EnginnTring
Thermal Dehydrohalogenation of
pocess deveIopment I C. W. HUSKINS'
AND PAUL TARRANT UNIVERSITY OF FLORIDA, GAINESVILLE, FLA.
J. F. BRUESCH AND J. J. PADBURY AMERICAN CYANAMID CO., STAMFORD, CONN. f
In an effort to develop a more satisfactory method for the preparation of certain haloethylenes, the thermal dehydrohalogenation of some chlorofluoroethanes was studied. The effect of such factors as temperature, contact time, use of diluents, and material of construction of the reaction tube on the vapor phase thermal dehydrohalogenation of six chlorofluoroethanes has been investigated. Special attention was devoted to the preparation of unsym.dichlorodifluoroethylene,for which conversions up to 78qo were obtained at 600' C. Other olefins prepared by thermal dehydrochlorination of the appropriately substituted
ethanes included 2-chloro-l,l-difluoroethylene, trichlorofluoroethylene, 1,2-dichloro-l-fluoroethylene,and chlorotrifluoroethylene. There seems to be a certain temperature at which the maximum conversion to olefin is obtained for each saturated compound at a particular input rate; higher temperatures caused degradation, and at low temperatures less dehydrohalogenation resulted. Lower conversions were obtained when hydrogen chloride was removed from ethanes in which the hydrogenwas bound to a carbon atom holding fluorine. Hydrogen-containing olefins were produced in lower yields.
C
a
ONSIDERABLE interest has been shown in the fluorohydrogen chloride from certain fluorochloroethanes-for example, and fluorochloroethylenes because of their relative revinyl fluoride was prepared by heating 1-chloro-1-fluoroethane. activity. For example, they undergo addition reactions with a Again details on apparatus, conditions, and yields were lacking. variety of organic compounds t o give widely diversified products: A British patent ( 7 ) gives brief details of the preparation of 1,lAlcohols add across the carbon-to-carbon double bond of tbe difluoroethylene and tetrafluoroethylene by thermal dehydrofluoroethylene t o give ethers (8, 10, l a ) , amines add in the preshalogenation of the appropriate chlorofluoroethanes. I n 1949, ence of water t o give amides ( l a ) , while certain fluoro-olefins patents (1, 8)were issued on a process for preparing fluoroethyldimerize t o give cyclobutane derivatives (6, 14) and higher enes b y passing chlorofluorethanes over aluminum fluoride or polymers (9). Reactions such as these make the fluoro-olefins alumina at temperatures around 300' C. particularly important in the preparation of fluorine compounds The present research was initiated to find a convenient method containing functional groups. for the preparation of unsyrn.-dichlorodifluoroethylenefrom 1,2,2The methods generally used (3) for the preparation of fluorotrichloro-1,l-difluoroethane, a compound readily prepared by ethylenes are ( a ) the removal of halogen by zinc from a 1,2the fluorination of pentachloroethane ( 3 ) . When it was found dichloro compound or ( b ) the reaction of alcoholic potassium that a good yield of dichlorodifluoroethylene could be obtained hydroxide on a compound containing hydrogen and a halogen conveniently by the thermal method, the preparation of other on adjacent carbon atoms. These methods are not always satisfluoro-olefins by this reaction was studied, factory; small yields of the desired olefin are sometimes obtained MATERIALS because of side reactions or because of the lengthy syntheses Because the saturated compounds used in this investigation involved. were not available commercially, they were prepared by the I n 1945 Torkington and Thompson (16) described the preparaand fluorination of the corresponding chloro compound with antimony tion of vinylidene fluoride from 1-chloro-1,l-difluoroethane 1-chloro-1-fluoroethylene from 1,l-dichloro-1-fluoroethane by trifluoride. Pentachloroethane was treated with a mixture of antimony trithermal dehydrohalogenation conducted at 750 o C. They stated that l,l-dichloro-2,2-difluoroethylene"was obtained by fluoride and anbimony dichlorotrifluoride at about 