Environ. Sci. Technol. 1994, 28, 1248-1253
Destructive Adsorption of Chlorinated Benzenes on Ultrafine (Nanoscale) Particles of Magnesium Oxide and Calcium Oxide Yong-Xi Ll, Hui LI, and Kenneth J. Klabunde'
Department of Chemistry, Kansas State University, Manhattan, Kansas 66506 The thermodynamically favored but kinetically inhibited exchange of oxide for chloride in the reaction of MgO/ CaO with chlorinated benzenes has been investigated. In order to enhance kinetic parameters, MgO/CaO particles with very large surface areas have been employed. The presence of MgO/CaO allows the destruction of chlorinated benzenes at lower temperatures than simple pyrolysis or combustion processes. The presence of hydrogen as a carrier gas allows still lower temperatures to be employed. Main reaction pathways have been deduced for mono-, di-, and trichlorobenzenes over MgO and CaO of varying surface areas and using helium, air, and hydrogen as flow gases. Significant differences between MgO and CaO have been realized, perhaps due to the fact that MgO can be prepared in higher surface areas. For example, CaO induces more carbon formation. A search for certain trace toxins as products (particularly the dibenzo-p-dioxin backbone with 0-3 chlorine substituents) was carried out. Under our analysisconditions, it was determined that such products were not formed under any circumstances with MgO, and if oxygen in the carrier gas was absent, no dioxins were formed with CaO either. However, with low surface area CaO and air as a flow gas, dibenzo-p-dioxin (no chlorine substituents) and a monochloro derivative were produced in small amounts.
Introduction Chlorinated organics are used on a large scale in the chemical, petrochemical, and electronic industries. Especially polychlorobiphenyls (PCBs)have found many uses such as hydraulic fluids, heat transfer fluids, plasticizers, adhesives, dielectric fluids, lubricants, and flame retardants ( I ) . Disposal of chlorinated aromatics has become a major environmental and social problem. PCBs are especially toxic and thermally stable. Spills of PCBs and related materials have been particularly tragic and have happened numerous times in Europe and the United States (2-4). The sources of chlorinated aromatics are often surprising, for example combustion of treated woods and plastics (3, 5 ) ,and even from automobile exhaust from cars burning gasoline or diesel fuel containing halogenated additives (6). Furthermore, combustion of halogenated aromatics often leads to even worse toxins such as chlorinated dioxins and furans. In this paper, we report some new results concerning the destructive adsorption of chlorinated benzenes on fine particles of magnesium oxide and calcium oxide. A t this stage we do not consider PCBs, but simply try to lend some chemical understanding through work with a series of chloroarenes. Approaches for Destruction of Chlorinated Aromatics (Not Including PCBs). Bozzelli and coworkers (7) have studied the destruction of chlorobenzene and dichlorobenzene in H2 and HdO2 mixtures (7). Using a 1248
Environ. Scl. Technol., Vol. 28, No. 7, 1994
