s t a b l i s h e d fact I , as is the presence of these compounds on fly ash produced by these f a c i l i t i e s 2-5. It has been shown 6 that h o m o g e n e o u s gas-phase reactions cannot be the sole source of levels of PCDDs and PCDFs found in effluents of MWI's. Since gas-solid r e a c t i o n s at moderate t e m p e r a t u r e s are k i n e t i c a l l y favoured compared to gas-phase radical reactions, heterogeneous (in c a s u fly ash catalyzed) reactions must be considered. Accordingly, a full description of the f o r m a t i o n of PCDDs and PCDFs should involve both radical gas-phase reactions - modelling the p y r o l y t i c and flame c h e m i s t r y occurring in the c o m b u s t i o n c h a m b e r - and heterogeneous (fly ash m e d i a t e d ) reactions, especially the chemistry, at relatively low temperatures, in boiler s e c t i o n s and in the e l e c t r o s t a t i c precipitator. Due ~o the p r e s e n c e of different reaction stages in m u n i c i p a l w a s t e incinerators (MWIs) until now no c l e a r - c u t relations between normal operating conditions of various MWIs and their P C D D (and/or 9CDF) content of effluent gas flows have been found. Some testing programs, e m p l o y i n g extreme operating conditions, have revealed a c o r r e l a t i o n between CO emissions and 9 C D D / g C D F effluent gas contents as well as a relationship between operating t e m p e r a t u r e and P C D D / P C D F emissions 7,8. Three general p a t h w a y s have been advanced for explaining the e m i s s i o n of dibenzodioxins and d i b e n z o f u r a n s : i) incomplete c o m b u s t i o n of trace levels a l r e a d y present in the feed 9 ; li) g e n e r a t i o n f r o m k n o w n p r e c u r s o r s such as p o l y c h l o r o b i p h e n y l s I0 and chlorophenolsll; iii) de n o v o f o r m a t i o n from organic compounds and inorganic chlorine 12. Several l a b o r a t o r y studies involving formation and degradation, 13,14 as well as (de)chlorination 15, of PCDDs and PCDFs on fly ash have revealed that P C D D / P C D F is readily produced from p r e c u r s o r c o m p o u n d s a l r e a d y present and/or ( d e ) c h l o r i n a t e d on fly ash at ca. 150 - 600 °C. Three p r e r e q u i s i t e s for this effect are: i) presence of t r a n s i t i o n or heavy metal cations; ii) p r e s e n c e of o x y g e n in the reaction system and iii) (in)organic carbon and chlorine sources. A d e c r e a s e of the P C D D / P C D F c o n t e n t on fly ash is o b s e r v e d upon thermal treatment
1629
1630
in an inert (oxygen deficient)
atmosphere.
In order to learn more about the role of fly ash in producing PCDDs and PCDFs under relevant conditions we have d e c i d e d to employ fixed beds of MWI fly ash, c o n t i n u o u s l y passing over o x y g e n / n i t r o g e n m i x t u r e s together with relevant organic compounds and chlorine derivatives. We now wish to report on results obtained with orthochlorophenol per se, and p h e n o l / c h l o r o p h e n o l mixtures, in the temperature region of 350 - 400 °C.