120' to 150' C. removal of hydrogen chloride from ~ , l ~ d ~ ~ u o r o ~ l , ~ , ~in~ a~n riron ~ c reaction ~ ~ o r ovessel ~ equi ped with a stirrer and a distillin column. fluorinated distilledacid. as they forme! ethane. B.p. 19" C." It is therefore not definitely known and were The collected underproiucts dilute hwere drochloric The conwhether the compound was made by a thermal reaction or by use versions t o CHClzCFzCland CHCl28FCl2were of the order of 65 of alkali. Because Torkington and Thompson were primarily and 20%, respectively. sum-Tetrachloroethane was fluorinated interested in the infrared spectraof compounds containing the in a similar manner and CHClzCHFz and CHCl&HFCl were obtained in yields of 30 and 15%, respectively; these are somewhat =CF2 group, they did not describe the apparatus used or the lower than those previously reported 0). methods employed in conducting the thermal dehydrohalogenaVinylidene chloride was chlorinated t o unsym-tetrachloroethane, which was fluorinated as-described above. The conversion tions. I n 1945 reports were received from Germany (6) that to CH2ClCFzCl averaged 51% for three runs; some CHzCICFCle chemists there were preparing fluoro-olefins by the removal of was also formed. 1 Present address, Citrus Products Station, U. S. Department of AgriculThe l,l-dichloro-1,2,2-trifluoroethane was prepared from ture, Winter Haven, Fla. CHClzCHFz by the chlorination of the latter compound to CCII-
1253
1254
INDUSTRIAL AND ENGINEERING CHEMISTRY
CHF, ( d ) , which was converted to CFCIZCHF, by treatment with antimony trifluoride. APPARATUS
The apparatus used in the small-scale experiments is shown in Figure 1. Liquid materials were fed from a displacement-type feeder ( 1 1 ) . The rate of fall of the piston was controlled by a small electric motor geared down to a final drive shaft speed of 0.18 r.p.m. A small-diameter Nichrome wire supporting the piston unwound from the drive shaft, thus lowering the piston and displacing liquid from the reservoir into the glass vaporizer. The latter was
Vol. 43, No. 5
action tube (38 mm. in inside diameter). In each case, a gear pump, driven by a variable-speed transmiasion, pumped feed from a graduated glass reservoir, through a rotameter, to the vaporizer which was heated to about 200' C. by an electric furnace. Both the metal and the glass reaction tubes were used in a vertical position. The metal tube \\-as packed with Raschig rings in some runs, while the borosilicate glass tube was packed in all runs. Both tubes were heated by two-section electric furnaces controlled by variable transformers. Thermocouple wells extended axially through the entire heated section (180 em.) of each tube. The apparatus for product collection was similar to that described above, except that a water scrubber for removing hydrogen chloride was used follo.ivingthe dry ice traps. DEHYDROHALOGEYATION OF 1,2,2-TRICHLORO-1,1DIFLUOROETHANE
The procedure used for the larger scale runs was the same for both glass and metal apparatus. Temperature distributions were taken several times during each run a t points 4 inches apart throughout the heated section of the reaction tube. There was a considerable range of temperature for each distribution, with lower temperatures neai the ends of the furnace sectiontx The reaction zone way arbitrarily selected, eliminating points at which the temperature was obviously too low for any reaction to take place. The arithmetical mean of temperatures of the remaining points was then calculated. and it is this value which appears in Table I. At the end of the feed period, the apparatus was s m p t out with nitrogen and the products were weighed. The organic product was washed with water to remove acid, The amount of acid in the scrubber and the product wash water was determined by titration with standard base and calculated as moles of hydrogen chloride. The Tashed product was dried over calcium chloride and fractionated through a 100-cm. column packed with glass helices. Refrigerated acetone was circulated through the total reflux head.