tubular flow reactor and 560-1000 "C, complete conversion to benzene, carbon solids, HCl, and small amounts of CHI and C2H6 was possible. Atomic hydrogen appeared to be responsible, and the addition of small amounts of O2 initiated a chain mechanism (Hz + 0 2 HO2 + H') for H* formation and allowed lower temperatures to be effective. It was pointed out that at higher temperatures in the presence of 0 2 the formation of chlorinated dibenzop-dioxins and dibenzofurans was more likely. In related work, Manahan and co-workers have used a reverse burn gasification method for treatment of a variety of hazardous wastes with some success, including organohalides (8). Cullis and Manton (9) studied thermal decomposition of chlorobenzene in a flow reactor. Between 770 and 800 "C the main products were HC1, H2, and p,p'-dichlorobiphenyl. Chorine atoms and chlorophenyl radicals appeared to be the principal reaction carriers involved in a chain reaction cycle and with little rupture of the aromatic ring. However, at 800-850 "C ring rupture was more prevalent, CHZ=CHCl was a major product, and CzHCl was believed to be an important intermediate. Louw and co-workers (10) reported a study on the reactions of chlorobenzene with radicals such as 'CGH~CI, Cl*, C6H5*, resulting in chlorobiphenyl and dichlorobiphyenyls as important products. Later work (11, 12) employing a H2 atmosphere indicated that hydrocarbon formation may involve a methycyclopentadiene intermediate. Hydrocarbons such as C2H2, CZH4, and C2H6 were also observed. Cui and co-workers (13) have used a shock tube to generate H' from hexamethylethane, studied the H'/C6H,Cl, reaction, and determined relative rate constants for CP displacements. Generally, the more highly chlorinated the aromatic species, the more rapidly the displacement reaction proceeded. Also, Maurer and co-workers (14) have treated halogenated hydrocarbons with CaO(40% )-Ca(OH)2(10 % )iron oxides (50%) at 700 "C. Some CaCl2 was formed as well as iron compounds. The residue appeared to be free of volatile organics. Potential Reactions of MgO and CaO with Chlorinated Aromatics. Considering thermodynamics only, the reaction of CaO or MgO with chlorocarbons such as CC4, CHC13, and 62Cl4 are energetically quite favorable (15). For chlorinated aromatics, the energetics are also favorable, for example:
-
-
4C6H,Cl + 2Mg0 3C6W6 2MgC1,
+
+ H20 + CO + 5C (1)
AHrx,,= -387 kJ For this reaction, the products that might be expected are MgC12, H20, c o , C6H6, and graphite. However, the reaction of MgO with gaseous CsH5C1 at elevated temperature would be a gadsolid reaction, and the MgCl2 0013-936X/94/0928-1248$04.50/0
0 1994 American Chemical Society
product would be expected to cover the surface of the remaining MgO, probably inhibing further reaction. Therefore, kinetics may completely control this or similar reaction pathways in spite of exothermicity. In order to encourage extensive reaction so that a high capacity for chloroarene decomposition would be possible, ultrahigh surface area MgO (or CaO) would be desirable. The experimentsdescribedhereinweredesignedtoshow what products are actually formed, the effect of CaO or MgO surface area, the effect of carrier gas (He, air, Hz), and the effect of temperatures. In order to collect data rapidly, two reactor-GC/MS systems were constructed (16, 17), and small amounts of chlorinated benzenes were pulsed over a bed of CaO (or MgO). We have chosen to study C6HaC1, '&&C~Z,and CeHsCls primarily to gain an understanding of their chemistry on MgO and CaO particles, but we also realize that they serve as models of chlorbiphenyls and related chloroarenes. Experimental Section
Figure 1. Automatic reactor-GClMSdevice: 1. mass spectrometer; 2. Gc; 3. injector; 4. flow rate stabillzer; 5. autosampler; 6. reagent 7, heating tape; 8. valve; 9, flow meter; IO. flow rate control valve; 11. furnace; 12, reactor; 13, oxlde bed: 14, thermal couple; 15 and 16, temperature controller; 17, caplilaly column; 18. vacuum pump; 19, printer; 20, Computer.
