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Conclusions Cobalt hydridocarbonyl is a rather unspecific catalyst for the reaction of methanol and synthesis gas (methanol homologation). We have identified more than 20 products from this reaction, which is considerably more than have been previously acknowledged in the literature. Therefore many yields and selectivities reported previously for this reaction may be misleading. Continuous unit operation represents a more reliable way of evaluating the potential of the methanol homologation reaction. We have found by comparing similar batch and continuous unit experiments that continuous operation increases selectivity to ethanol potential from 45 to about 65%. This represents a significant increase in potential ethanol yield from this reaction. The possible commercial exploitation of this process is hampered by high reaction pressure, lack of reaction specificity, and the overall low reaction rate. Future work should be directed toward overcoming these obstacles. Acknowledgment The authors wish to acknowledge the technical assis-
tance of Messrs. L. A. Miller, R. W. Jarrett, and R. Cuevas in analytical methods and for GC-mass spectral analyses and Mr. C. H. Floyd for help in carrying out the experimental work. Literature Cited Albanesi, G., Chim. Ind. (Milan),55, 319 (1973). Berty, J., Markol, L., Chem. Tech. (Berlin), 8,260 (1956). Brooks, R. E. (to DuPont), US. Patent 2457204 (Dec 28, 1948). Burns, G. R., J. Am. Chem. SOC.,77, 6615 (1955). Emmett, P. H.,in "Catalysis", Vol. V, p 73, Reinhold, New York, N.Y., 1957. Mizoroki, T., Nakayama, M., Bull. Chem. SOC.Jpn., 37, 236 (1964). Piacenti, F., Bianchi, M., "Organic Synthesis via Metal Carbonyls", VoI. 2, pp 1-42, I. Wender and P. Pino, Ed., Wiley, New York, N.Y., 1977. Pruett, R. L. (to Union Carbide), US. Patent 3 833 634 (Sept 3, 1976). Riley, A. D., Bell, W. 0. (to Commercial Solvents), US. Patent 3 248432 (Apr 26, 1966). Slaugh, L. H. (to Shell), Belgian Patent 842430 (Feb 11, 1977). Sternberg, H., Wender, I., Proc. Int. Conf. Coord. Chem., Chem. Soc. Spec. Publ., No. 13, 35 (1959). Wender, I., Friedel, R. A., Orchin, M., Science, 113, 206 (1951). Wender, I., Levine, R., Orchin, M., J. Am. Chem. SOC.,71, 4160 (1949). Wender, I., Catal. Rev. - Sci. Eng., 14, 97 (1976). Wilson, J. (to DuPont), US. Patent 2555950 (June 5, 1951). Ziesecke, K. H., Brennst. Chem., 33, 385 (1952).
Received for review January 16, 1978 Accepted May 30, 1978
Methylene Chloride from Chloroform by Hydrochlorination Dwain A. Dodson and Howard F. Rase' Department of Chemical Engineering, The University of Texas at Austin, Austin Texas 78772
Three catalysts (Pd-on-charcoal, Pt/A1203,and Pt-Re/A1203 reforming catalyst) were compared on the basis of activity, activity maintenance, and selectivity in the hydrodechlorination of chloroform to yield methylene chloride. The palladium catalyst deactivated rapidly because of coke formation and sintering of the palladium. Both R/AI2O3 and Pt-Re/AI2O3gave promising life, but Pt/A1203was more active at the conditions studied. Decline in activity was primarily caused by a coke deposit consisting of a chlorinated polymeric hydrocarbon. The deposit was successfully removed by regeneration using oxygen.