rRuPS & PRY /r.c NCKTONE 5 4 7H5
COLLKCT/ON FLCISK
a
PRY IC€ BHTN
ucrroffr
Figure 1. Thermal Dehydrohalogenation Apparatus
heated by an oil bath to about 200" C. A small flow (0.5 liter per hour) of nitrogen was introduced a t the top of the feeder through a tube drawn down to a capillary tip to prevent vaporized material from diffusing back into the feeder. The seal a t the top of the feeder consisted of a rubber stopper drilled (while frozen by liquid nitrogen) with a very small hole for the piston-supporting wire. I n those runs in which nitrogen was used in larger amounts as a diluent, the gas was metered from a capillary-type flowmeter and the flow was divided, most of it going directlv into the vaporizer. The Vycor reaction tube was 76 em. long and 29.5 mm. in inside diameter and was packed with small Raschig rings. The packed heated section had a volume of 170 ml. The electrically heated furnace for the reaction tube had three separate heating units, each of which was adjusted to approximately the same temperature by means of resistors. The temperature of the furnace was maintained a t the desired point by means of a controlling pyrometer, with the thermocouple located in the Vycor reaction tube a t the center of the heating section nearest the vaporizer. Under these conditions, the pyrometer control was based on the temperature in only one section of the furnace, and it is possible that the temperature varied slightly in each section. The products passed from the furnace into a water-cooled condenser leading into a flask cooled in a mixture of dry ice and acetone. Two dry ice and acetone traps connected in series completed the collection apparatus. I n the larger scale dehydrohalogenations of 1,2,2-trichloro-l,ldifluoroethane, two sets of apparatus were used. The f i s t had a stainless steel vaporizer and reaction tube (1.5 inch I.P.S.) and the second a borosilicate glass vaporizer and re-
The first runs TT ith 1,2,2-trichloro-L,l-difluoroethanewertl made in the larger scale metal apparatus (Table I). Material balances decreased considerably as the temperature of the reaction tube was raised and as the amount of hydrogen chloride increased. The conversions to dichlorodifluoroethylene based on distillations were r~lativelylow ( 5 to 43%). Examination of the apparatus after four runs revealed severe corrosion of both the stainless steel reaction tube and the water-cooled iron condenser. To avoid these difficulties, glass apparatus was used for the subsequent experiments.
TABLE I. DEHYDROHALOGENATIOS O F 1,2,2-TRICHLORO-l,1DIFLCOROETHANE cou-
ver4v. hIasion terial Acid in Based ConierReaction Input Tube CHC12CF2CI PlodBal- Product, on sion to Temp., ucts. anoe, Moles HC1, CCh= RunQ C. Giams Moles Grams % €IC1 7G CF2, W 2 446 308 1 82 300 97 0 04 12 2 29 334 0 28 465 388 86 311 77 590 1 63 68 2.38 403 3 13 58 76 600 5 39 694 914 1532 1470 7 44 82 96 9 04 510 67 P 7 11 1410 1371 86 97 8 32 520 66 2 45 96 2.01 416 400 82 535 a Runs 1 2 , 3, and 4 were carried out in a stainless steel reaction tube a t a n input oi triohlorodifluoroethane of 0.05 mole/min. Runs 5 , 6, and 7 were made in a borosilicate glass reaction tube packed with Raschig ring5 (volume of reaction zone 830 mi.) a t 0.025 mole/min. Reaction tube unpacked, volume of reaction zone S50 ml. C Reaction tube packed with Raschig rings, volume of reaction zone 600 ml . d Products f r o m rung 5 and 6 combined for distillation.