Saftety Issues. The starting chlorocarbons and some of the possible byproducts of these reactions are toxic. All chemicals must be handled with care and with good ventilation, and effluents from GC/MS and GC experiments must be vented to a hood. Two in situ flow reactor GC/MS systemswere employed for these experiments. In the fvst reactor, described earlier (17),usuallyO.lOgofMgOorCaOfinepowderwaspacked in a stainless steel U-tube reactor (6-mm o.d., 5-mm i.d., 200-mm long). Silica-alumina wool was used to bold the powder sample in place. The flow of carrier gas (He, air, or Hz) was 50 mL/min. A t the exit of the U-tube reactor, a He gas inlet was added (1-5 mL/min) in order to help carry the products through the GC and jet separator and into the MS. The GC column used was simply an empty tube in cases where product separation was still adequate. In other cases, a 10% SE-30 60-80 mesh chromosorb G (6-m long with 6-mm id.) packed column was employed. The MgO and CaO samples were prepared as described earlier (16,17). Chlorobenzene, 1,3-dichlorobenzene, 1,4dichlorobenzene,and 1,3,5-trichlorobenzenewere obtained from Aldricb Chemical Co. and used without further purification. The latter two were solids and so were dissolved in n-heptane (28% w t 5% chlorocarbon). Most experimentswere carried out by injecting 1-2-pL samples of the chlorocarbon or heptane/chlorocarbon solution over a 230 O C injection port. The sample then passed into the flow reactor tube at some desired temperature and then intothe GC/MS. Alltransfer lines were heated to 140"C. This MS system was a Finnigan 4021-C controlled by aFmiganIncos datasystem. The ionization energy was set at 20 eV, and the multiplier voltage was set at 1700 V. The ionizing chamber temperature was 250 "C, the manifold was set at 110 "C, and the manifold vacuum was set at 3.0 X 10-7-9.0 X 10-8 Torr. A second in situ flow reactor-GC/MS system was constructed so that more computer automation could be used to advantage (Figure 1). This device could only be used withHeorHzasflowgases,anditsautomatedfeatures allowed long-term testinginorder to determinedestructive adsorption capacity of the MgO and CaO samples. An autosampler injected 1-rL samples of chlorocarbon onto an injector block at 230 OC and then into a U-tube flow reactor. Volatile decomposition products were split into
two fractions, and a small portion was sent to a GC capillary column (HP-1, cross-linked methyl silicone, 12 m X 0.200.33 mm thickness operating a t 230 "C). Eluted products were analyzed by MS on a Perkin-Elmer Q-Mass 910 instrument, and datawere stored on a 386 IBM-compatible personal computer. Typical conditions were analyzer chamber vacuum at 1X 10-6 Torr, analyzer temperatures at 250 "C, multiplier voltage at 1200 V, lens 1 voltage at 35 V, and lens 2 voltage at 75 V. Volatile products were identified by retention time, fragmentation pattern, and Selected ion-monitoring (SIM) experimolar mass (M+). ments were carried out using conditions of He as flow gas at 30 mL/min and 900 "C reactor temperature. XRD and XPS Experiments. XPS experimentswere carried out on a Leybold-Horeaus LHS-11 spectrometer. The X-ray source (MgKa=1253.6sv)was operated at 240 W. The vacuum during analyses was less than 1X lo-' Torr. All samples analyzed after decomposition experiments were adhered on double-sided tape on a sample rod and then placed in the analysis position. The CI. peak (284.6 eV) was used for the calibration of binding energies. Photoelectron lines for Mgzp. C2p. Cas, OI.,and CI. were acquired for all samples. Usually more than 50 scans were used for the measurements. An XDS-2000 spectrometer using CuK. radiation (Scintag, Inc.) was employed for the powder X-ray diffraction (XRD) studies. The tube voltage was 40 kV, the current was 45 mA, and the scanning rate was 2.5"/min. TGA-GC/MS Experiments. A ShimadzuTGA-50was connected to the GC/MS system (see below). Air (or He) flow was stabilized with a flow stabilizer a t 50 mL/min. A used sample (about 20 mg) of MgO or CaO (after use for decomposition of chlorbenzenes) was placed in the t h e r m a l b a l a n e e i n a p l p a n . Thesamplewasheated at 20 OC/min up to 900 "C. The weight loss was continuously monitored. The gaseous effluent was passed into a heated stainless steel capillary tube (as short as possible) and then into a 30 m X 0.25 m i.d. capillary GC column and into the MS. When air was used as the flow gas, the initially black samples turned white. When He was the flow gas, the samples remained darkly colored. However, pure air could not be allowed to enter the mass spectrometer, so TGA-GUMS experiments were only possible with He or with 10% air-OO% He as flow gas. Of Envlron. Sd.T&mcJ.. VoI. 28. No. 7, 1994 f249
