Decomposition characteristics and reaction mechanisms of methylene chloride and carbon tetrachloride...

ELSEVIER Applied Catalysis B: Environmental 8 (19961 157-182

Decomposition characteristics and reaction mechanisms of methylene chloride and carbon

tetrachloride using metal-loaded zeolite catalysts

Bala Ramachandran, Howard L. Greene * , Sougato Chatterjee Department of Chemical Engineering, The University of Akron, Akron, OH 44325-3906, USA

Received 11 July 1995; revised 18 September 1995; accepted 21 September 1995

Abstract

The catalytic activities and selectivities of four zeolite-Y catalysts (H-Y, Co-Y, Na-Y and Co-Y/CA) for the oxidation of metbylene chloride and carbon tetrachloride were compared. Reactor experiments were carried out in a fixed bed reactor with temperatures ranging from 150 to 350°C and a space velocity of 2400 hh’ at atmospheric pressure. Other catalyst characteristics, including oxygen adsorption capacities, surface area, acidity and catalyst composition were measured and compared. The Co-Y catalyst showed excellent activity compared to the other catalysts with complete conversion of both feeds at temperatures as low as 2OO”C, and therefore was used as a model catalyst for reaction mechanistic investigations. Variable space velocity reactor runs (100 to 47000 hh’) were conducted at 350°C with this catalyst to distinguish series/parallel reaction mechanisms. It was found that CO was the predominant deep oxidation product ( > 95% selectivity) and that the formations of CO and CO, occurred by parallel reactions in the oxidation of methylene chloride. Also, during the oxidation of carbon tetrachloride, phosgene was found to be a reaction intermediate. To elucidate the surface reaction mechanisms, in situ FTIR experiments were performed using a transmission reaction cell at 300°C. The results indicated that the CVOCs adsorbed on the Bronsted acid sites of the zeolite. The FTIR results also suggested the formation of an unstable intermediate (COHCl) during the oxidation of methylene chloride. The formation of phosgene as a reaction intermediate during the formation of CO, was observed for the oxidation of carbon tetrachloride, consistent with the variable space velocity experiments. Based on these results and the current literature, deep oxidation reaction mechanisms have been proposed for these two systems.

Keywords: CVOC oxidation; Reaction mechanisms; Metal loaded zeolites; In situ FTIR

* Corresponding author.

0926.3373/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0926-3373(95)00060-7

158 B. Ramachandran et al./Applied Catalysis B: Environmental 8 (1996) 157-182

1. Introduction

Chlorinated volatile organic compounds (CVOCs) have been produced com- mercially and used for many purposes such as the manufacture of herbicides, plastics and solvents in the chemical industry. Soil and ground water contamina- tion due to improper disposal of these chlorinated volatile organic compounds such as methylene chloride, carbon tetrachloride and trichloroethylene has been a major concern.

In recent years, catalytic oxidation of the CVOCs has become a widely used method of destruction and is a strongly favored alternative for thermal incineration. Since, catalytic oxidation can be carried out at temperatures below 500°C it proves to be a highly energy efficient process when compared to thermal incineration. Another major advantage of the catalytic oxidation process is the potential for excellent selectivity towards the formation of harmless reaction products.

The decomposition of 1,2 dichloroethane was studied utilizing various acid catalysts such as zeolite Y, various types of mordenites, ZSM-5, silica/alumina and titania/silica by Imamura and Tarumoto [I]. Various other studies on the complete oxidation of CVOCs have been reported [2-51. Zeolite Y catalysts, namely H-Y, Cr-Y and Ce-Y, washcoated on low surface area honeycomb supports were previously utilized by Chatterjee and Greene [6] for the oxidation of methylene chloride. Complete catalytic oxidation of trichloroethylene, carbon tetrachloride and methylene chloride using Co-Y, Cr-Y and Mn-Y catalyst were also studied [7]. A cation exchanged/impregnated catalyst was used with great success for the oxidation of the above mentioned CVOCs. The effects of catalyst composition on these dual site catalysts were investigated [8]. The deactivation of the metal exchanged zeolite catalysts during exposure to chlorinated hydro- carbons under oxidizing conditions was also studied [9].

The objective of this study was three-fold: [l] to determine the reaction characteristics of four different zeolite catalysts (H-Y, Na-Y, Co-Y and Co- Y/CA) for the oxidation of methylene chloride and carbon tetrachloride; [2] to propose reaction mechanisms for the deep catalytic oxidation of methylene chloride and carbon tetrachloride; [3] to investigate the possible interactions between CVOC and support for the case where no hydrogen was present either in the feed molecule (Ccl,) or in the feed itself (H,O).

The catalyst characterization included determination of conversions and selectivities of the four catalysts for the oxidation of methylene chloride and carbon tetrachloride as well as 0, adsorption capacities, surface area measurements, cation loading and acidity measurements. The poor performance of the Na-Y catalyst compared to the H-Y and Co-Y catalysts, quite clearly indicated the importance of Brprnsted acid sites for the oxidation of the CVOCs. Since, the results showed that Co-Y had good conversion for the oxidation of methylene chloride and carbon tetrachloride, it was employed to gain insight into the deep

B. Ramachandran et al./Applied Catalysis B: Environmental 8 (1996) 157-182 159

oxidation reaction mechanisms, which utilized the variable space velocity reactor runs and the in situ FTIR experiments.

The variable space velocity reactor runs were carried out to distinguish between series and parallel reactions during the oxidation of the CVOCs. The conversion and selectivity trends were carefully analyzed during the oxidation of both CVOCs. The selectivity trend for methylene chloride suggested that the formations of CO and CO, most likely occurred in parallel. Similarly, for carbon tetrachloride, COCl, was found to be the intermediate product with CO, as the deep oxidation product.

A long-term deactivation of Co-Y at 250°C using dry Ccl, as the feed, suggested that the absence of hydrogen in the feed stream was detrimental to the catalyst, causing chlorination and eventually the loss of crystallinity of the catalyst. Additional experiments were conducted using the transmission cell in the FTIR to aid in deciphering the reaction mechanisms for the oxidation of both methylene chloride and carbon tetrachloride. The in situ FTIR experiments were systematically performed and the results were analyzed to draw conclusions regarding the oxidation reaction mechanisms for both CVOCs. It was observed that both methylene chloride and carbon tetrachloride molecules adsorbed on the Bronsted acid sites with the formation of an intermediate species of the type COHCl during the oxidation of CH,Cl, and COCl, during the oxidation of Ccl,. From these results, and consistent with supporting literature, reaction mechanisms are proposed for the oxidation of both CH,Cl z and Ccl,.