The data for three runs made in the larger borosilicate glass apparatus are contained in the lower part of Table I. These runs were made primarily to obtain a supply of dichlorodifluoroethylene. The material balances were satisfactory. The conversions as determined by distillation of the product were some-
May 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
what lower than those based on the acid found. This difference was for the most part caused b y losses of dichlorodifluoroethylene (boiling point 19 C.) during the predistillation washing and drying operations, as only minor amounts of by-products were obtained from the distillation. I n subsequent experiments with the smaller apparatus, i t was shown t h a t losses resulting from washing and drying of the product decreased conversion figures 6 t o 10%. For this reason, washing of the product was discontinued. O
80-
The reaction was also studied at several temperatures in a series of experiments in which the 1,2,2-trichloro-l,l-difluoroethane was added a t 0.013 mole per minute. With this reduced rate of feed and consequently longer contact time, conversions were somewhat higher at the lower temperatures. Under these conditions the highest conversion, 67%, was obtained at 550" C. A few runs were made (Table 111) in which the vaporized 1,2,2-trichloro-l,l-difluoroethanewas diluted with nitrogen. At a feed rate of 0.022 mole per minute, diluted with a n equal volume of nitrogen, the conversion was 68% a t 650'; with increasing proportions of nitrogen, conversions were reduced slightly.
TABLE111. THERMAL DEHYDROHALOGENATION O F 1,2,2-TRICHLORO-~,~-DIFLUOROETHANE AT 650' C. IN PRESENCE OF KITROGEN
? z
z 1 cn
f
1255
CHCIzCF?Cl, Mole/Min. 0,022 0.020 0,015
5z 40-
8 20-
Nitrogen, Mole/Min. 0.021 0.036 0,027
CHClzCFzCI Added, G. 360 268 244
Conversion t o CClz=CF%,
CClz=CFz Yield, G . 187 128 114
% 68 61 69
PREPARATION OF MISCELLANEOUS CHLOROFLUOROETHY LENES
I
400
I
I 500
t
I
I
600
I
700
I n order t o determine the influence of other variables on the reaction, several series of experiments were made using the smaller reactor (Figure 1). The procedure was essentially that described above, except that the reaction products were fractionated directly without first washing with water to remove hydrogen chloride. Conversions are based on the amount of CC12=CF2 (boiling point 18-20' C.) obtained by fractionation of the crude reaction products in a 100cm. vacuum-jacketed column packed with glass helices and equipped with a refrigerated head. I n the f i s t set of experiments, the CHClzCF&l was added a t a constant rate of 0.03 mole per minute while the temperature of the reaction tube was varied. The results shown in Table I1 and Figure 2 reveal that a t 450' C. little reaction occurred, but a t 550" to 600' C. conversion to CC12=CF2 reached a maximum of 78y0,. Above this temperature range most of the starting material reacted, but conversion to the desired product fell off rapidly. At 700' C. only a very smxll amount of product was collected and no unreacted CHC12CFzCl was recovered. Extensive polymerization and degradation took place.
A less detailed study was made of the preparation of other chlorofluoroethylenes by pyrolysis of the appropriate ethanes. I n all these experiments the smaller apparatus (Figure I) was used, and products were fractionated directly, without washing, to determine conversions. 2-CHLORO-~,~-DIFLUOROETHYLENE. It Was Of interest to prepare 2-chloro-l,l-difluoroethylenefrom two different chlorofluoroethanes in order t o observe the influence of structure on the conversions. CHzClCFzCl
\CHCECHF~ /
CHCl=CF2
+ HCl
I n one of these ethanes the hydrogen atom is removed from the carbon atom containing no fluorine, whereas with the other compound, hydrogen is removed from a carbon holding two fluorine atoms. I n Tables IV and V are summarized the data for dehydrochlorination of 1,2-dichloro-l, 1-difluoroethane and 1,l-dichloro2,2-difluoroethane, respectively. I n these experiments the a
TABLE IV. THERMAL DEHYDROHALOGENATION OF 1,2-D1CHLORO-1, 1-DIFLUOROETHANE AT 0.04 MOLEPER MINUTE
Run No.
Temp., O
c.
CHzClCFzCI Added, Recovered, G. G.
CHCl=CFz Yield,
G.
Conversion
t o CHCl=CFz,
70
TABLE11. THERMAL DEHYUROHALOGENATION OF
R~~ NO.