Table 2. Decomposition of Chlorinated Benzenes over MgO (130 mz/g)asb
HCl(1)
900 (air) 500 (Hz) 700 (Hz)
900c
course, normal TGA experiments were carried out when pure air was used. Results
Thermal Decompositionin Absence of MgO or CaO. An empty stainless steel reactor tube with He flow was employed to obtain data on strictly thermal decomposition. With a flow rate of 50 mL/min, the calculated residence time in the 200-mm reactor tube is 7.5 s. Under these conditions, with He as the carrier gas, no decomposition took place at reactor temperatures of 500 or 700 "C. A t 900 "C, chlorobenzene was destroyed and benzene, biphenyl, and HC1 were products (Table 1). Note that, in the case of 1,4-C~H&12,terphenyl was also formed. However, in the case of 1,3,5-CsH&13 only HC1 and C2H4 were detected, indicating that much of the arene moiety was converted to polyaromatics probably including graphite. Using air as the carrier gas, no decomposition took place at 500 "C while minor partially dechlorinated products were observed at 700 "C with 1,4-C&&12 and 1,3,5-C&C13. A t 900 "C, complete destruction of the starting material took place. Interestingly, however, some partially chlorinated benzenes were still present. As expected, oxygen-containing products CO, HzO, and COz were also formed (Table 1). With Hz as the carrier gas, a small amount of decomposition/reduction occurred at 500 "C, and at 700 "C nearly complete thermal decomposition/reductionhad occurred. Major products were benzene and HC1. At 900 "C, complete decomposition/reduction took place, and large amounts of C6H6, HC1, and CzH4 were detected. As expected, no oxidation products were observed. Destructive Adsorptionon MgO andCaO. ( A )MgO (130 m2/g)with He, Air, Hz,and Chloroarenes. When the 1250
Environ. Sci, Technol., Vol. 28, No. 7, 1994
HzO (13) CGHB (84) C6H6 (82) CO (12) CO (15) HzO (4) HzO (3) CeHiClz (74) CsHiClz (75) CsH&1(6) C&cl(20) C6H6 (20) C6H6 (5) CeH6C1(9) CaH4Clz (7) CgHe (60) CizHio (10) C6H.&l(5) CzHi (40) C&I&H (6) CeH60H (4) CeH6 (64) CsHs (54) CzH4 (11) CzHi (30) CisHiz (10) C6H6 (48) C6H6 (18) CeH6 (47) CzH4 (52) CzHi (87) CZH4 (43) CsHs (81) coz (4) HzO (10) CaHsCl(94) C6H6 (6)
c6&
(80)
co (12)
HzO (8) CeH&ls (59) C&Clz (30) C6H6 (11) CG& (16) CzHi (84)
c& (16) C2H4 (84)
a Numbers in parentheses indicate the relative G U M S peak area for that product. When a higher surface area MgO (390 m2/g) was used, similar products were formed. At 900 OC,CsHe,COz, CO, and HzO were formed.
reactor tube was charged with 0.1 g of metal oxide, the calculated residence time through the oxide bed is 0.38 s. Even with these short contact times substantially more decomposition took place; usually 4-6 pL of chlorinated benzenes could be completely decomposed at lower temperatures when MgO or CaO were present. Table 2 summarizes the volatile products and their relative abundance according to GC peak areas for the MgO (130 m2/g)sample. Note that at 500 "C and especially at 700 "C substantial decomposition took place with He as the flow gas. At 900 "C, decomposition was total. Oxygen containing compounds CO and HzO became major products at 700 and 900 "C, indicating that the chlorine stripping by MgO yielded oxygen: MgO
+ 2ArC1MgC1,
+ "0" + 2[Arl-
CO + H20 (2) organic fragments
When 0 2 was provided (air as flow gas), product distributions changed although not dramatically. No phenol or biphenyl were ever detected, but on occasion C02 was a product. However, benzene was the major product at the higher temperatures with both He or air. Hydrogen aided the decomposition somewhat, and ethylene became a major product along with benzene. Some water was also observed. We can conclude from these comparisons that the presence of MgO is quite beneficial. Lower decomposition temperature was possible vs strictly thermal decomposi-
Chart 1
Table 3. XPS Analysis of MgO (390) and CaO (10) after Decomposition of Chlorobenzene molar mass = 184
DD
sample
region
CaO (10)
Cazp Cl2p C18
molar mass = 218 for "CI isotope
018
moncchloro-DD
clg&I)cl
MgO (390)
Mg2p c12p
c1.