2. Experimental

The H-Y catalyst was received in the form of l/16 in. extruded pellets (LZ-Y-62), using alumina binder, from UOP, Tarrytown, NY. This catalyst formed the basis for the preparation of the other three catalysts, namely Na-Y, Co-Y and Co-Y/CA.

For the cation exchanges, the H+ ion in the H-Y catalyst was first replaced with the NH: ion, which was then replaced with the preferred cation. For the ammonium exchange, 120.5 g of ammonium chloride were dissolved in 1 1 of deionized water and 100 g of H-Y zeolite were added to this solution and stirred continuously at 90°C for 2 h. The ammonium chloride solution was then drained off the H-Y pellets which were then washed thoroughly with deionized water. This process was repeated twice more to yield about 90% exchange to the NH,-Y form.

Na-Y was obtained from the NH,-Y by sodium exchange, for which 340 g of sodium nitrate were dissolved in 1 1 of water and 20 g of NH,-Y pellets were then added to this solution with continuous stirring and heating at 90°C. After 8 h of exchange, the sodium nitrate solution was drained off and the pellets were washed with deionized water. The catalyst pellets were then dried at 120°C for 2


h and then calcined at 500°C for 10 h. Two more exchanges were performed and the catalyst pellets were calcined at the end of every exchange in order to prevent the sodium cations from exchanging back into the precursor solution. Since, the amount of Naf exchanged into the zeolite could not be quantified, review of literature on exchange of mono-valent cations in zeolites indicated that three successive exchanges would result in about 90% exchange of the protons by the cation. An almost complete exchange of the Hf ions by the Na+ ions was expected and which was necessary to remove all the Brprnsted acidity of the catalyst. This was necessary to find out if the Brransted acid sites in a zeolite were responsible for the oxidation of both the CVOCs tested.

About 20 g of NH,-Y was cobalt exchanged with a solution containing 16 g of Co(NO,), . 6H,O dissolved in 1 1 of deionized water, and this solution was stirred continuously for 48 h at 90°C. As opposed to the Na+ exchange in which complete exchange of the sodium ions with the H+ ions in H-Y was desired, only a 1.5% (by weight as cobalt) loading of cobalt on the zeolite was needed for the catalyst to be active for the oxidation of both methylene chloride and carbon tetrachloride. Therefore, only a single exchange was performed with the necessary amount of cobalt ions in the fresh exchange solution. Also, the duration of the exchange was increased in order to facilitate the exchange of all the cobalt ions in the cobalt nitrate solution with the H+ ions in the zeolite catalyst. The pellets were then thoroughly washed with deionized water, dried at 120°C for 2 h and then calcined at 500°C for 10 h to obtain the Co-Y catalyst. It was calculated that a 13% exchange of the available H+ ions in the zeolite by the cobalt cations was accomplished.

Chromium impregnation of the cobalt exchanged zeolite catalyst was performed to form the Co-Y/CA catalyst. The chromium impregnation was carried out with a solution containing 19 g of chromic acid in 100 ml of deionized water. The Co-Y catalyst was immersed in this solution for 1 h and then dried at 120°C for 2 h followed by calcination of the catalyst at 500°C for 10 h thereby forming the Co-Y/CA catalyst. It was noticed that the cobalt content of the catalyst was reduced after the impregnation with chromium. It is possible that during the impregnation there is some exchange of chromium ions into the zeolite and washing away of cobalt ions out of the zeolite during the process.

All the reactor runs were carried out with a fixed bed continuous reactor which is schematically depicted in Fig. 1. The reactor, made of Pyrex, was 0.032 m od by 0.028 m id and 1 m in length. The flow rate through the reactor was set at 500 ml/min and the space velocity was maintained at 2400 h-’ (calculated at 25°C and 1 atm based on empty reactor volume). The CVOC feed concentration at the reactor inlet was about 1500 ppm; reactor runs were performed at temperatures between 200 and 350°C and at atmospheric pressure.

A Hewlett-Packard 5890 gas chromatograph and 5970B mass selective detector were used for the quantification of the reactor feeds and effluents. The gas samples from the reactor inlet and outlet sampling ports were transported to

B. Ramachandran et ok/Applied Catalysis B: Environmental 8 (1996) 1.57-182 161

n

6-

4 13

8 a

Fig. 1. Schematic of the reactor. (1) Sampling ports, (2) catalyst section, (3) reactor furnace, (4) heat tape and insulation, (5) preheater furnace, (6) main air inlet to the reactor, (7) N, to the CVOC bubbler, (8) CVOC bubbler, (9) outlet to vacuum pump, (10) HCI scrubber, (11) thermocouples, (12) manometer tap and (13) glass wool.

the GC/MS by means of a 200 ~1 constant delivery gas syringe made by Hamilton Co. For the quantification of CO, Cl, and phosgene, detector tubes made by Mine Safety Appliances (MSA) were used along with a model A MSA Samplair pump. HCl was quantified by determining the pH of a known amount of deionized water through which the reactor effluents were passed for a known amount of time. It was found out by experiments earlier that the amount of chlorine from the effluent stream that dissolved in the HCl scrubber was very negligible.

FI’IR experiments were performed using a Bio-Rad model FIS-7 FI’IR with a purged bench option. The in situ experiments were performed using a variable temperature transmission reaction cell (HTC- 100) manufactured by Harrick Scientific. The catalyst samples for the reactions were made in the form of self supported wafers 13 mm in diameter using a pellet die also made by Hanick Scientific. The reaction cell was equipped with a set of three heater elements which can uniformly heat up the cell to about 500°C. A high vacuum system with a capability of producing vacuum as low as 10e5 torr was used to evacuate the cell.

162 B. Ramachmdran et al./Applied Cutalysis B: Environmental 8 (1996) 157-182

During the in situ FfIR experiments, the CVOC stream in N, with a concentration of about 1000 ppm in nitrogen was passed into the reaction cell with the catalyst wafer maintained at 300°C. Scans of the catalyst wafer were taken prior to and after the adsorption of the CVOC by the wafer. After adsorbing the CVOC for 1 h the cell was evacuated to lo-’ Torr for 1 h to remove physisorbed reactant. A scan taken after this would show the spectrum of the sample with only the chemisorbed CVOC remaining. For the in situ oxidation of the CVOC, 0, was passed over the CVOC chemisorbed wafer and repeated scans were collected with time to look for any chemical transforma- tions on the surface of the catalyst wafer. Analyses of the collected spectra were carried out later.