1,2,2-TRICHLORO-1,1-DIFLUOROETH.4NE CHCIzCFzC1 T : ~ ~ , , ,Input CHClzCF5g CC1z=CF2 Yield, Mole/min. G. G. G.
"z-
ConverCCh= to CFz, %
a
Addition rate 0.018 mole/min.
TABLE V . THERMAL DEHYDROHALOGENATION OF 1,~-DICHLORO2,2-DIFLUOROETHANE AT 0.04 MOLE PER MINCTTE Run No.
a
Residue in still pot. contained some CHC~ZCFLCI. Insufficient amound of product condensed for distillation.
a
Temp.,
c.
CHClzCHFz Added, Recovered, G. G.
'Rate of delivery 0.075 mole/min.
CHCl=CFz Yield, G.
Conversion to CHCl= CFz, %
INDUSTRIAL AND ENGINEERING CHEMISTRY
1256
products were fractionated in a vacuum-jacketed column equipped with refrigerated head, and 2-chloro-1,l-difluoroethylene was collected at -17" C. I n the preparation of 2-chloro-1,ldifluoroethylene, conversions are lower than those obtained for dichlorodifluoroethylene, discussed in the previous section. This is not unexpected, because 2-chloro-l,l-difluoroethylene might be further dehydrohalogenated t o give other materials.
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Vol. 43, No. 5
1,2-DICHLORO-l-FLUOROETHYLENE. Dehydrochlorination of 1,1,2-trichloro-l-fluoroethanegave 1,2-dichloro-l-fluoroethylene. which was separated from the reaction products by distillation, (boiling point 34-36' C.). As shown in Table VI1, conversions were low, and recovery of starting material was poor.
TABLEVII. TIIERMAL DEHYDROHALOGENATION OF 1,1,2-TRICHLORO-~ - F L U ~ R ~ E T H A N E CHzC1-
6
T ~ ~ ~ CHzClCFClz . , Added C. Mole/min. G. 450 0.040 55 550 0.040 43 050 0 068 82
CFClz R ~ ered, G. 31
8 19
CHCI= CFCl
~ Yield, ~ G. 4 9.5 13
Conver-
sion t o ~ CHC1= -
CFCL %
10 29 21
CHLOROTRIFLUOROETHYLENE. This compound was prepared by pyrolysis of I , l-dichloro-1,2,2-trifluoroethane The maximum temperature used in these runs (Table VIII) was only 545 a t which point the conversion t o chlorotrifluoroethylene was 27%. Inasmuch as 60% of the starting material was recovered in this run and only a small amount of higher boiling by-product was obtained, it is probable that much better conversions would have been realized had the experiments been extended to higher temperatures.
8
O,
I 400
I
I
I
I
I
I
700
600
500
TEMPERATURE, OC.
TABLEVIII.
Figure 3. Variations of Percentage Conversion of Dichlorodifluoroethane to Chlorodifluoroethylene with Temperature
I n a comparison of the conversions obtained from the two starting materials, l,l-dichloro-2,2-difluoroethane and 1,2dichloro-1,l-difluoroethane,a t 550" and a feed rate of 0.04 mole per minute, it is evident t h a t hydrogen chloride is more easily removed from the latter. Conversion to CHCI=CFz,
% 17 60
Compound CHCIzCHFz CHzClCFzCl
THERMAL DEHYDROHALOGENATION OF I,1-DI0.0065 MOLE PER &fINUTEa
CHLORO-1,2,2-TRIFLUOROETHANEAT
Recovered Starting Materials, %
53 15
Comparison of conversions to 2-chloro-1,l-difluoroethylene from both starting materials is shown graphically in Figure 3. 1,1,2-TRICHLORO-%FLUOROETHYLENE. This compound was prepared by pyrolysis of 1,1,2,2-tetrachloro-l-fluoroethane at temperatures in the range of 450" t o 650" C. Conversions were determined in the usual manner by distillation and the fraction boiling a t 71-73' C. was collected as product. Table VI shows that the maximum conversion (62%) was obtained a t 650" C., the highest temperature employed in this series of runs. These data also reveal t h a t the conversion to olefin is much less dependent on temperature than was observed in the pyrolysis of the other chlorofluoroethanes. T h a t the reaction is insensitive to variations in contact time is shown in a comparison of runs 4,6, and 7, all made a t 600" C. A fourfold variation in the rate of feed of the starting material made no significant change in the conversion.