EXPERIMENTAL H e t e r o g e n e o u s g a s - s o l i d reactions were conducted in a tubular quartz reactor (length 100 cm, internal d i a m e t e r 8 mm) packed with fly ash obtained from a D u t c h m u n i c i p a l solid waste incinerator. A fixed bed of fly ash (length 40 cm) was situated in the middle part of the reactor by m e a n s of quartz wool. The reactor was mounted v e r t i c a l l y in a W.C. Heraeus Hanau RO 7/75 e l e c t r i c a l l y heated tubular oven. The upper and lower ends of the oven were insulated by quartz wool. The temperature was controlled by a RKC Rex F-9 temperature regulator. T e m p e r a t u r e was m e a s u r e d by chromel-alumel thermocouples inserted in the oven, and d i s p l a y e d on a digital t h e r m o m e t e r (Therma I, type ST-861-I07), equipped w i t h ambient t e m p e r a t u r e compensation. Once installed, the fly ash bed described above was pretreated at 500 "C in a gentle stream of pure oxygen for 60 hours to remove all organic compounds and particulate organic carbon initially p r e s e n t on the fly ash. By m o n i t o r i n g the p r o d u c t i o n of CO and C02, the completion of this process was determined. (Chloro)phenol substrates (Phenol: M e r c k 99.5%; o-chlorophenol: M e r c k 98%; m-chlorophenol: A l d r i c h 98%; p - c h l o r o p h e n o l : Fluka 98%) were used as such and - if n e c e s s a r y - liquefied by adding ca. I0 w% water. Using a glass 10 ml syringe driven by a m o t o r i z e d syringe pump (B. Braun Melsungen, P e r f u s o r VI type 871222/0), the m i x t u r e was introduced via a gas-tight rubber s e p t u m in a n i t r o g e n gas flow and vapourized before entering the reactor. Nitrogen (Hoekloos: S-80-V) and oxygen (Hoekloos: Z-80-V) gases were purchased in standard cylinders. Flows were r e g u l a t e d by needle valves and m e t e r e d via calibrated flowmeters. The exit tube of the reactor was m a i n t a i n e d at 150 - 200"C by w r a p p e d heating tape to prevent the c o n d e n s a t i o n of less volatile products. Products and u n r e a c t e d starting c o m p o u n d s were c o l l e c t e d for a period of 15-30 min. using a trap cooled w i t h liquid nitrogen. Ca. I00 mg of a stock solution of bromobenzene (93 w%) and l-bromonaphthalene (7 w%) was a c c u r a t e l y w e i g h e d d i r e c t l y into a C h r o m p a c k sample vial and m i x e d homogeneously w i t h the c o l l e c t e d sample to serve as an internal standard. Samples were q u a l i t a t i v e l y analyzed for PCDDs and PCDFs employing a Hewlett Packard 5890 GC e q u i p p e d w i t h a 5970 MSD. A n a l y s i s of the various isomers of DDs and DFs produced, was p e r f o r m e d on a C P - S i l - 5 - C B (50 m) capillary column. Typical injection v o l u m e s were I-2 ~i of sample. P r o d u c t s were q u a n t i t a t e d using a Hewlett Packard 5890 GC/FID. A b s o l u t e amounts were based on p e a k surface areas relative to that of l - b r o m o n a p h t h a l e n e of the internal standard m i x t u r e in c o n j u n c t i o n w i t h independently determined m o l a r responses. Permanent gases such as CO and CO 2 were analyzed using a packed column equipped w i t h a m e t h a n l z e r and a flame i o n i z a t i o n d e t e c t o r by injection of 0.50 ml samples of exit gas via a gas tight syringe. A b s o l u t e a m o u n t s were based on peak surface areas relative to those given by injection of standard gas mixtures.
RESULTS Experiments I and II of Table I outline the results of the slow combustion chlorophenol. The rate of formation of the various products is expressed as:
of
o-
amount product formed (micrograms) w e i g h t of fly ash bed (grams) • time (hour) It a p p e a r s that o x i d a t i o n of o - c h l o r o p h e n o l proceeds readily at 350 - 400 "C, and almost c o m p l e t e d e s t r u c t i o n seems p o s s i b l e at temperatures well b e l o w 500 "C. The m a j o r products are c a r b o n d i o x i d e and carbon m o n o x i d e in a ratio of ca. I0. O t h e r d e g r a d a t i o n products such as c y c l o p e n t a d i e n e and naphthalene, n o r m a l l y found upon h o m o g e n e o u s slow combustion of c h l o r o p h e n o 1 1 6 , are not observed.
1631
Table
I.
F l y a s h m e d i a t e d o x i d a t i v e c o n v e r s i o n of m o n o c h l o r o p h e n o l s .
EXP (no.) Temp ('C) Res. time (s) Fly a s h (g)
I 375 5.1 9.0
II 326 5.5 9.0
III 367 3.4 i0.7 a
IV 367 3.2 I0.7 a
V 367 3.2 i0,7 a
1.83 24.4 138
1.83 24.4 138
0.952 1.85 0.106 48.2 160
0.952 1.85 0.106 57.0 164
0.952 1.85 0.106 54.5 161
INTAKE (mmol/h) water phenol o-CIPhenol m-CiPhenol p-CiPhenol Oxygen Nitrogen PRODUCTS (~mo!/h) CO CO 2 Phenol ~-CIPhenol m-CIPhenol p-CiPhenol PRODUCTS (~g/(g-h) PhCI o-PhCl 2 m-PhCl 2 p-PhCl 2 1,3,5-PhCI 3 1,2,4-PhCI 3 1,2,3-PhCI 3 1,2,3,5-PHCI 4 1,2,4,5-PHCI 4 1,2,3,4-PHC14 PhCl 5 PhCl 6 Z M I C B F s de Z D2CBFsde Z T 3 C B F s de Z T 4 C B F s de Z P 5 C B F s de Z H 6 C B F s de DF DD 2-MCDD I-MCDD D2CDDse T3CDDse T4CDDse P5CDDs e H6CDDs e HTCDDse 08CDD
a b: c: d. e.