molar mass = 252
dichloreDD c1;
tion. And in the presence of air or Hz (vs He), temperature requirements were even lower. With air, as would be expected, more oxidation products were formed, and with Hz more dechlorinated hydrocarbons were formed. With He as the carrier gas, much of the chloroarene formed a polymer on the surface of the MgO. ( B )Effect of Surface Area of MgO. Two samples MgO (130 mZ/g) and MgO (390 mZ/g) were directly compared. Differences were encountered with MgO (390); oxygencontaining products CO and HzO were detected in higher relative concentrations under He flow. This indicates that surface -OH and lattice oxygen of the MgO do provide oxygen and that this oxygen is more accessible on the high surface area samples (see eq 2). In the presence of air the MgO (390), considerably more CO and HzO were formed, suggesting that, even when air is present, the surface of MgO is still necessary for the formation of CO and HzO. (C) Comparison of CaO with MgO. A low surface area sample of CaO (10 m2/g)was directly compared with the MgO samples. Inherent surface reactivity seemed to be slightly higher for CaO as indicated by more decomposition at 500 OC under He. At 700 and 900 "C, product distributions were similar to the MgO experiments. However with air as a flow gas, dibenzo-p-dioxin (DD) (molar mass = 184, no chlorine substituents) was detected for the first time in these studies. This product was not observed under any conditions with MgO and not with He or Hz carrier gases overall. Only the combination of CaO and air led to its formation. For example, with 1,3-C6H4Clz the products at 900 "C with CaO/air were C6H6 (75 relative amount), CO (22), and dibenzo-p-dioxin (3). With 1,3,5-C&3C13, the products were C6H6 (98) and DD (2). We undertook selected ion-monitoring experiments searching for DD and chloroanalogs. Indeed, we searched for all chlorinated analogs of polychlorodibenzodioxanes and polychlorodibenzofurans by high sensitivity MS scans (detection limits = 0.01 ng). Under certain conditions, very small peaks corresponding to DD (no chlorine substituents) and monochloro-DD were detected. Therefore, we undertook SIM experiments by "locking on" to the masses for these two species as well as dichloro- and trichloro-DD (see Chart 1). Selected Ion-Monitoring (SIM) Experiments for Detection of DD and Chlorinated DD. Experimental conditions were employed that encouraged their formation: low surface area CaO and air flow. The automatic injector system and Perkin-Elmer Q-Mass 910 were employed with 10% air in He as flow gas at 30 mL/min and 900 OC reactor temperature. Under these conditions, very small amounts of DD and monochloro-DD were
01.