Many commercial chlorination processes involve photochemical or thermal reactions which produce a variety of products in proportions that are not always ideal in terms of current demands. This is particularly true in the case of chlorinated methanes. Existing processes combined with the practice of treating waste streams of chlorinated high molecular weight hydrocarbons by thermal degradation tend toward overproduction of carbon tetrachloride and chloroform. At the same time, interest and demand for methylene chloride has increased significantly. Unlike chloroform and carbon tetrachloride, methylene chloride (CH,Cl,) is nontoxic to test animals in levels up to 3500 ppm (Chem. Eng. News, 1977). Methylene chloride is valuable as a solvent, flame suppressant, vapor-pressure depressant, and a substitute for the controversial and more expensive chlorofluorocarbons in aerosol products. There is incentive to develop viable processes for converting carbon tetrachloride and chloroform to methylene chloride. One possible procedure is catalytic hydrodechlorination of these higher chlorinated compounds. This study was planned as a preliminary screening of likely candidate catalysts with the purpose of attaining some insights on performance, deactivation characteristics, and directions for future work. Three commercial catalysts were selected for study: palladium-on-granular carbon, platinum-on0019-7890/78/1217-0236$01.00/0
alumina, and platinum-rhenium-on-alumina. Each of these is an important hydrogenation-dehydrogenation and hydrogenolysis catalyst. The latter two are dual-function catalysts with both hydrogenation-dehydrogenation and acid-catalyst functions, and they are used primarily in catalytic reforming of petroleum naphthas to aromatics. Previous Work Various transition metals have been claimed in the patent literature as catalyts for hydrodehalogenation. These include Ru, Rh, Pd, Os, Ir, Pt, Cu, Ag, and Au (Mullin and Wymore, 1968; Ward, 1974). The metals Cu, Ag, Fe, Co, Ni, Pd, and Pt have been studied individually as deposits on silica in the dehalogenation of haloethanes and halopropanes. A pulse reactor was used under conditions in which the metals were halogenated by the haloalkanes (Mochida et al., 1972). Deactivation was very rapid. Weiss and Krieger (1966) conducted a detailed study of the chemistry and mechanism of vapor phase hydrodechlorination of dichloroethylenes using a 0.5 % Pt-onalumina reforming catalyst. Above 370 "C extensive pore diffusion and bulk mass-transfer resistances were encountered, and subsequent studies on carbon tetrachloride utilized an alumina support upon which Pt had been
0 1978 American Chemical
Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 3, 1978 VENT
237
t
A I R BATH - I
FLUIDIZING A I R
Figure 1. Microreactor system flow diagram.
deposited only on the peripheral region. Weiss et al. (1971) used this catalyst to develop a free-radical mechanism for hydrodechlorination of carbon tetrachloride. Chloroform and methane were the main products, but the ratio between these products varied with catalyst preparation. Differences in the preparation were not detailed. A study by Gambhir and Weiss (1972) involved dichloroethylenes and hydrogen on Pt-alumina and demonstrated the importance of surface diffusion in transporting reactants and products to and from the highly dispersed Pt sites. The major portion of these reactants and products were apparently physically adsorbed on the alumina surface. None of the studies attempted to measure catalyst life or define the deactivation characteristics of the catalysts. Experimental Plan and Purposes The purpose of this study was to compare three commercial catalysts on the basis of activity, selectivity, and catalyst life or stability for the hydrodechlorination of chloroform. A rapid screening test was utilized involving a differential reactor. Constant mass velocitv was used for most of the tests in order to minimize the effects, if any, of bulk mass transfer. Initial temperature screening was followed by studies on the effect of hydrogen partial pressure. Life studies were made and deactivated catalysts studied using sensitive surface techniques. Regeneration of deactivated catalyst was also investigated. Experimental Equipment and Techniques Reactor System. The hydrodechlorination of chloroform was studied using the automatic precision precision microreactor which has been described in detail by Harrison et al. (1965). With reference to Figure 1, liquid chloroform was pumped through the mixing tee, where hydrogen, which had been metered by a Hastings-Raydist Model LF-300 mass-flow transducer, was added. The meter was calibrated using the moving soap-film technique. The microreactor used was a standard Swagelok fitting as shown in Figure 2. Catalyst volume was 0.95 cm3. Chromatographic Analysis. The product from the reactor was sampled through a Biotron sampling valve to
R E F R A S I L
TALYST
0 25"
1-05.4
316
S
x
CLOTH
BED I 25"
STEEL
Figure 2. Microreactor fitting.