A thermogravimetric analyzer TGA 2950 made by TA Instruments was used for all the adsorption and TPD experiments. For the oxygen adsorption, the samples were degassed at 500°C and then oxygen chemisorption was performed at 300°C. A continuous monitoring of the weight change of the sample was measured. A Philips PV9550 X-ray fluorescence spectrometer (XRF) was used to measure the composition of the catalysts. In particular, the amount of metal loading on the catalysts and the silica/alumina ratio of the zeolite catalysts were determined using the XRF. The surface areas of the catalysts were determined using the Quantasorb Jr. BET surface area analyzer.

3. Results and discussion

3.1. Catalyst characterization

Reactor runs were performed over temperatures ranging from 150 to 350°C at atmospheric pressure and a space velocity of 2400 h-‘, to understand the variations in the conversion and selectivity of the four catalysts. Oxygen adsorption capacities of the various catalysts along with their surface areas are tabulated in Table 1. Table 2 depicts the chemical composition of the four catalysts and Fig. 2 shows the NH, TPD acidity curves for those catalysts.

Table 1 Catalyst characteristics

Catalyst Surface 0, adsorption 300°C (mg/g cat.)

CH,CI, 300°C

Conversion (o/o)

CCI, 300°C

Conversion (o/o)

H-Y 660 1.9 33.4 71.9 Na-Y 540 0.8 15.8 17.2 CO-Y 612 2.3 38.4 71.2 Co-Y/CA 103 1.6 27.0 60.7

B. Rumuchundran et d/Applied Catalysis B: Environmental 8 (1996) 157-182 163

Table 2 Catalyst compositions

Catalyst Catalyst composition

Si/AI % co % Cr

H-Y 2.60 Na-Y 2.89 _ _ CO-Y 2.86 1.62 _

Co-Y/CA 3.27 0.54 5.54

The results of the reactor runs for the oxidation of methylene chloride are tabulated in Table 3, and the conversions of the catalysts as a function of the temperature are plotted in Fig. 3. The conversion of Co-Y was higher than for all other catalysts except H-Y which had as high a conversion as the Co-Y at temperatures of 300°C and above, although, at temperatures of 250°C and lower, Co-Y had slightly better conversion than H-Y. Even though the total number of acid sites in the H-Y was more than that of Co-Y (as indicated by the area under the curve in Fig. 21, due to the presence of the divalent cobalt cation, Co-Y had slightly stronger acid sites than H-Y (as indicated by the slightly higher NH, TPD peak temperature in Fig. 2). This difference between the two catalysts might have added to the slightly better performance of Co-Y over H-Y.

H-Y showed significantly higher conversion than Na-Y as can be seen from Fig. 3. This result points out that Brsnsted acidity appeared to play an important role in the oxidation of methylene chloride, since, the only difference between H-Y and Na-Y was the presence of a considerably higher number of Bronsted acid sites in the former as was evident from the acidity curves of Fig. 2.

The oxygen adsorption capacity of Co-Y was slightly higher than that of H-Y as seen in Table 1. This factor could also have added to the better performance of Co-Y than H-Y for the oxidation of methylene chloride.

The catalytic conversion of the only cation exchanged/impregnated catalyst studied, namely Co-Y/CA, was lower than that of Co-Y. It is possible that

z 3, I

Fig. 2. Comparison of acidity of catalysts by NH, PD.

164 B. Rrrmachandrun et d/Applied Cutalysis B: Environmental 8 (1996) 157-182

Table 3 Catalytic activity of the catalysts for the dry oxidation of methylene chloride

Run no. Temperature Feed Conversion Chlorine Carbon Cl, /HCl Reaction (“0 cont. (%I balance balance product rate

(ppm) (o/o) (o/o) ratio (mol/h cm3 cat) x IO’

Catalyst: H-Y pellets MCLlOl 350 996 MCLIO2 300 778 MCL103 250 1040 MCL104 200 1190 Catalyst: Na-Y pellets MCLIOS 350 1716 MCL106 300 1909 MCL107 250 2028 MCLIOS 200 2137 Catalyst: Co-Y pellets MCL109 350 1406 MCLIIO 300 1191 MCLlll 250 1206 MCLl12 200 1543 Cutalyst: Co-Y/CA pellets MCL113 350 1073 MCL114 300 1010 MCL115 250 992 MCL116 200 1104 Homogeneous run MCL117 350 1611

100.0 72.3 89.5 0.00 9.78 97.8 89.3 88.5 0.00 7.47 85.5 96.4 73.2 0.00 8.73 34.4 16.7 83.8 0.00 4.02

83.5 68.3 110.7 0.44 14.06 32.2 87.3 111.0 0.11 6.03

5.6 96.8 114.1 0.13 1.12 1.0 99. I 99.9 - 0.21

100.0 44.1 80.3 0.00 13.80 99.9 47.2 16.6 0.00 I 1.68 95.9 45.2 74.9 0.00 I I .35 65.1 45.3 70.8 0.00 9.86

100.0 64.6 84.9 0.39 10.53 100.0 97.2 91.0 0.44 9.91 69.9 91.6 110.8 0.18 6.81 23.5 95.3 106.7 0.10 2.55

24.4 75.7 76.0 0.00 3.86

during the impregnation some of the pores in the catalyst might have been clogged. This was also supported by the drastic loss in surface area of this catalyst from 612 m*/g in Co-Y to 103 m*/g in Co-Y/CA as shown in Table 1. However, the primary reason for the subdued performance of Co-Y/CA is probably the loss of crystallinity in the zeolite due to the highly acidic nature of

100 90 so

8 70

V0

1”

-o- c4bwcA Fwlets

s 40 -x-HomogeneousRun

v 30 20 Spacz Velocity 2400 h-’

x

10 0

0 100 200 300 400

= Tcmpc~ Cc)

Fig. 3. Comparison of conversion trends of catalysts for methylene chloride oxidation.