MgO (390)
Mgzp Clzp C18
01,
position, eV
app, %
349.8 200.3 284.6 533.5 50.3 199.5 284.6 531.9 49.6 198.7 284.6 531.4 49.5 198.4 284.6 531.2
11 2 46 41 18 1 38 33 25 1 38 36 29 1 33 37
[Clzp + C181 Cazp(Mgzp)l, flowgas % employed 4.4
He
2.2
He
1.5
He
1.2
He
reproducibly detected from reactions of chlorobenzene. However, no di-, tri-, or tetrachloro-DD were detected. When MgO (130 mZ/g) was used under these same conditions, no DD or substituted DD were produced. In addition, with CaO in the absence of air as a carrier gas, no DD analogs were produced. It is increasingly clear that in order to produce DD the conditions of low surface area CaO and the presence of air are necessary. Actually, it is quite easy to avoid these unwanted products by employing MgO and/or by avoiding air as a flow gas. X-ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), and Thermal Gravimetric Analysis Experiments (TGA) on Used MgO and CaO Samples. In order to identify the remaining solid products on the used MgO and CaO samples,a series of spectroscopic analyses were carried out. Powder XRD of used samples clearly indicated that large amounts of MgClz and CaClz were formed. The spectra were essentially overlays of MgO/MgClz or CaO/ CaC12. No other crystalline products were detected (e.g., graphite). Data from XPS are shown in Table 3. Further support for the presence of MgClz and CaC12 can be inferred from the binding energies of 198.4-200.3 eV for ClzPand shift to higher binding energies for MgZp (49.5 eV 50.3 eV) and Cazp (349.8 ev). And since XPS only analyzes the surface, it would appear that MgC12 and CaClz are the major solid surface components, as would be expected from the reaction of surface MgO/CaO with chloroarenes. Carbon was also a prominent element present, and a ratio of the intensities of (Clap+ C1,)/Cazp (or Mgzp)could be obtained. As expected, this ratio was considerably higher when CaO with He flow was used, indicating that considerably more organic polymeric residue was formed under these conditions. With MgO (390 m2/g) and He flow this ratio was about half of the CaO value. Under Hz and air, this ratio was much smaller still, indicating that the amount of carbonaceous residue was much smaller. Material Balance Studies by TGA-GC/MS. Quantitative determination of small amounts of organic residue on the surface of inorganic particles often involvesthermal gravimetric analysis (TGA) experiments (18-21). However, it is also useful to know how much weight loss is due to water, and so GUMS studies of the volatiles involved would be helpful. In order to carry out such experiments,
-
Envlron. Scl. Technol., Vol. 28, No. 7, 1994
1251
____I___
I I
t
Table 4. TGA Results for MgO (130 m2/g) after Decomposition of 1,2-Dichlorobenzene amount of 1,2-dichlorobenzene injected
FL 38 36 18 10 4 2 OC
TGA measurement weight of residual
weight, mi??
weight loss, %
species on MgO (130)*
49.6 47.0 23.5 13.1 5.2 2.6 0
11.9 9.26 6.37 4.91 3.80 2.60 1.89
23.8 18.5 12.7 9.8 7.6 5.2 3.8
*
1,2-Dichlorobenzenedensity = 1.305. Based on calculations from TGA weight loss measurement on 0.200 g of MgO (130). Fresh MgO (130).
is estimated to be about 15 pL.) Our TGA experiments where 0, 2, or 4 pL had been injected are tabulated in Table 4. Shown are the masses of 1,2-dichlorobenzene used, TGA weight losses, and the calculated mass of residue that was left on the MgO sample (0.200 g of MgO was the starting mass). We averaged the 2- and 4-pL results and took into account the weight loss of the sample with no exposure to 1,2-dichlorobenzene: If we assume that all C,
t
L
+ 2.5 mg/2 = 3.9 mg of starting dichlorobenzene 7.6 mg + 5.2 mg/2 = 5.2 mg
2.6 mg residue lost as gaseous products
I
1 0
600
300
900
"C Figure 2. TGA spectra of (a) fresh MgO (130), (b) MgO (130) exposed to 10 pL pulses of 1,2dichlorobenzene, and (c) MgO (130) exposed to 36 pL pulses of 1,2dichlorobenzene(700 O C reaction temperature under He flow).