a Perkin-Elmer Model 154A vapor phase chromatograph equipped with a thermal conductivity detector. The column consisted of a 6-ft X l/g in. 0.d. tubing packed with Porapak Q. Operating conditions were 144 "C and 9.5 p i g with a helium flow of 35 cm3/min. Total elution time was approximately 9 min, and samples were taken every 30 min. A Disc Instruments, Inc., Model 201 mechanical chart integrator was used in conjuction with the recorder. The chromatographic column was calibrated before each run using known samples. Catalyst Characterization. In order to study changes in catalyst characteristics, samples of both fresh and used catalyst were subjected to various measurements employing the following apparatus: surface area, Aminco Sor-BET, Model 5-7300; differential thermal analysis, Tracor-Stone Model 202 with chromel-alumel thermocouple; scanning electron microscope, JEOL Electronics Model JSM-2 with a Canabera dispersive X-ray detector; ESCA and SIMS, Physical Electronics Model 548; X-ray diffractometer, Phillips Electronics Type 12215/0 with
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Table I. ProDerties of Catalvsts Tested Pd/carbon
Pt/alumina
nominal composition
0.5 wt % on granular carbon
0.292 wt % Pt on alumina
size
4-10 U.S. mesh
total surface area, m2/g bulk density, glcm' pore volume, cm3/g
PtRe/alumina
1200
0.3 wt % Pt and 0.3 wt % Re on alumina extrudates: 0.18 cm X 0.54 cm av length 200 200
0.45
0.68
0.64
0.6
0.63
__
A.M.R. goniometer having a graphite monochromator and photomultiplier detector. Catalysts Tested. Properties of the catalysts tested are summarized in Table I. The Pd-on-carbon (E-141), a standard commercial hydrogenation catalyst, was supplied by the Calsicat Division of Mallinkroft, Inc., Erie, Pa. The Pt/alumina (Aeroform PHF) and the Pt-Re (Aeroform PR-6), which are standard naphtha-reforming catalysts, were supplied by American Cyanamid Co. Reactants. Technical grade chloroform was used in order to simulate a typical feedstock. Impurities were 50 ppm of 2-methyl-2-butene, 50-90 ppm of vinylidene chloride, 100-150 ppm of 1,l-dichloromethane, 300 ppm of bromochloromethane, 250 ppm of carbon tetrachloride, 200 ppm of methylene chloride, and 1-2 ppm of hydrogen chloride. Chromatographic grade hydrogen, 99.998+ % pure, was used both for catalyst pretreatment and for the reaction studies. Reaction Procedure. In each case approximately 0.5 g of catalyst was loaded into the microreactor and the catalyst was tamped down to reduce channeling effects. Quartz cloth was placed on the shoulders of the reactor fitting above and below the catalyst bed as a means of distributing flow and preventing movement of the bed. The filled reactor was attached to the associated tubing and the bayonet placed in the sand bath and connected to the system. Heating of the bed to reaction temperature was then accomplished in a 2-h period under a nitrogen purge. After heating, hydrogen was passed over the sample for 2 h as a standard activation procedure. Other times were also studied. After activation, chloroform at 8 liquid cm3/h was admitted along with the hydrogen and the run was begun. Periodic blank runs without catalyst were made to check thermal reactions and possible activation of the reactor walls. Such activation did not occur. Run time, which was limited by the holding capacity of the pump, was normally 6 h. Experimental Results Initial screening for each catalyst involved an ascending temperature scan using a 3:l ratio of hydrogen-tochloroform for the purpose of determining the apparent optimum temperature range. Then activation and life tests and studies of other variables, where warranted, followed. In all cases catalyst pretreatment was carried out at the planned reactor operating temperature. Noncatalytic tests were made, and a small amount of methyl chloride was detected. These results without catalyst and the constancy of the small amount of methyl chloride formed with each catalyst indicate that methyl chloride is primarily the product of a thermal reaction. A general comparison of the three catalysts is given in Table 11. Details for each catalyst follow.