300 1860 1.8 321 b b
70 450 0.35 618 b b
420 2540 45 b b I0
460 1770 401 8.2 19 Ii
325 1630 315 33 b 5.8
) 3.83 182 2.07 0.933 c 50.0 24.8 c 25.9 6.42 9.15 c 11.9 4.64 49.4 7.48 26.8 44.3 c 67.0 26.6 299 550 169 59.1 c c c c
1.19 27.7 0.363 0.174 0.449 10.5 4.69 c 2.39 32.5 5.38 39.2 4.15 c 8.15 12.0 7.01 8.65 c 20.6 6.92 52.2 160 27 5.26 c e c c
29.1 2.11 0.528 2,61 6.52 6.33 16.0 26.0 c 9.57 13.5 8.38 16.7 12.8 26.8 44.5 7.71 10.8 106 24.0 3.64 10.2 2.60 c c c c c c
N e w fly ash bed. B e l o w d e t e c t i o n limit (ca. 0.I mmol/h). B e l o w d e t e c t i o n limit (ca. 0.2 ~g/(g-h)). N.B. M o n o b e n z o f u r a n s I E l u t i o n o r d e r u n k n o w n a n d / o r isomers could e m p l o y e d ( C P - S i l - 5 CB).
28.7 1.41 e 0.620 3.03 4.33 c 7.90 c 4.34 24.2 34.1 13,1 2.29 5.93 43,2 9.67 21,0 166 36.3 4,44 14,3 31.8 35.9 54.1 26,5 38,0 c c
not
be
43.5 0.568 0.240 0.545 4.11 1.75 c 4.83 c 1.66 4.21 4.10 52.9 I.i0 1.44 6.13 1.64 7.41 101 25.8 0.617 1.82 2.33 1.68 6.48 c c c c
separated
with
the
column
1632
In addition, numerous isomers of three types of compounds, comprising chlorinated benzenss, m o n o b e n z o f u r a n s and d i b e n z o - p - d i o x i n s , are formed. This, too, contrasts w i t h the product pattern arising from the h o m o K e n e o u s slow combustion of o-chloropheno116. In the latter case mainly dichlorodibenzofurans (DCDFs) are formed, whereas (monochloro)dibenzo-p-dioxins ((MC)DDs) are only m i n o r products. Using GC/MS, traces of di- and t r i c h l o r o p h e n o l s toEether with p h e n o x y - d i b e n z o - p - d i o x i n s could be identified. A l t h o u g h the formation of d i b e n z o - p - d i o x i n s per se has been documented beforel3), our e x p e r i m e n t s show that fly ash causes production of PCDDs, polychlorinated benzenes, and monobenzofurans) w h e n passin@ over monochlorophenol. Apparently, (organic) chlorine is "collected" in the aromatic compounds escaping from oxidative breakdown. In a second series of reactions (exp. III-V, Table I) the behaviour, of all three monoc h l o r o p h e n o l s s e p a r a t e l y is outlined. The chlorophenol feed is diluted (tenfold) by added phenol. The results are c o m p a r a b l e to those found on combustion of pure (ortho)chlorophenol. Apparently, phenol does not interfere with the "concentration" of organic chlorine as m e n t i o n e d above. Phenol itself is independently converted, in part, into unsubstituted d i b e n z o f u r a n (DF). As in absence of fly ash the degree of c o n v e r s i o n is negligible at T < 400 °C, this r e a c t i o n appears to be catalyzed by fly ash as well.