we have designed our apparatus so that a part of the gaseous effluents could be passed into an GUMS apparatus (22, 23). Using He as the TGA purge gas, we were able to determine weight loss due to water, carbon oxides, and organics. Figure 2 demonstrates typical weight loss curves for MgO. The GUMS results indicated that the weight loss before reaching 150 OC was mainly due to HzO loss. The loss at higher temperatures was due to a mixture of HzO, CO, Con, and an ion mle = 64 or 66 (probably HCOCl+). Using air as a flow gas, we could not route the gases to the GC/MS due to filament burnout in the presence of large amounts of oxygen. In order to get at material balance, we can only use the data from experiments where starting material (in this case 1,2-dichlorobenzene) was not being eluted in the volatile products. Generally, on the 5th or 6th injection of 1-pLsamples, 1,2-dichlorobenzene could be detected as a smallpart of the effluent. (Total decomposition capacity 1252
Envlron. Sci. Technol., Vol. 28, No. 7, 1994
H, and C1 deposited are lost by one process or another, these results show that at least two-thirds of the starting 1,2-dichlorobenzene is initially trapped as residues on the MgO surface and that about one-third forms volatile products during the pulsed reactions. More data will be necessary before material balances and TGA effluents will be more precisely known. We have also carried out such TGA experiments under air flow, and analysis of these results suggest that about 80-9092 of the chloroarenes form nonvolatile residues. Thus, we can conclude that, depending on the chloroarene, approximately 70-90% of the mass of the material remains in the solid residue and 10-30% is evolved as volatiles. In fact, this should not be surprising since it is expected that, at least under oxidative conditions, all of the C1 should remain as MgClZ/CaClz. (Note that chlorobenzene has 32 % C1 by mass, dichlorobenzene has 48 % C1, and trichlorobenzene has 59% Cl.) Also, the speculative stoichiometric reactions discussed earlier predict that some graphite will be a necessary product (for example, see eq 1). Discussion
We have shown that the presence of MgO or CaO allows chloroarenes to be completely decomposed under somewhat milder conditions compared with straight thermolysis. Chlorine-containing volatile products were never emitted from the MgO reactor bed; not chloroaromatics, HCI, or Cl2. Only easily combustible products such as benzene and CO along with COz and HzO were observed. The carrier gases He, air, and Hz were compared, and it was determined that air and Hz allowed for slightly more and cleaner volatile products. The higher surface area of the MgO was also beneficial, as might be expected for surface-gas reactions.
Solid residues were also formed on the MgO and CaO surfaces. However, the low surface area CaO samples allowed the formation of much more carbonaceous residues that may contain some chlorine. However, the major chlorinated products are MgCldCaClz. One potential problem using CaO and air carrier gas was encountered where small amounts of DD and monochloro-DD were produced. These products could be avoided by using MgO and avoiding air as the carrier gas. Certainly the surface chemistry involved is complex in these reactions. It is interesting that a rather simple product mixture resulted. Based on the observation of benzene, CO, COz, HzO, and MgClz (CaC12) as products, some major reaction pathways can be written down. Without MgO or CaO.
+
6C6H4C1,.-* 2C6H6+ 12HC1 24C (no oxygen in carrier gas) 24C + 120, 24C
+ 24H,
-
-
W i t h MgO.