Table 11. Comparison of Catalyst Performance Pd/carbon Pt/Al,O, Pt-Re/Al,O, selectivity to CH,Cl, (rate) (1000)@ 188 'C, 1 atm, and 3 : l H,CHCl,, mol of CH,Cl, formed/ (h)(g of cat.) activity characteristics
-75% 5.5
-85-90% 11
unstable
stable
=loo% (22.5") 2.5 @ 160 "C
stable
@ 188 "C @ 188 "C
@ 160 "C
a Extrapolated. Actually at 188 "C excessive coking occurred.
60
180
120 REXTION TIME
24
(MINUTES)
Figure 3. Pd-charcoal activation times. Activation and operation at 188 "C, 1 atm. and 3:l H2/CHC13.
Palladium-on-Carbon. A 3:l hydrogen-to-chloroform molar feed ratio and an activation time for the catalyst of 30 min in hydrogen were used for the temperature scan between 100 and 215 "C a t 1 atm system pressure. Methylene chloride was first detected at 187 "C, but the conversion declined over relatively short run times (60 min). Hydrogen pretreatment of the catalyst for 2-3 h improved the activity and activity maintenance, but activity declined rapidly after only 200 min of use (Figure 3). Surface area measurements indicated a sharp decline in charcoal area for the used catalyst (1100 fresh to 230 m2/g for used), and electron microscope scans of the deactivated catalyst revealed deposits of chlorides that were not present on the fresh catalyst. This observation suggests a possible polymeric deposit of chlorohydrocarbon which seems to be further substantiated by the observed decline in surface area. Debye-Scherrer X-ray diffraction patterns of fresh and deactivated catalysts revealed no detectable Pd crystallite growth, but the small amount of Pd present in the catalyst made such measurements difficult. The results of P d aggregation, however, were observed by scanning electron microscope studies. Regions of larger Pd aggregates were observed on the deactivated catalyst which were not present on the fresh catalyst. The rapid decline in activity of P d catalyst in hydrodechlorination would appear to be due to both a polymeric coke deposit of high chloride content and some sintering of the active palladium. The loss in area of the original granular carbon support suggests a partial plugging of the pore structure by a deposit. Platinum-on-Alumina. All runs were made a t 1atm system pressure using a volumetric space velocity of 9 h-' and a constant vapor velocity. Pretreatment for 2 h in
Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 3, 1978 239 15 14
A 3 2 . I
L .
L
60
.
1
I20
.
I
180
.
I
.
240
REACTION TIME
I
*
300
I
RUN 33 6 I
R U N 53
2
1
4
RUN 3 4 6 I
7
RUN 5 6
2
I
RUN 35 4 51
I
360
RUN 31 3 1 RUN 3 2 3 I
*
420
4
RUN36 =RUN
6
180
240
REACTION TIME
.
1
I20
300
360
,
,
180
.
I
I
.
39
4 5 I 4 5 I
B I
RUN40
6 1
1 R U h 41
5 1
4 2 0
480
.
300
240
REACTION TIME
A RUN 37
120
60
(MHUTSS
Figure 4. Reaction rates over Ptialumina at 188 "C and 1 atm.
60
1
.
480
I
,
360
I
.
420
I
.
480
(MINUTES)
Figure 6. Reaction rates over Pt-Re/alumina at 144 "C and 1 atm.
t
A
48 3 I RUN49 3 I 2 I
8 RUN 52
'F
6
7
80
120
(MINUTES)
180
240
300
REACTION T I M E
360
RUN 5 4 RUN55
420
2 I 3 I
400
(MNUTES)
Figure 5. Reaction rates over Pt/alumina at 208 "C and 1 atm.
