Claims
- 1. A continuous process for preparing 1,1,1-trifluoro-2,2-dichloroethane which comprises reacting perchloroethylene with HF in a gas phase in the presence of catalysts comprising Cr.sub.2 O.sub.3 carried on AlF.sub.3, without Al.sub.2 O.sub.3, in the gamma and/or beta form.
- 2. The process according to claim 1, wherein the Cr.sub.2 O.sub.3 amount ranges from 1 to 15% by weight calculated as chrome on the catalyst.
- 3. The process according to claim 1, wherein the surface area of AlF.sub.3 ranges from 15 to 30 m.sup.2 /g.
- 4. The process according to claim 1, wherein AlF.sub.3 is in the form of powders having particle size ranging from 20 to 200 microns.
- 5. The process according to claim 1, wherein the AlF.sub.3 carried can contain also the delta phase up to amounts of 30% by weight.
- 6. Process according to claim 1 wherein said catalyst is prepared by the steps of:
- (a) impregnating AlF.sub.3 with a solution of a soluble chrome salt;
- (b) drying the catalyst;
- (c) activating the catalyst with air or nitrogen, at temperatures ranging from 200.degree. C. to 600.degree. C.
- 7. Process according to claim 6, wherein the activation step is conducted in the presence of steam.
- 8. The process according to claim 6, wherein the activation temperature ranges from 330.degree. to 500.degree. C.
- 9. The process according to claim 1, wherein the reaction temperature ranges from 300.degree. C. to 400.degree. C.
- 10. The process according to claim 1, wherein the contact time between reagents and catalytic mass ranges from 1 second to 30 seconds.
- 11. The process according to claim 1, wherein the molar ratio between reagents HF and C.sub.2 Cl.sub.4 is of at least 3:1.
Priority Claims (1)
| Number |
Date |
Country |
Kind |
| 19622 A/87 |
Mar 1987 |
ITX |
|
DETAILED DESCRIPTION OF THE INVENTION
This application is a continuation of application Ser. No. 163,659, filed Mar. 3, 1988, now abandoned.
The process described in U.S. Pat. No. 3,755,477 leads to yields of the mentioned product equal to about 20% and it cannot be utilized on a commercial scale because of the too high amount of by-products.
An alternative process (U.S. Pat. No. 4,145,368 and U.S. Pat. No. 4,192,822) for preparing the compound of the invention comprises reacting 1,1,1-trifluoro-2-chloroethane with chlorine. The yields of the compound of the invention are very low. However, to increase the useful product yields, it is possible, according to the above-cited patents, to react the obtained reaction mixture with further starting product 1,1,1-trifluoro-2-chloroethane in the presence of catalysts such as Cr.sub.2 O.sub.3, oxyfluorides or activated carbon.
Also when using this method, the yields of useful product 1,1,1-trifluoro-2,2-dichloroethane are of the order of 14%.
This process is not utilizable on an industrial scale as it requires two reaction steps plus a third step, which is necessary to prepare the starting product. Furthermore, the contact time of the reagents used in the step where the catalyst based on Cr.sub.2 O.sub.3 or the ones cited above is of the order of 90 seconds. These times are too long for an industrial process in the gas phase, because it requires too large reactors.
Processes for the liquid-phase-hydrofluorination of halogenated olefins for preparing chlorofluorohydrocarbons are well known in the art, such processes using, as catalysts, for example TaF.sub.5, BF.sub.3, SbCl.sub.3 F.sub.2.
When perchloroethylene is utilized as a starting product, it is not possible to obtain the product of the invention.
This product is obtainable with very low yields, of about 10%, by previously reacting perchloroethylene with HF and by subsequent hydrofluorination in the presence of TaF.sub.5. See, for example, Journal Fluorine Chemistry, 13 (1979), 7-18, "Chemistry of Hydrogen Fluoride v. Catalysts for Reaction of HF with Halogenated Olefins", A. E. Feiring.
In the art there are known many modifications of the various types of fluorination catalysts which are utilized for preparing perhalo-compounds of chlorofluorohydrocarbons.
There are known, in fact, fluorination catalysts based on chrome oxides, oxyfluorides, fluorides, or copper, iron, nickel, manganese, cobalt fluorides, etc. carried on Al.sub.2 O.sub.3, AlF.sub.3, activated carbon, CaF.sub.2.