DISCUSSION The high C 0 2 / C 0 ratio and the absence of intermediate d e g r a d a t i o n products proves that catalytic c o m b u s t i o n is responsible for the c o n v e r s i o n of c h l o r o p h e n o l s at relatively mild conditions. In the h o m o g e n e o u s gas-phase slow combustion of (chloro)phenols, CO is a much more prominent product. In our v i e w two c o m p e t i t i v e processes occur: Catalytic oxidation of (chloro)phenols and chlorination employinE the chlorine m i n e r a l i z e d in the c o m b u s t i o n process. The more c h l o r o p h e n o l is converted, the more chlorine is available for a declining amount of organic compounds. This effect can 8reatly enhance the chlorine content of the persisting aromatic compounds, and can result in "optimal" p r o d u c t i o n of PCDDs, etc. under certain conditions. As yet it is u n c l e a r w h e t h e r c h l o r i n a t i o n advances due to oxidative "Deacon-llke" formation of C12, or by some form of activated chlorine, e. E. Cu(II)-Cl derivatives, on the fly ash surface. HCI and precursors such as alkyl chlorides - may also engender this type of (oxy)chlorination. W h e n feeding phenol as in exps. Ill - V, t o s e t h e r w i t h HCl, substantial p r o p o r t i o n s of o and p - C l P h O H are observed. The product c o m p o s i t i o n in exps I I I - V is not m u c h dependent on the p o s i t i o n of C1 in the starting chlorophenol. Likewise, considerable isomerization in the "unconverted" c h l o r o p h e n o l occurs, r a t i o n a l i z i n g that the presence of ortho chlorine is not essential for d i b e n z o - p - d i o x i n formation. Dickson and K a r a s e k 17 have found that orthochlorine is not essential for d i b e n z o - p - d i o x i n formation upon pyrolysis of t r i c h l o r o p h e n o l s on fly ash. In our v i e w a c h l o r o p h e n o l molecule, after c h e m i s o r p t i o n on the fly ash catalyst, appears to be subject to v a r i o u s reactions: isomerization, oxidative breakdown, or "conducted tour" m e c h a n i s m s of (oxy)chlorination, before distinct products (C02, c h l o r i n a t e d benzenes, PCDD) are released. Adding an excess of phenol ensues K e n e r a t i o n of dibenzofuran. Since no chlorinated d i b e n z o f u r a n s could be detected, c h e m i s o r b e d phenol seems to be subject either to oxidative breakdown, or c o n d e n s a t i o n to dibenzofuran, rather than to (oxy)chlorination. The scope, and m e c h a n i s m s of these fly ash catalyzed (oxidative) c o n v e r s i o n s are further i n v e s t i g a t e d in our laboratory. In order to assess the importance for the o p e r a t i o n of MWI's it should be kept in m i n d that ashes hang on, e.g. in boilers, and m a y be instrumental in converting P_roducts of I_ncomplete C_ombustion in a way as u n c o v e r e d by our model experiments.
1633
ACKNOWLEDGEMENT This research has been made possible through financial Housing, Physical Planning, and Environment (VROM).
support
from the Dutch Ministry
of
REFERENCES I) 2) 3) 4) 5) 6) 7) 8) 9) i0) ii) 12) 13) 14) 15) 16) 17)
K. Olie, P.L. Vermeulen and O. HutzinEer , Chemosphere, 6, 455-459 (1977). H.R. Buser, H.P. Bosshardt and C. Rappe, Chemosphere, ~, 165-172 (1978). G.A. Eiceman, R.E. Clement and F.W. Karasek, Anal. Chem., 51, 2343-2350 (1978). L.L. Lamparski and T.J. Nestrick, Anal. Chem., 52, 2045-2054 (1980). A.C. Viau, S.M. Studak and F.W. Karasek, Can. J. Chem., 62, 2140-2145 (1984). W.M. Shaub and W. TsanE, Environ. Sci. Technol., 17, 721-730 (1983). F. Hasselriis, Waste Hana E. & Res., 5, 311-326 (1987). J.R. Visalli, Hazard. Waste HanaE. , 37, 1451-1463 (1987). H.M. Tosine, R.E. Clement, V. Ozvacic and G. WonE, Chemosphere, l_4&, 821-827 (1985). H.-R. Buser and C. Rappe, Chemosphere, 8, 157-174, (1979). B. Jansson, G. Sundstrom and B. AhlinE, Sci. Total Environ., IO, 209-217 (1978). L. StieElitz and H. VoEE, presented at the Int. Symposium on Chlorinated Dioxins and Related Compounds, Dioxin 87, (1987). H. VoEE and L. StieElitz , Chemosphere, 15, 1373-1378 (1986). H.-P. HaEenmaier , M. Kraft, H. Brunner and R. HaaE, Environ. Sci. Technol., 21, 10801084 (1987). G.A. Eiceman and H.O. Rshei , Environ. Sci. Technol., 16, 53-56 (1982). J.G.P. Born, R. Louw and P. Mulder, Chemosphere, submitted for publication, (1989). L.C. Dickson and F.W. Karasek, J. ChromatoEr. , 389, 127-137 (1987).