Figure 7. Reaction rates over Pt-Re/alumina at 166 "C and 1 atm.
hydrogen at the temperature and flow rate required for the reaction test followed. Initially a temperature scan between 140 and 290 "C was made using a 3:l hydrogen-to-chloroform molar feed ratio. Conversion to methylene chloride was first detected at 177 "C. Conversion continued to increase up to 233 "C and dropped rapidly at higher temperatures. Since the fresh catalyst was white, it was easy to see that a black deposit (coke) occurred on the catalyst in high-temperature runs. In fact, above 208 "C coking seemed to occur readily. In order to study reaction characteristics without the intrusion of coking and rapid deactivation, subsequent runs were made a t 208 and 188 "C. Rate plots are presented for various ratios of hydrogen-to-chloroform in Figures 4 and 5. A t a ratio of 6:l it appeared that good activity maintenance was possible. Catalyst particles removed from the reactor after runs at these conditions were only slightly gray. The rate data were fitted to a rate form which was first order in chloroform partial pressure and zero order in hydrogen partial pressure. The energy of activation was found to be 5.2 kcal/g-mol. This low value strongly suggests large intrapellet diffusional resistances. To check this hypothesis a smaller size catalyst sample was tested which was prepared by breaking as-received extrudates into smaller pieces using a pressing action so as to fracture the catalyst and avoid dust producing attrition. A 40 to 60 mesh sample of this broken catalyst was tested and the observed rate, 0.0108 mol/h g of cat., was compared with
Table 111. Pt-Alumina Bulk Compositions elemental concentration" catalyst
A1
c1
Pt
fresh partially deactivated severely deactivated
1.00 0.77 0.79
1.00 1.81 5.07
1.00 1.15 0.92
Elemental concentrations are referenced to unit values in the fresh catalyst.
that of unbroken catalyst, 0.007 mol/h g of cat. The significant difference in rate between the two sizes at 188 "C and a 6:l hydrogen-to-chloroform ratio suggests an effectiveness factor of the unbroken catalyst of 0.6 or less and a true activation energy of 1 2 kcal/g-mol. The apparent low effectiveness factor was substantiated by visual inspection of used catalyst which had been operated under coking conditions. When broken to reveal the cylindrical cross section, a black outer ring 0.3 mm thick and a central white core of 1.0 mm in diameter were observed, indicating that the core region was ineffective. Platinum-Rhenium/Alumina.A temperature scan between 144 and 277 "C revealed measurable rates beginning at 144 "C. Above 205 "C the conversion declined markedly and above 188 "C activity declined very rapidly. The effects of hydrogen partial pressure were studied at 144 and 166 "C where catalyst life was good (Figures 6 and 7). High hydrogen partial pressures (6:l) reduced catalyst life a t 166 "C, but a ratio of 2:l gave good sustained life.
240
Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 3, 1978
Table IV. Pt-Alumina Surface Compositions ~~
~~~~~~
~
~~
atomic orbital concentration" 1.00
fresh
1.00
1.00
partially 0.68 3.06 1.99 deactivated 3.04 deactivated 0.43 3.26 " Concentrations are referenced to unit values in the fresh cat.alyst.