B. Ramuchundran et ol./Applied Curalysis B: Environmental 8 (1996) 157-182 165

the chromic acid solution which was used in the impregnation of chromium on the Co-Y catalyst. Chatterjee [lo] studied the XRD crystallinity patterns of various pH treated Y-zeolites and concluded that a highly acidic medium reduced the crystallinity of zeolites. The crystallinity loss may also be a reason for the observed loss of surface area in the Co-Y/CA catalyst. The poor activity showed by the Co-Y/CA catalyst with low crystallinity proved that the zeolite structure is also very essential for the oxidation of methylene chloride to be effective on the catalyst.

The only catalyst containing chromium (Co-Y/CA), showed substantial selectivity towards the formation of Cl, as would be expected from a catalyst which favors the Deacon reaction. Also, the amount of chlorine produced, increased with an increase in the reaction temperature because the Deacon reaction (where HCl and 0, form Cl, and H,O) was favored at temperatures higher than 300°C. Poor chlorine balances that were observed with all the reactor runs carried out were attributed to chlorination of the catalyst during oxidation without water addition. However, it is not quite clear as to the reasons for the wide variations in the chlorine balances.

Reactor runs were also carried out with Ccl, as the feed for the four catalysts. The results of these runs are tabulated in Table 4. A plot of the conversions of the various catalysts as a function of temperature is also shown in Fig. 4. The trends in conversion of the catalysts for Ccl, oxidation were quite similar to that of methylene chloride; however, one major difference between the two CVOCs was the formation of COCl, as a reaction product in the oxidation of Ccl,. Ccl, with no hydrogen in its structure seemed to be a relatively easier feed to destroy, probably, because the strength of the C-Cl bond in Ccl, was 70.4 kcal/mol and the bond strength of the C-H bond in CH,Cl, was 99.0 kcal/mol. Hence, the lower strength of the C-Cl bond might have been the reason for the enhanced destruction of Ccl, as compared to CH,Cl,.

Co-Y seemed to be the most active catalyst of the group, outperforming even H-Y at temperatures of 200°C and less. However, H-Y showed much higher activity (87% at 150°C) compared to that of Na-Y (7% at 15O”C), indicating that Brprnsted acidity does play an important role in the oxidation of Ccl,.

As with the oxidation of methylene chloride, Co-Y/CA had low conversion compared to that of H-Y or Co-Y for the same reasons mentioned earlier with methylene chloride oxidation. All the catalysts had an high selectivity for HCl rather than Cl, except Co-Y/CA which had Cl, as the predominant chlorinated product. However, there was also some formation of COCl, by all the catalysts at temperatures of 200°C and below. The chlorination of the catalyst was the reason for the poor chlorine balances in all the reactor runs conducted without water in the feed stream. XRF measurements on the catalysts samples after the dry runs indicated that chlorine was present in the catalysts thereby proving the catalyst chlorination hypothesis.

166 B. Ramachnndrun et d/Applied Catalysis B: Enuironmentul8 (1996) 157-182

3.2. Reaction mechanisms for the oxidation of methylene chloride and carbon tetrachloride

3.2.1. Variable space velocity experiments As was observed in the reactor runs performed on the four catalysts for the

oxidation of both the CVOCs, Co-Y definitely did have the best conversion among the catalysts studied. Therefore, all the experimentation that follows was carried out utilizing this catalyst. The variable space velocity reactor runs were carried out over a wide range of space velocities from 100 to 47000 h -’ to differentiate between series and parallel reaction mechanisms. Since, the conversion of the catalyst decreased with the increase in space velocity, a suitably high temperature was needed to achieve sufficient conversion at very high space velocities. A temperature of 350°C proved to be a good choice for the experiments. Due to limitations on the maximum flowrate through the reactor and to have a reasonable diameter-to-height ratio for the catalyst bed and at the same

Table 4 Catalytic activity of the catalysts for the dry oxidation of carbon tetrachloride

Run no. Temperature Feed Conversion Chlorine Carbon Cl, /HCI Reaction (“0 cont. (%I balance balance product rate

(ppm) (o/o) (%I ratio (mol/h cm3 cat) x 103

Cutalyst: H-Y pellets CCL101 350 1366 CCL102 300 1333 CCL103 250 1351 CCL104 200 1181 CCL105 150 1460 Cutalyst: Na-Y pellets CCL106 350 918 CCL107 300 933 CCL108 250 869 CCL109 200 1045 CCL1 10 150 1087 Catulyst: Co-Y pellets CCL111 350 1604 CCL112 300 1509 CCL113 250 1471 CCL114 200 1424 CCL1 15 150 1532 Catalyst: Co-Y/CA pellets CCL116 350 1657 CCL117 300 1662 CCL118 250 1691 CCL119 200 1703 CCL120 150 1556 Homogeneous run CCL121 350 1553

100.0 72.9 83.4 100.0 44.9 75.6 100.0 67.9 86.8 96.8 66.1 93.3 87.0 14.4 77.1

100.0 10.1 90.6 97.3 21.9 77.2 87.4 36.6 109.4 28.1 93.0 88.9

6.5 95.7 96.8

I Co.0 31.3 79.5 100.0 30.4 77.9 100.0 34.9 85.8 1 CO.0 34.1 76.7 92.4 23.7 81.1

100.0 90.9 91.5 100.0 68. I 82.5 100.0 70.0 79.9 84.0 99.5 91.8 45.8 81.5 99.1

28.2 73.5 74.3

0.003 13.41 0.002 13.08 0.000 13.26 0.000 11.22 0.000 12.47

0.600 9.01 0.196 8.91 0.216 7.46 0.025 2.88 0.000 0.69

0.011 15.74 0.008 14.81 0.009 14.44 0.010 13.98 0.016 13.90

1.378 16.26 1.110 16.31 1.660 16.60 1.192 14.04 0.220 7.00.

0.000 4.30

B. Ramachandran et ul./Applied Catalysis B: EnuironmentalS (1996) 157-182 167

-O-Na-YPdktS

u) ---u-co-YICA

x 20 -. space Veloci~ 2400 li’ 10 --

0 50 loo 150 200 250 300 350 400

w Tmpvrtpn Q

Fig. 4. Comparison of conversion trends of catalysts for carbon tetrachloride oxidation.

time have a very high space velocity, l/16 in. alumina pellets were added to the catalyst as diluent. The addition of alumina the reactor runs over a wide range of space runs were conducted both with and without feed stream.

as the diluent aided in performing velocities. In this case, the reactor the addition of water vapor in the

3.2.1 .l. Methylene chloride oxidation. Comparison of the oxidation conversion of the Co-Y catalyst with and without the presence of water vapor in the methylene chloride feed stream, as a function of space velocity, is represented in Fig. 5 and showed that the conversion decreased with increase in the space velocity because the residence time for each feed molecule through the catalyst bed decreased with increase in the space velocity. Also, the runs carried out with water vapor in the feed stream showed a slightly lower conversion than the runs without water vapor in the feed stream, perhaps because the presence of water vapor in the feed stream caused competitive adsorption between the water molecules and the CH,Cl, molecules on the surface of the catalyst.

t 0”’ a”““““‘-“”

0 10000 20000 30000 40000 50000 Space Velocity (h-l)

Fig. 5. Conversion of Co-Y as a function of space velocity for methylene chloride oxidation.