(Received
in
Germany
25
April
1989;
accepted
8 September
1989)
pp
1629-1633,
1989
0045-6535/89 $3.00 Pergamon Press plc
+
.OO
F O R M A T I O N OF D I B E N Z O D I O X I N S AND C H L O R O B E N Z E N E S IN FLY ASH C A T A L Y Z E D REACTIONS OF MONOCHLOROPHENOLS.
J.G.P.
Born, R. Louw and P. Mulder*.
C e n t e r for C h e m i s t r y and the Environment, Gorlaeus Laboratories, L e i d e n University, P.O. Box 9502, 2300 RA Leiden, The Netherlands.
ABSTRACT Partial a u t o x i d a t i o n of m o n o c h l o r o p h e n o l s to carbon dioxide and carbon m o n o x i d e proceeds at 350 - 400 °C m e d i a t e d by Municipal W_aste l_ncinerator (MWI) fly ash. M o r e o v e r a wide range of (poly)chlorinated benzenes, monobenzofurans and d i b e n z o - p - d i o x i n s is produced, with a c o n s i d e r a b l e f r a c t i o n of the original organic chlorine concentrated into these remaining aromatic rings.
KEYWORDS PCDDs, polychlorobenzenes, chlorination.
polychlorobenzofurans,
fly
ash
catalysis,
oxidation,
INTRODUCTION The f o r m a t i o n of p o l y c h l o r i n a t e d d i b e n z o - p - d i o x i n s (PCDDs) and d i b e n z o f u r a n s (PCDFs) in M W I ' s is a well e s t a b l i s h e d fact I , as is the presence of these compounds on fly ash produced by these f a c i l i t i e s 2-5. It has been shown 6 that h o m o g e n e o u s gas-phase reactions cannot be the sole source of levels of PCDDs and PCDFs found in effluents of MWI's. Since gas-solid r e a c t i o n s at moderate t e m p e r a t u r e s are k i n e t i c a l l y favoured compared to gas-phase radical reactions, heterogeneous (in c a s u fly ash catalyzed) reactions must be considered. Accordingly, a full description of the f o r m a t i o n of PCDDs and PCDFs should involve both radical gas-phase reactions - modelling the p y r o l y t i c and flame c h e m i s t r y occurring in the c o m b u s t i o n c h a m b e r - and heterogeneous (fly ash m e d i a t e d ) reactions, especially the chemistry, at relatively low temperatures, in boiler s e c t i o n s and in the e l e c t r o s t a t i c precipitator. Due ~o the p r e s e n c e of different reaction stages in m u n i c i p a l w a s t e incinerators (MWIs) until now no c l e a r - c u t relations between normal operating conditions of various MWIs and their P C D D (and/or 9CDF) content of effluent gas flows have been found. Some testing programs, e m p l o y i n g extreme operating conditions, have revealed a c o r r e l a t i o n between CO emissions and 9 C D D / g C D F effluent gas contents as well as a relationship between operating t e m p e r a t u r e and P C D D / P C D F emissions 7,8. Three general p a t h w a y s have been advanced for explaining the e m i s s i o n of dibenzodioxins and d i b e n z o f u r a n s : i) incomplete c o m b u s t i o n of trace levels a l r e a d y present in the feed 9 ; li) g e n e r a t i o n f r o m k n o w n p r e c u r s o r s such as p o l y c h l o r o b i p h e n y l s I0 and chlorophenolsll; iii) de n o v o f o r m a t i o n from organic compounds and inorganic chlorine 12. Several l a b o r a t o r y studies involving formation and degradation, 13,14 as well as (de)chlorination 15, of PCDDs and PCDFs on fly ash have revealed that P C D D / P C D F is readily produced from p r e c u r s o r c o m p o u n d s a l r e a d y present and/or ( d e ) c h l o r i n a t e d on fly ash at ca. 150 - 600 °C. Three p r e r e q u i s i t e s for this effect are: i) presence of t r a n s i t i o n or heavy metal cations; ii) p r e s e n c e of o x y g e n in the reaction system and iii) (in)organic carbon and chlorine sources. A d e c r e a s e of the P C D D / P C D F c o n t e n t on fly ash is o b s e r v e d upon thermal treatment
1629
1630
in an inert (oxygen deficient)
atmosphere.