3C6H4C1,+ MgO 5 c + 30, 5C + 5H,
-
24CO
12C,H4
-
2C6H6
4CO + CO, 2.5C,H4
(oxygen in carrier gas) (hydrogen as carrier gas)
+ MgC1, + co + 5 c (no oxygen in carrier gas) (oxygen in carrier gas) (hydrogen as carrier gas)
The production of carbon as a solid product seems unavoidable when He is used as the carrier gas. In the presence of 02,this nascent carbon could be easily converted to CO and COz, and in the case of Hz it could be converted to CZH4 (which seems to be a main product along with benzene). As discussed above, the presence of MgO (or CaO) is beneficial, especially if high surface areas are employed, and allows a better thermodynamic advantage. However, the details of this surface chemistry remain to be elucidated. Conclusions
Although the chemistry is complex, it is clear that chloroarenes can be destructively adsorbed on the surface of MgO and CaO. However, capacity depends on surface area, and therefore, high surface area samples of these oxides are necessary. Acknowledgments
The research described herein was funded in part by the U.S.Environmental Protection Agency under As-
sistance Agreement R-815709,to Kansas State University, through the Hazardous Substance Research Center for U.S. EPA Regions 7 and 8. It has not been subjected to the Agency’s peer and administrativereview and, therefore, may not reflect the views of the Agency, and no official endorsement should be inferred. The partial support of the Army Research Office is also acknowledged with gratitude. Literature Cited (1) Safe, S. In Hazards, Decontamination, and Replacement of PCB; Crine, J.-P., Ed.; Plenum Press: New York and London, 1988. (2) Tucker, R. E., Young, A. L., Gray, A. P., Eds. Human and Environmental Risks of Chlorinated Dioxins and Related Compounds; Plenium Press: New York, 1983. (3) Hutziner, O., Frei, R. W., Merian, E., Pocchiari, F., Eds. Chlorinated Dioxins and Related Compounds;Pergamon Press: Oxford, 1982. (4) Exner, J. H., Ed. Solving Hazardous Waste Problems: LearningfromDioxins;ACS Series 338, ACS: Washington, DC, 1987. ( 5 ) Thoma, H. Chemosphere 1988,17, 1369. (6) Rappe, C.; Anderson, R.; Borggvist, P.-A.; Brohede, C.; Hansson, M.; Keller, L.-0.; Lindstrom, G.; Marklund, S.; Nygren, M.; Swanson, S. E.; Tysklind, M.; Wiberg. K. Chemosphere 1987,16, 1603. (7) Ritter, E.; Bozzeli, J. W. Combust. Sci. Technol. 1990, 74, 117-135. (8) Kinner, L. L.; McGowin, A.; Manahan, S. E. Environ. Sci. Technol. 1993, 27, 482. (9) Cullis, C. F.; Mantan, J. E.J.Chem. Soc., Trans. Faraday SOC.1958, 54 (3), 381. (10) Louw, R.; Rothuizen, J. W.; Wegman, R. C. C. J. Chem. SOC.,Perkin Trans. 2 1973,12, 1635. (11) Louw,R.;Dijks, J. H.; Mulder,P. J. Recl. J.R. Neth. Chem. SOC.1984, 103, 271. (12) Manion, J. A.; Dijks, J. H.; Mulder, P.; Louw, R. Red. Trav. Chim. Pays-Bas 1988,107, 434. (13) Cui, J. P.; He, Y. Z.; Tsang, W. J.Phys. Chem. 1989,93,724. (14) Maurer, P. G.; Neupert, D. German Patent, DE, 1986, 3,447,337. (15) Koper, 0.; Li, Y.-X.; Klabunde, K. J. Chem. Mater. 1993, 5,500. (16) Li, Y.-X.; Koper, 0.;Atteya, M.; Klabunde, K. J. Chem. Mater. 1992,4, 323. (17) Li, Y.-X.; Klabunde, K. J. Chem. Mater. 1992, 4, 611. (18) Warne, S. S. J. Thermochim. Acta 1991,192, 19-28. (19) Leinweber,P.;Schulten, H. R.; Horte, C. Thermochim. Acta 1992,194, 175-87. (20) Leinweber, P.; Schulten, H. R. Thermochim. Acta 1992, 200, 151-67. (21) Gallagher, P. K. J. Thermal Anal. 1992,38, 17-26. (22) Knuemann, R.; Scheussner, M.; Bockhorn, H. Znt. Annu. Conf.ICT, 22nd 1991,36,1-36. (23) Szekely, G.; Nebuloni, M.; Zerilli, L. F. Thermochim. Acta. 1992,196, 511-32. Received for review August 13, 1993. Revised manuscript received December 29, 1993. Accepted April 4, 1994. e Abstract published in Advance ACS Abstracts, May 15, 1994.
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