A- Co Dry Run

E 60 -& CO Vitb Vatcr Vapor Addition (15.000 ppm)

h .s: -$- Co2 Dry Run .5 ?; -@- 032 6lh lakr Vapor Addilioa (15.000 ppm) s 91

40

Temperature 3SPC

20

20000 30000

Space Velocity (h-’ )

Fig. 6. Selectivity of Co-Y as a function of space velocity for methylene chloride oxidation.

Fig. 6 shows the trends in the CO and CO, selectivities during the oxidation of methylene chloride at different space velocities. It can be noticed immediately that the selectivities for both CO and CO, did not change significantly or follow any particular pattern over the whole range of the space velocities studied. At very low space velocities, the CO selectivity tended to diminish very slightly but overall, the CO and CO, selectivities remained relatively constant. If the formation of CO, was due to the oxidation of CO, (suggesting a series reaction) then with an increase in the space velocity, the selectivity of Co-Y for the formation of CO, should decrease, and vice versa for the formation of CO. It can be seen that this is clearly not the case. Conversely, if the formations of CO and CO, occurred from parallel reactions, any variation in space velocity would not be expected to affect the selectivity of the catalyst towards the formation of these products. This is what was observed in Fig. 6 and is consistent with the reactions being parallel. It was also observed that the selectivity of the catalyst towards the formation of CO, was negligible ( < 5%).

The addition of water vapor did not significantly alter the trend observed in the dry runs. The addition of H + ions in the form of water vapor was not necessary for the deep oxidation of methylene chloride because there was sufficient hydrogen in the molecule itself to form the stoichiometric HCl. This is evident in Table 3 from the selectivity of Co-Y to form HCl with almost no Cl, in the dry oxidation runs. The same trend, with HCl as the predominant chlorinated product, was also observed in the wet oxidation runs [ 1 I]. Thus, no significant difference in selectivity trends for the carbon as well as the chlorine deep oxidation products was observed during the variable space velocity wet versus dry oxidation of methylene chloride.

3.2.1.2. Carbon tetrachloride oxidation. The effects of the change in space velocity on the oxidation of Ccl, were also investigated. A plot of the


20 1 -+- lith Water Vapor Addition (15.000 ppm)

Temperalure 350°C

0 *L~‘~*~‘~,~‘~ 0 10000 20000 30000 40000 50000

Space Velocity (h-l )

Fig. 7. Conversion of Co-Y as a function of space velocity for carbon tetrachloride oxidation.

conversion of Co-Y as a function of space velocity for the oxidation of CCI, with/without the addition of water vapor in the feed stream is shown in Fig. 7 and the selectivity trend of Co-Y is plotted in Fig. 8. These runs were performed at a stretch with the same bed of catalyst, which is important, since, it pertains to the deactivation of the catalyst during Ccl, oxidation, discussed later.

The conversion pattern for Ccl, was quite different from what was observed for the reactor runs with CH,Cl,. For Ccl,, it was noticed that Co-Y showed higher conversion as a catalyst with water vapor than without water vapor in the feed stream. Another important observation was that the major chlorine product during the dry oxidation of Ccl, was HCl and not Cl, as one would expect in a CVOC that does not contain hydrogen in its molecular structure. The feed stream into the reactor contained less than 8 ppm of water; therefore, the hydrogen in the product HCl could only have come from the zeolite structure. Gallinskii et al. [12] studied the chlorination of faujasites by Ccl,, in which they

0 tp-y---y;,. -~..?...^..I?.. 0 11000 22000 33000 44000 55000

Space Velocity (Ii’ )

Fig. 8. Selectivity of Co-Y as a function of space velocity for carbon tetrachloride oxidation.

170 B. Ramachandran et al./Applied Catulysis B: Environmental 8 (1996) 157-182

suggest the formation of aluminum and silicon monochlorides and the formation of COCl, due to chlorination. The dealumination of zeolites by halogen-containing reagents has also been studied by Fejes et al. [ 131. In this study, they suggested the adsorption of COCl, on a Brprnsted acid site and the subsequent removal of hydrogen as HCl.

The above studies clearly support the limited production of HCl in the dry oxidation of Ccl,. Also, in the reactor runs during dry oxidation, the removal of the Bransted acid sites would most likely cause the catalyst to deactivate, with a resultant decrease in conversion. A dry run at 3160 h-’ was repeated at the end of the set of runs to check for deactivation in the catalyst. The conversion at this space velocity dropped from 100% initially, to 89% for the benchmark run, indicating that the catalyst did deactivate, even though the selectivity of the catalyst did not change (perhaps because the loss in crystallinity of the catalyst at that time was still low). In the presence of water, the H+ ions in the reactor feed replenished the Brransted acid sites of the catalyst making it stable.

The selectivity patterns with space velocity for Ccl, oxidation, as seen in Fig. 8, were again different from those for CH,Cl,. In the absence of water vapor in the Ccl, feed stream the selectivity to CO, decreased and that of COCl, increased with an increase in space velocity. This observation suggested that COCl, was the intermediate in the formation of CO, during the oxidation of Ccl,. However, the variable space velocity reactor runs performed with the presence of water vapor in the feed stream did not show any appreciable change in the product selectivities, with CO, always being the predominant product, implying that the replenishment of the proton on the catalyst surface provided by the water vapor, enhanced the reaction path for the formation of this deep oxidation product.

Since, it was observed that the Co-Y catalyst deactivated slightly during the dry variable space velocity experiments, a separate dry deactivation run with this catalyst and Ccl, as the feed was needed to confirm the earlier results.