In order to learn more about the role of fly ash in producing PCDDs and PCDFs under relevant conditions we have d e c i d e d to employ fixed beds of MWI fly ash, c o n t i n u o u s l y passing over o x y g e n / n i t r o g e n m i x t u r e s together with relevant organic compounds and chlorine derivatives. We now wish to report on results obtained with orthochlorophenol per se, and p h e n o l / c h l o r o p h e n o l mixtures, in the temperature region of 350 - 400 °C.
EXPERIMENTAL H e t e r o g e n e o u s g a s - s o l i d reactions were conducted in a tubular quartz reactor (length 100 cm, internal d i a m e t e r 8 mm) packed with fly ash obtained from a D u t c h m u n i c i p a l solid waste incinerator. A fixed bed of fly ash (length 40 cm) was situated in the middle part of the reactor by m e a n s of quartz wool. The reactor was mounted v e r t i c a l l y in a W.C. Heraeus Hanau RO 7/75 e l e c t r i c a l l y heated tubular oven. The upper and lower ends of the oven were insulated by quartz wool. The temperature was controlled by a RKC Rex F-9 temperature regulator. T e m p e r a t u r e was m e a s u r e d by chromel-alumel thermocouples inserted in the oven, and d i s p l a y e d on a digital t h e r m o m e t e r (Therma I, type ST-861-I07), equipped w i t h ambient t e m p e r a t u r e compensation. Once installed, the fly ash bed described above was pretreated at 500 "C in a gentle stream of pure oxygen for 60 hours to remove all organic compounds and particulate organic carbon initially p r e s e n t on the fly ash. By m o n i t o r i n g the p r o d u c t i o n of CO and C02, the completion of this process was determined. (Chloro)phenol substrates (Phenol: M e r c k 99.5%; o-chlorophenol: M e r c k 98%; m-chlorophenol: A l d r i c h 98%; p - c h l o r o p h e n o l : Fluka 98%) were used as such and - if n e c e s s a r y - liquefied by adding ca. I0 w% water. Using a glass 10 ml syringe driven by a m o t o r i z e d syringe pump (B. Braun Melsungen, P e r f u s o r VI type 871222/0), the m i x t u r e was introduced via a gas-tight rubber s e p t u m in a n i t r o g e n gas flow and vapourized before entering the reactor. Nitrogen (Hoekloos: S-80-V) and oxygen (Hoekloos: Z-80-V) gases were purchased in standard cylinders. Flows were r e g u l a t e d by needle valves and m e t e r e d via calibrated flowmeters. The exit tube of the reactor was m a i n t a i n e d at 150 - 200"C by w r a p p e d heating tape to prevent the c o n d e n s a t i o n of less volatile products. Products and u n r e a c t e d starting c o m p o u n d s were c o l l e c t e d for a period of 15-30 min. using a trap cooled w i t h liquid nitrogen. Ca. I00 mg of a stock solution of bromobenzene (93 w%) and l-bromonaphthalene (7 w%) was a c c u r a t e l y w e i g h e d d i r e c t l y into a C h r o m p a c k sample vial and m i x e d homogeneously w i t h the c o l l e c t e d sample to serve as an internal standard. Samples were q u a l i t a t i v e l y analyzed for PCDDs and PCDFs employing a Hewlett Packard 5890 GC e q u i p p e d w i t h a 5970 MSD. A n a l y s i s of the various isomers of DDs and DFs produced, was p e r f o r m e d on a C P - S i l - 5 - C B (50 m) capillary column. Typical injection v o l u m e s were I-2 ~i of sample. P r o d u c t s were q u a n t i t a t e d using a Hewlett Packard 5890 GC/FID. A b s o l u t e amounts were based on p e a k surface areas relative to that of l - b r o m o n a p h t h a l e n e of the internal standard m i x t u r e in c o n j u n c t i o n w i t h independently determined m o l a r responses. Permanent gases such as CO and CO 2 were analyzed using a packed column equipped w i t h a m e t h a n l z e r and a flame i o n i z a t i o n d e t e c t o r by injection of 0.50 ml samples of exit gas via a gas tight syringe. A b s o l u t e a m o u n t s were based on peak surface areas relative to those given by injection of standard gas mixtures.