IL
a
L
e
L
A0
& ' & ' & ;o0 I
l
l
REACTION TME
'
600
700
800
903
1000
(MINUTES)
Figure 8. Regeneration of Pt/alumina after operation at 224 " C , 1 atm, and 3:l H2/CHC13. A 1:l ratio produced very low rates indicating a possible mechanism involving two moles of hydrogen. As with platinum-on-alumina, the rate was first order in chloroform and zero order in hydrogen. The apparent activation energy was found to be 32 kcal/g-mol. Regeneration. Since the Pt/alumina catalyst produces higher rates at higher operating temperatures, its possible use seems favored provided regeneration of the catalyst is possible after prolonged operation. To answer this question quickly and tentatively a run was made at 224 "C and 3:l hydrogen-to-chloroform molar feed ratio so that rapid deactivation would occur. As conversion declined the chloroform feed was stopped and hydrogen flow continued for 1 h at reaction temperature. Then chloroform feed was reintroduced. As shown in Figure 8, the hydrogen treatment did not restore the activity. The reactor was then purged with nitrogen and heated to 455 "C at which point pure oxygen was introduced for 1 h. After cooling to the original reaction temperature, the feed was reintroduced and, as shown in Figure 8, a substantial portion of the initial activity was restored. The coke deposit apparently had been removed. Catalyst obtained under the same operating conditions was black when not regenerated and largely white when regenerated. Differential thermal analysis under oxidizing conditions of a deactivated catalyst revealed a strong exotherm in the region of 450 to 480 "C which further confirmed the existence of a coke deposit and the temperature region required for its removal by burning. N a t u r e of Coke Deposit. Samples of fresh, partially deactivated, and deactivated catalysts were examined by scanning electron microscopy, electron spectroscopy for chemical analysis (ESCA), and secondary ion mass spectrometry (SIMS) as means for detecting possible differences which might better define the nature of the coke deposit and the deactivating mechanism. By using the dispersive X-ray detector in conjunction with the
1.00 0.75
1.00 0.79
0.72
1.00
1.12
0.48
0.41
scanning electron microscope, it was possible to obtain relative amounts of Al, C1, and Pt at various positions on the sample. Typical bulk values are summarized in Table 111. Similarly relative values of surface elements and hydrocarbon fragments found by ESCA observations are summarized in Table IV. It would appear that there has been some minor agglomeration of Pt as shown by the increase in observed Pt at various regions encountered during the scanning by the electron microscope. Both these observations and the surface observations afforded by ESCA show an increase in chloride and carbon as deactivation proceeds, and the SIMS data confirm this increase in chloride along with the appearance of hydrocarbon fragments removed by the bombardment along with chloride. Since platinum agglomeration is not marked and since much of the activity was restored by regeneration, it must be concluded that the coke deposits are responsible for the observed deactivation. Based on the qualitative analytical results from ESCA, X-ray scanning, and SIMS, it appears that this coke is a chlorinated polymeric hydrocarbon. Conclusions Based on these preliminary screening tests Pt/A203 reforming-type catalyst appears to be the most promising candidate for a practical hydrodechlorination process. It exhibits a reasonable rate at atmospheric pressure and would certainly show improved rates and life a t higher total pressure as indicated by the apparent kinetics and the observed inhibition of coking by increased hydrogen partial pressure. It is also readily regenerable. Platinum-rhenium/A120~catalyst had good life characteristics at lower temperature where the rate was also less. This lower rate could be overcome by higher operating pressures. The improved selectivity and life and the higher tolerance for coke which has been reported for Pt-Re catalyst in reforming should encourage additional work on hydrodechlorination. In fact, both Pt/A1203 and Pt-Re/A1203 deserve further study with special reference to optimizing catalyst pore structure, catalyst pretreatments, and operating pressure. Acknowledgment We are grateful to Mr. J. W. Rogers, Jr., for the ESCA and SIMS studies and to the National Science Foundation (Grant CHE 76-05172) for partial support of this instrumentation. Literature Cited Chem. Eng. News, 55, 6 (May 9, 1977). Gambhir, B. S.,Weiss, A. H., J . Catal., 26, 82 (1972). Harrison, D. P., Hall, J. W. Rase, H. F., Ind. Eng. Chem., 57, 18 (1965). Mochda, I., Yamamoto, H., Kato, A,, Seiyama, T., Bull. Chern. SOC.Jpn., 45,
2319 (1972). Mullin, C. R., Wymore, C. E. (to Dow Chem. Co.), US. Patent 3 579 596 (March
29, 1968). Ward, J. A (to Dow Chem. Co.), US. Patent 3927 131 (June IO, 1974). Weiss, A. H., Gambhir, B. S.,Leon, R. E., J . Catal., 22 245 (1971). Weiss. A. H.. Krieger, K. A,, J . Catal., 6, 167 (1966).
Received for review May 4 , 1978 Accepted May 30, 1978