Fig. 9 shows a plot of the conversion at 250°C of Co-Y as a function of the time of deactivation. It can be seen that this conversion dropped very rapidly after about 10 h of operation and plummeted to about 3% after 50 h of deactivation. The rapid fall of the catalyst conversion at around 10 h of deactivation, correlates well with the time calculated (10.9 h) for the removal of all the H+ ions (Bronsted acid sites) as HCl during the reaction. However, not all the chlorine atoms in the feed molecule were converted to HCl, as part of it was converted to phosgene. So essentially, most but not all of the H+ ions were removed from the zeolite after about 10 h of deactivation.

It was also noticed that the selectivity of the catalyst changed during the deactivation. Fig. 10 shows a plot of the catalyst selectivity with the time of deactivation. The selectivity of Co-Y towards the formation of CO, decreases during the time of deactivation and vice versa for COCl, formation. Since, there was no water vapor present in the feed stream, the protons from the Bronsted

B. Ramuchundrun et d/Applied Cutulysis B: Environmental 8 (1996) 157-182 171

Temperalure 250°C

Space Velwitg 2100 C’

-0 10 20 30 40 50 60

Time of Deactivation (h)

Fig. 9. Deactivation of Co-Y during dry carbon tetrachloride oxidation.

acid sites were utilized in the formation of HCl after the adsorption of carbon tetrachloride on the Brernsted acid sites. Thus, as the time of deactivation increased, the quantity of Brernsted acid sites left on the catalyst surface was reduced. Since these sites are believed necessary for the adsorption and further decomposition of phosgene to CO, the changes in selectivity are reasonable.

The catalyst’s surface area dropped almost 50% (from 612 to 310 m*/g> and also showed a significant loss in its acidity during deactivation. In addition, the FIIR spectrum (discussed later) of the deactivated catalyst showed that the bands corresponding to the hydroxyl region were lost, confirming the removal of the Bronsted acid sites from the catalyst during the dry oxidation of Ccl,. The absence in the deactivated sample of most of the XRD peaks characteristic of fresh Co-Y clearly showed the loss in crystallinity of the deactivated Co-Y.

Therefore, the conversion and selectivity changes associated with the variable

+ co2

-e- cock2 0

Space Velaity 2400 li’

1

20 30 40

Tie of Deactivation (h)

Fig. IO. Selectivity of Co-Y during deactivation by dry carbon tetrachloride oxidation.

172 B. Rumochandran et d/Applied Cutulysis B: Environmentul8 (1996) 157-182

space velocity experiments were partially masked by the deactivation of the Co-Y catalyst. From the results obtained, it is very clear that the absence of a hydrogen atom in the feed molecule almost completely altered the reaction mechanism and making the catalyst vulnerable to chlorine attack. The interaction between Ccl, and Co-Y was much more pronounced because of the absence of hydrogen in the Ccl, molecule. Deactivation studies of Co-Y by Ccl, or CH,Cl, in the presence of water vapor in the feed stream, was not performed in this study. These studies were previously carried out with the Co-Y catalyst in the presence of moisture and have been reported elsewhere [9]; the catalyst showed no similar deactivation tendencies under these conditions.

3.2.2. In situ FTIR experiments The adsorptions and oxidations of both the CVOC feeds over the Co-Y

catalyst were also performed in situ in the FAIR utilizing the transmission reaction cell. The Co-Y catalyst in the form of a self supported wafer was placed inside the reaction cell and evacuated to lo-’ torr at 350°C. The normal procedure followed was to leave the wafer under these conditions overnight for removal of impurities. The CVOC feed with a concentration of 1500 ppm in a nitrogen atmosphere was then passed into the reaction cell and scans were collected in the FI’IR at different time intervals to check the progress of the adsorption. After the adsorption was complete, the cell was evacuated at 1O-5 torr for 1 h to remove any physisorbed CVOC.

3.2.2.1. Methylene chloride experiments. Fig. 11 shows the spectra of methylene chloride adsorbed on Co-Y at 300°C. During the adsorption as seen in spectrum 1 of Fig. 11, it was noticed that IR bands at 3730, 3652, 3586 and 3550 cm-’ decreased in intensity, indicating the adsorption of methylene chloride at the sites represented by these bands. Since, the background for these spectra was that of pure Co-Y, the above mentioned bands appear with negative intensities.

The band at 3730 cm-’ is believed to correspond to the silanol (Si-OH) groups on the outer surfaces of the crystallites (OH groups which terminate the faces of the zeolite crystallites at positions where bonding in the interior would otherwise occur with adjacent tetrahedral Si or Al ions). The band appearing at 3652 cm-’ was associated with a hydroxyl group showing Brransted acid properties, which has been shown to protrude into the supercage [ 143. The next band appearing at 3586 cm-’ corresponded to very strong hydroxyl groups, and appeared due to the presence of amorphous silica-alumina formed on the sample from extra-framework aluminum (EFAL) remaining in the zeolite after dealumination, and being concentrated mainly on the surface. Support for this explanation is that a hydroxyl band at 3610 cm-’ corresponding to very strong acid sites has been observed in amorphous silica-alumina with a high SiO,/Al,O, ratio [15]. A similar study on hydroxyl groups on non-framework aluminum species in dealuminated Y zeolites [ 161, also suggested the presence

B. Ramachandran et d/Applied Cutulysis B: Enuironmentul8 (1996) 157-182 173

3o;o Wavenumbers (cm-l)

2obo

Fig. 1 I. IR Spectra of adsorbed methylene chloride on Co-Y at 300°C. (1) CH,CI, Adsorbed (physisorbed and chemisorbed), (2) CH,CI, Chemisorbed (after removal of physisorbed CVOC by evacuation).

of AlOH groups within the non-framework species and the presence of the corresponding absorption band at 3606 cm-‘. Another band at 3550 cm-’ also appeared after the sample was evacuated (spectrum 2). This band was again believed to be associated with an OH group that protruded into the sodalite cages or hexagonal prisms [ 171.

In addition, two small peaks are seen at 2976 and 2850 cm-’ which correspond to the asymmetrical C-H stretching ( v,,CH*) and symmetrical C-H stretching (v,CH,) of methylene chloride, respectively. A band appearing at 16 14 cm- ’ may be caused by the C= C stretching due to coking on the catalyst wafer.