RESULTS Experiments I and II of Table I outline the results of the slow combustion chlorophenol. The rate of formation of the various products is expressed as:
of
o-
amount product formed (micrograms) w e i g h t of fly ash bed (grams) • time (hour) It a p p e a r s that o x i d a t i o n of o - c h l o r o p h e n o l proceeds readily at 350 - 400 "C, and almost c o m p l e t e d e s t r u c t i o n seems p o s s i b l e at temperatures well b e l o w 500 "C. The m a j o r products are c a r b o n d i o x i d e and carbon m o n o x i d e in a ratio of ca. I0. O t h e r d e g r a d a t i o n products such as c y c l o p e n t a d i e n e and naphthalene, n o r m a l l y found upon h o m o g e n e o u s slow combustion of c h l o r o p h e n o 1 1 6 , are not observed.
1631
Table
I.
F l y a s h m e d i a t e d o x i d a t i v e c o n v e r s i o n of m o n o c h l o r o p h e n o l s .
EXP (no.) Temp ('C) Res. time (s) Fly a s h (g)
I 375 5.1 9.0
II 326 5.5 9.0
III 367 3.4 i0.7 a
IV 367 3.2 I0.7 a
V 367 3.2 i0,7 a
1.83 24.4 138
1.83 24.4 138
0.952 1.85 0.106 48.2 160
0.952 1.85 0.106 57.0 164
0.952 1.85 0.106 54.5 161
INTAKE (mmol/h) water phenol o-CIPhenol m-CiPhenol p-CiPhenol Oxygen Nitrogen PRODUCTS (~mo!/h) CO CO 2 Phenol ~-CIPhenol m-CIPhenol p-CiPhenol PRODUCTS (~g/(g-h) PhCI o-PhCl 2 m-PhCl 2 p-PhCl 2 1,3,5-PhCI 3 1,2,4-PhCI 3 1,2,3-PhCI 3 1,2,3,5-PHCI 4 1,2,4,5-PHCI 4 1,2,3,4-PHC14 PhCl 5 PhCl 6 Z M I C B F s de Z D2CBFsde Z T 3 C B F s de Z T 4 C B F s de Z P 5 C B F s de Z H 6 C B F s de DF DD 2-MCDD I-MCDD D2CDDse T3CDDse T4CDDse P5CDDs e H6CDDs e HTCDDse 08CDD
a b: c: d. e.
300 1860 1.8 321 b b
70 450 0.35 618 b b
420 2540 45 b b I0
460 1770 401 8.2 19 Ii
325 1630 315 33 b 5.8
) 3.83 182 2.07 0.933 c 50.0 24.8 c 25.9 6.42 9.15 c 11.9 4.64 49.4 7.48 26.8 44.3 c 67.0 26.6 299 550 169 59.1 c c c c
1.19 27.7 0.363 0.174 0.449 10.5 4.69 c 2.39 32.5 5.38 39.2 4.15 c 8.15 12.0 7.01 8.65 c 20.6 6.92 52.2 160 27 5.26 c e c c
29.1 2.11 0.528 2,61 6.52 6.33 16.0 26.0 c 9.57 13.5 8.38 16.7 12.8 26.8 44.5 7.71 10.8 106 24.0 3.64 10.2 2.60 c c c c c c
N e w fly ash bed. B e l o w d e t e c t i o n limit (ca. 0.I mmol/h). B e l o w d e t e c t i o n limit (ca. 0.2 ~g/(g-h)). N.B. M o n o b e n z o f u r a n s I E l u t i o n o r d e r u n k n o w n a n d / o r isomers could e m p l o y e d ( C P - S i l - 5 CB).