Temp.,
c.
470 530 545
Input CFCIzCHFz, Moles
0.53 0.60
1.13
HC1, % of Theory Trace
12 33
Conversion to CFCl=CFs,
%
... 'jib
Reactor made of borosilicate glass combustion tubing packed with glass beads. Volume of reaction zone was 60.5 ml. Reaction product was fractionated in a low-temperature column and CFCI=CFz collected a t -27.5' to -26.5' C. a
*
ACKNOWLEDGMENT
The portion of this research which was done a t the University of Florida was supported by funds from the Office of Naval Research. The authors acknowledge with thanks the help of J. A. Price and Calvin Martin in the preparation of some of the chlorofluoroethancs. LITERATURE CITED
Bratton, F. H., and Wyman, G. M. (to Allied Chemical & Dye Corp.), U. S. Patent 2,478,933 (Aug. 16, 1949). Hanford, W. E., and Rigby, G. W. (to E. I. du Pont de Nemours &Go.), I b i d . , 2,409,274 (Oct. 15, 1946). Henne, ,4.L., in "Organic Reactions," Vol. 11, pp. 49-93, New York, John Wiley & Sons, 1944. Henne, A. L., and Ladd, E. C., J . Am. Chem. Soc., 58,402 (1936). Henne, A. L., and Ruh, R. P., I b i d . , 69,279 (1947). Jonas, "Fluoroethylenes," U. S. Dept. Commerce, Office of Publication Board, Rept. PB 722 (1943). Kinetic Chemicals, Inc., Brit. Patent 581,405 (Oct. 11, 1946). Miller, C. B., and Bratton, F. H. (to Allied Chemical & Dye Corp.), U. S. Patent 2,478,932 (Aug. 16, 1949). Miller, W.T., Jr., Dittman, A. L., Ehrenfeld, R. L., and Prober, M.,IND.ENG.CHEM.,39, 333 (1947). Miller, W. T., Jr., Fager, E. W., and Griswold, P. H., J. Am. Chem. Soc., 70,431 (1948). Oblad, A. G., Marschner, R. R., and Heard, L., Ibid., 62, 2066
.----,
11940).
TABLE VI. THERMAL DEHYDROHALOGENATION OF 1,1,2,8-TETRACHLORO-l-FLUOROETHANE
Run NO.
I n p u t CHC12CFCh Mole/min. G.
TT?
0,036 0.036 0,036 0.036 0,036 0.015 0.058
162 164 177 135 160 114 172
CHClZCFCh Recovered,
G. 68.5 31 36 36 39 38 37.5
CCh= CFCl Yield,
G. 57 63 78 63 80 50 75
Conversion to CC1CFCI, %
44 48 54 58 62 54 54
Park, J. D., Vail, D. K., Lea, K. R., and Lacher, J. R., Ibid., 70,1550 (1948). Rigby, G. W.,and Schroeder, H. E. (to E. I. du Pont de Nemours & Co.), U. S. Patent 2,409,315 (Oct. 15, 1946). Scherer. "Fluorine Derivatives of Propane and Butane." U. S. Dept. Commerce, Office of Publication Board, Rept. PB 776 (1943). Torkington, P., and Thompson, H. W'., Trans. Faraday SOC.,41, 236 (1945). RECEIVED March 9,1950. Presented before the Division of Industrial and Engineering Chemistry, Symposium on Fluorine Chemistry, a t the 116th Meeting of the AMERICAN CHEMICAL SOCIETY, Atlantic City, N. J.