The oxidation of methylene chloride previously chemisorbed on a Co-Y wafer was carried out by passing pure oxygen over it in the FI’IR reaction cell. Fig. 12 shows the spectra collected as the oxidation proceeded. The background for the spectra was the spectrum of methylene chloride chemisorbed on Co-Y (spectrum 2, Fig. 11). Here it was seen that, as the time of oxidation increased, the hydroxyl bands appeared again and gradually increased in intensity. This suggested that as methylene chloride (previously chemisorbed on the hydroxyl sites) was oxidized and removed as products from those sites, the hydroxyl bands were evident again. It can also be noticed that the band appearing at 163 1 cm-’ gradually increased in intensity, then decreased in intensity and finally disappeared (spectrum 5). This band may be due to the adsorption of water on

174 B. Ramachantlran et al./Applied Catalysis B: Environmental 8 (1996) 157-182

Warenumbers (cm-f)

Fig. 12. IR Spectra of methylene chloride oxidation on Co-Y at 300°C. 1 . . . 5 Increase in time of oxidation (10 min intervals).

the surface, which was present in the oxygen stream sent into the reaction cell (about 10 ppm of water) as impurity.

Fig. 13 magnifies a region of interest where some peaks seem to form and then gradually diminish and disappear. The double peaks appearing at 2363 and 2326 cm- ’ correspond to CO,. These peaks gradually diminished with the increase in time of oxidation (spectra 1 to 8), suggesting the formation and subsequent desorption of CO, during the reaction. Another peak appeared at 2168 cm-’ which increased in intensity as the time of oxidation increased, then diminished slowly and finally disappeared. As suggested by Kniizinger and Zaki [ 181, this band corresponds to the C-O stretching band, implying the formation of CO during the reaction.

Fig. 14 shows the region in which two bands (at 1790 and 1740 cm-‘) appeared. The band at 1790 cm-’ gradually increased in intensity and finally diminished (spectrum 8) as the time of oxidation increased. The peak at 1740 cm -I did not change intensity significantly but finally disappeared as seen in spectrum 8. Fejes et al. [ 191 in their infrared spectroscopic study of adsorption of phosgene and carbon tetrachloride on zeolites have seen the formation of two bands of equal intensity at 1710 and 1800 cm-’ corresponding to the C=O stretching and Ccl, asymmetric stretching vibration based on Fermi resonance which is caused by the interaction between fundamental vibrations and over- tones. It is reasonable that the peak at 1740 cm-’ (which corresponded to the Ccl, asymmetric stretching vibration) did not increase in intensity, since during

B. Ramachandran et al./Applied Catalysis B: Environmental 8 (1996) 1.57-182 175

0

Z4DrJ ZixJo

Warenumber~ (cm-l)

Fig. 13. IR Spectra of methylene chloride oxidation on Co-Y at 300°C. 1 . . . 8 Increase in time of oxidation (IO min intervals).

lab0 li50 W~rcnumbert (cm-l)

Fig. 14. IR specka of methylene chloride oxidation on Co-Y at 300°C. 1 . 8 Increase in time of oxidation (I 0 min intervals).


4000 3000 2000

Wavenumbers (cm- 1)

Fig. 15. IR spectra of adsorbed carbon tetrachloride on Co-Y at 300°C. (1) CCI, adsorbed (physisorbed and chemisorbed), (2) CCI, chemisorbed (after removal of physisorbed CVOC by evacuation).

the reaction the formation of a species with a Ccl, group is very unlikely because no intermediates such as phosgene were detected during the high space velocity reactor runs. However, the increase in intensity of the band at 1790 cm-’ is probably due to the formation of an intermediate compound containing a carbonyl group, such as COHCl as suggested by Fevrier et al. [20]. They have also speculated that this highly unstable compound would undergo rapid decomposition into CO and HCl prior to any other reaction.

3.2.2.2. Carbon tetrachloride experiments. The adsorption of Ccl, on Co-Y at 300°C is shown in Fig. 15. Again, decrease in the intensities of the hydroxyl bands indicates the adsorption of Ccl, on the Bronsted acid sites of Co-Y. Fig. 16 shows the spectra during oxidation, collected at different durations of oxidation. The hydroxyl bands in the higher wave number region increased in intensity as time of oxidation progressed, indicating the removal of Ccl, from the hydroxyl sites. A band at 1633 cm-’ which was also present in the oxidation of methylene chloride was seen. This band, as suggested earlier for methylene chloride oxidation, may be due to the adsorption of water from the oxygen stream on the surface of the catalyst. Fig. 17 shows the region where the doublet peak corresponding to CO, at 2363 and 2328 cm-’ first appears and then gradually diminishes with increasing elapsed time. Fig. 18 shows the region where the peaks due to the Fermi resonance appeared. The peaks corresponding to the C=O stretching and Ccl, asymmetric stretching were observed at 1790


.l t % c 0

p” 4

3000 2000 Ravenumb+a (cm-l)

-7 Q

P

Fig. 16. IR spectra of carbon tetrachloride oxidation on Co-Y at 300°C. I . . . 3 Increase in time of oxidation (IO min intervals).

and 1711 cm-‘. Since, the intensities of both the peaks increased with increase in the time of oxidation, this suggested the formation of phosgene on the surface of the catalyst during the oxidation of Ccl,. The band at 1624 cm- ’ represents

Wavenumbers (cm-l)

Fig. 17. IR spectra of carbon tetrachloride oxidation on Co-Y at 300°C. 1 . . 5 Increase in time of oxidation (10 min intervals).


.1

Wavcnumben (cm-l)

Fig. 18. IR spectra of carbon tetmchloride oxidation on Co-Y at 300°C. 1 . . . 18 Increase in time of oxidation (10 min intervals).

C=O stretching and suggests an oxidative product such as coke being formed during Ccl, adsorption and oxidation.

3.2.3. Reaction mechanisms Based on the results presented, as well as the literature available in this area

of research, reaction mechanisms for the oxidation of methylene chloride and carbon tetrachloride are now proposed.

3.2.3.1. Methylene chloride oxidation. The reaction mechanism proposed for the oxidation of methylene chloride on the Co-Y catalyst is given in Fig. 19. It is believed that initially methylene chloride adsorbs on the Bronsted acid site. This is in agreement with the in situ FTIR adsorption data of methylene chloride on Co-Y, where the decrease in the intensities of the bands corresponding to the hydroxyl region of the zeolite catalyst clearly indicated the adsorption of methylene chloride on Co-Y. The methylene chloride molecule is thought to combine with the proton from the hydroxyl group in the Bronsted acid site to form the carbonium ion.