28.7 1.41 e 0.620 3.03 4.33 c 7.90 c 4.34 24.2 34.1 13,1 2.29 5.93 43,2 9.67 21,0 166 36.3 4,44 14,3 31.8 35.9 54.1 26,5 38,0 c c
not
be
43.5 0.568 0.240 0.545 4.11 1.75 c 4.83 c 1.66 4.21 4.10 52.9 I.i0 1.44 6.13 1.64 7.41 101 25.8 0.617 1.82 2.33 1.68 6.48 c c c c
separated
with
the
column
1632
In addition, numerous isomers of three types of compounds, comprising chlorinated benzenss, m o n o b e n z o f u r a n s and d i b e n z o - p - d i o x i n s , are formed. This, too, contrasts w i t h the product pattern arising from the h o m o K e n e o u s slow combustion of o-chloropheno116. In the latter case mainly dichlorodibenzofurans (DCDFs) are formed, whereas (monochloro)dibenzo-p-dioxins ((MC)DDs) are only m i n o r products. Using GC/MS, traces of di- and t r i c h l o r o p h e n o l s toEether with p h e n o x y - d i b e n z o - p - d i o x i n s could be identified. A l t h o u g h the formation of d i b e n z o - p - d i o x i n s per se has been documented beforel3), our e x p e r i m e n t s show that fly ash causes production of PCDDs, polychlorinated benzenes, and monobenzofurans) w h e n passin@ over monochlorophenol. Apparently, (organic) chlorine is "collected" in the aromatic compounds escaping from oxidative breakdown. In a second series of reactions (exp. III-V, Table I) the behaviour, of all three monoc h l o r o p h e n o l s s e p a r a t e l y is outlined. The chlorophenol feed is diluted (tenfold) by added phenol. The results are c o m p a r a b l e to those found on combustion of pure (ortho)chlorophenol. Apparently, phenol does not interfere with the "concentration" of organic chlorine as m e n t i o n e d above. Phenol itself is independently converted, in part, into unsubstituted d i b e n z o f u r a n (DF). As in absence of fly ash the degree of c o n v e r s i o n is negligible at T < 400 °C, this r e a c t i o n appears to be catalyzed by fly ash as well.
DISCUSSION The high C 0 2 / C 0 ratio and the absence of intermediate d e g r a d a t i o n products proves that catalytic c o m b u s t i o n is responsible for the c o n v e r s i o n of c h l o r o p h e n o l s at relatively mild conditions. In the h o m o g e n e o u s gas-phase slow combustion of (chloro)phenols, CO is a much more prominent product. In our v i e w two c o m p e t i t i v e processes occur: Catalytic oxidation of (chloro)phenols and chlorination employinE the chlorine m i n e r a l i z e d in the c o m b u s t i o n process. The more c h l o r o p h e n o l is converted, the more chlorine is available for a declining amount of organic compounds. This effect can 8reatly enhance the chlorine content of the persisting aromatic compounds, and can result in "optimal" p r o d u c t i o n of PCDDs, etc. under certain conditions. As yet it is u n c l e a r w h e t h e r c h l o r i n a t i o n advances due to oxidative "Deacon-llke" formation of C12, or by some form of activated chlorine, e. E. Cu(II)-Cl derivatives, on the fly ash surface. HCI and precursors such as alkyl chlorides - may also engender this type of (oxy)chlorination. W h e n feeding phenol as in exps. Ill - V, t o s e t h e r w i t h HCl, substantial p r o p o r t i o n s of o and p - C l P h O H are observed. The product c o m p o s i t i o n in exps I I I - V is not m u c h dependent on the p o s i t i o n of C1 in the starting chlorophenol. Likewise, considerable isomerization in the "unconverted" c h l o r o p h e n o l occurs, r a t i o n a l i z i n g that the presence of ortho chlorine is not essential for d i b e n z o - p - d i o x i n formation. Dickson and K a r a s e k 17 have found that orthochlorine is not essential for d i b e n z o - p - d i o x i n formation upon pyrolysis of t r i c h l o r o p h e n o l s on fly ash. In our v i e w a c h l o r o p h e n o l molecule, after c h e m i s o r p t i o n on the fly ash catalyst, appears to be subject to v a r i o u s reactions: isomerization, oxidative breakdown, or "conducted tour" m e c h a n i s m s of (oxy)chlorination, before distinct products (C02, c h l o r i n a t e d benzenes, PCDD) are released. Adding an excess of phenol ensues K e n e r a t i o n of dibenzofuran. Since no chlorinated d i b e n z o f u r a n s could be detected, c h e m i s o r b e d phenol seems to be subject either to oxidative breakdown, or c o n d e n s a t i o n to dibenzofuran, rather than to (oxy)chlorination. The scope, and m e c h a n i s m s of these fly ash catalyzed (oxidative) c o n v e r s i o n s are further i n v e s t i g a t e d in our laboratory. In order to assess the importance for the o p e r a t i o n of MWI's it should be kept in m i n d that ashes hang on, e.g. in boilers, and m a y be instrumental in converting P_roducts of I_ncomplete C_ombustion in a way as u n c o v e r e d by our model experiments.
1633
ACKNOWLEDGEMENT This research has been made possible through financial Housing, Physical Planning, and Environment (VROM).
support
from the Dutch Ministry
of
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(Received
in
Germany
25
April
1989;
accepted
8 September
1989)