The dissociative adsorption of oxygen on the cationic site is also proposed. The adsorption of oxygen on the cation as a dissociative species [Co-O] had been suggested by Tsuruya et al. [21] in their investigation of the oxidation of benzyl alcohol over Co-Y. This process oxidizes the cobalt from the 2 + to 3 + oxidation state. In the case of H-Y catalyst, which is somewhat active but does


Al Si Al Si

T T Ii+ I

Hcl co /O\ Cf+ Al si Al si

Fig. 19. Proposed reaction mechanism for methylene chloride oxidation.

not possess the metal cationic site, it is not clear at this moment what the oxygen adsorption site is. However, the presence of oxygen as an excess reactant in the feed and perhaps on the catalyst surface renders it possible for any catalyst to adsorb oxygen.

It is believed that the CH2C12H+ carbonium ion next forms the carbenium ion by the abstraction of a molecule of HCl, a process which has previously been suggested by Chatterjee [lo]. The carbenium ion would then interact with the O- at the cationic site to form the COHCl intermediate, as has been suggested by Fevrier et al. [20], which also brings about the simultaneous restoration of the proton on the Brprnsted acid site. The presence of the C=O stretching frequency as detected in the FTIR oxidation experiments aids in the confirmation of such an intermediate species.

The COHCl intermediate being very unstable, dissociates into CO and HCl as the final products of deep oxidation as per Fevrier et al. [20]. The small amounts of CO, detected in the product stream is not formed by the oxidation of CO as the variable space velocity runs suggest that the formations of CO and CO, occur by parallel reactions. The COHCl intermediate associates itself with another O- to form CO, and HCl. However, the feasibility of this step is very low as the COHCl is an unstable intermediate. This is also supported by the very low CO, selectivity in the variable space velocity runs.

3.2.3.2. Carbon tetrachloride oxidation. The proposed reaction mechanism for the oxidation of carbon tetrachloride is shown in Fig. 20. As with methylene chloride, carbon tetrachloride also adsorbs on the Bronsted acid site as was evident from the decrease in the intensities of the hydroxyl bands of the zeolite during the FTIR experiments. It is again proposed that oxygen adsorbs dissocia-


cl cl \/ o- Q ct

cl’ h 1 ‘c’ _..*)

p O-4 T ,i ’ I\

<,.h \

+::, ;,.. ,I 9’ ,” cl

a+ cox ? i \ ai aa //,’ ,,.” &:::;.::.. . . . . . T Ha

’ I ‘; ‘, ‘; 8 CITIJ

\,!zp’

/O\ I 1 ‘r T&- . . . . . . ?- -

! CL* ,<

/“\ /O\ Al Si /O\ /O\ /O\ 1

Al si Al Si Al Si

Al si At si

Cl

Fig. 20. Proposed reaction mechanism for carbon tetrachloride oxidation.

tively on the cationic site and increases the oxidation state of cobalt from 2 + to 3 + . As before, Ccl, interacts with the H+ ion of the Brprnsted acid site, this time to form a carbonium species CC14H+.

It should be noted that a major difference between carbon tetrachloride and methylene chloride oxidation mechanisms under dry conditions is the absence of a hydrogen source in the former. This deficiency, which eliminates the restoration of the Bronsted acid site, is believed to be a crucial difference leading to the eventual destruction of the catalyst matrix as outlined below.

After the abstraction of a molecule of HCl from the carbonium ion, the Ccl, species is believed to interact with another Brprnsted acid site (the only hydrogen source available) and with the dissociatively adsorbed oxygen on the cationic site to form phosgene, thereby releasing another molecule of HCl. The formation of phosgene as a reaction intermediate has been verified by the variable space velocity reactor runs as well as by the in situ FIIR experiments in which bands corresponding to C=O stretching and Ccl, asymmetric stretching have been detected. Furthermore, from the variable space velocity experiments it is also evident that the phosgene selectivity increases with increase in the space velocity as series intermediates generally do. Thus, at lower space velocities there is sufficient residence time for the phosgene molecule on the catalyst surface such that it adsorbs on the Bronsted acid site and undergoes further decomposition as shown below.

When the phosgene molecule adsorbs on an adjacent Brprnsted acid site, it is believed to form an unstable positively charged halo-acylium ion (COCl+ with the simultaneous removal of a molecule of HCl, as suggested by Fejes et al. [22] The halo-acylium ion then chlorinates the zeolite structure to from AlOCl and

B. Ramuchandran et al./Applied Catalysis B: Environmental 8 (1996) 157-182 181

releases a molecule of CO, utilizing a framework oxygen in the process. The structural damage is evident from the XRD results of the deactivated catalyst which show almost complete loss of crystallinity. The detection of Cl- on the catalyst by XRF also confirmed the proposed chlorination of the zeolite structure. Further adsorption of phosgene on the AlOCl site results in the formation of CO, and AlCl,. Mechanistic details of the case where water addition occurs during the oxidation of CVOCs has been discussed by Chatterjee [lo].

4. Conclusions

The Co-Y catalyst showed good activity compared to the other catalysts for the oxidation of methylene chloride and carbon tetrachloride. Variable space velocity runs indicated that CO was the predominant deep oxidation product and that the formations of CO and CO, occurred by parallel reactions during the oxidation of methylene chloride. Phosgene was identified as the reaction intermediate in the oxidation of carbon tetrachloride. The FTIR results suggested the formation of an unstable intermediate (COHCl) during the oxidation of methylene chloride. Based on the results and the current literature, deep oxidation reaction mechanisms were proposed for the two CVOCs.

Acknowledgements

The funding for this research was obtained from the US EPA, the US Air Force and SERDP, and is acknowledged with appreciation. Zeolite samples were supplied by U.O.P. and alumina samples were supplied by Norton Co.

References

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[lo] S. Chatterjee, Ph.D. Dissertation, The University of Akron, 1993. [l 11 B. Ramachandran, M.S. Thesis, The University of Akron, 1993. [12] A.A. Galinskii, N.A. Markova and P.N. Galich, Ukr. Khim. Zh., 52 (9) (1986) 934. [13] P. Fejes, 1. Kiricsi, A.K. Hannus and Gy. Schiibel, React. Kinet. Catal. Lett., 14 (4) (1980) 481. [ 141 B.C. Gates, J.R. Katzer and G.C.A. Schuit, Chemistry of Catalytic Processes, Chemical Engineering

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Decomposition characteristics and reaction mechanisms of methylene chloride and carbon tetrachloride...

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