However it is well known that carriers different from the fluorinated ones, in the presence of HF tend to give the corresponding fluorinated carriers. Therefore, in practice, the reaction which leads to fluorocarbons is carried out in the presence of a fluorinated carrier.
However, the product of the invention is not exemplified in any patent, except the ones mentioned above.
U.S. Pat. No. 3,258,500 describes various fluorination catalysts, among which is Cr.sub.2 O.sub.3. The 1,1,1-trifluoro-2,2-dichloroethane yields are of about 16% if it is operated at about 300.degree. C. and they sink to about 3.5% if it is operated at 400.degree. C. In said patent, column 7, lines 30-52, it is specified that it is essential, when using these catalysts, to operate at exactly defined temperatures to obtain good conversions to the desired useful product. As mentioned above, the best mode of this patent for the product 1,1,1-trifluoro-2,2-dichloroethane indicates 16% as the best result.
Furthermore, this patent indicates, as a possible alternative, the possibility of supporting chrome oxide on various inert supports; for example Al.sub.2 O.sub.3 (column 6, lines 23-54). Nevertheless the results so obtained are of little interest as the yields decrease.
The process according to said patent is in any case of low industrial value because the by-product amount is too high.
It has surprisingly been found that it is possible to prepare 1,1,1-trifluoro-2,2-dichloroethane by a process which is readily practicable on an industrial scale, if perchloroethylene is used as a starting product.
Thus, the object of the present invention is a process for preparing 1,1,1-trifluoro-2,2-dichloroethane, which comprises reacting perchloroethylene with HF in the gas phase in the presence of catalysts which comprise Cr.sub.2 O.sub.3 supported on AlF.sub.3, in the gamma and/or beta form.
Generally, the amount of Cr.sub.2 O.sub.3 ranges from 1 to 15% by weight, calculated as Cr on the catalyst. The percentage of Cr.sub.2 O.sub.3 is a function of the surface area of AlF.sub.3 in the gamma form.
Carriers having a high surface area, of the order of 25-30 m.sup.2 /g, are generally preferred. In this case it is possible to use Cr.sub.2 O.sub.3 amounts also lower than 5%.
If the surface area is lower, higher Cr.sub.2 O.sub.3 amounts are generally utilized.
The carrier can be in the form of powders having particle size generally ranging from 20 to 200 micron. If necessary, it can be also in the form of pellets.
AlF.sub.3 may contain, besides the gamma and/or beta form, also the delta form, generally also in percentages up to 30% by weight.
AlF.sub.3 in the alpha form can be present too, although it should be preferably limited because it was observed that this form is little active.
The catalyst of the invention can be prepared in various manners, one of the preferred methods being the following: the AlF.sub.3 carrier in the crystallographic forms is wet or dry impregnated, according to a technique known in the art, with a solution of a soluble trivalent chromium salt, for example CrCl.sub.3.6H.sub.2 O.
The catalyst is then dried to remove the water contained therein. Then it is charged into a reactor and subjected to an activation treatment with air or nitrogen either or not in the presence of steam.
The activation treatment is generally carried out at temperatures from 200.degree. to 600.degree. C., preferably from 350.degree. C. to 500.degree. C., to convert chrome to the oxide form.
The above-mentioned allotropic structures of AlF.sub.3 are known and are characterized by the X-ray diffraction pattern, as is reported in J.C.P.D.S. 1981 and in French patent No. 1,383,927 to Du Pont.
The above-considered gamma.sub.c, delta.sub.c and beta.sub.c phases are the ones described in French patent No. 1,383,927 by J. Christoph and J. Teufer. The alpha phase is described in Anal. Chem. 29, 984 (1957).
After a prolonged use, the catalyst activity can be restored by a treatment with air at high temperatures (from 350.degree. to 500.degree. C.), however, these regenerations are to be conducted cautiously as they can impair the catalyst life. It was observed, for example, that the heat treatments can promote the irreversible conversion of the aluminium trifluoride active phases to the alpha-AlF.sub.3 inactive phase.
The molar ratio between reagents HF and C.sub.2 Cl.sub.4 is at least of 3:1, in particular of 5:1. It was ascertained that lower molar ratios lead to low conversions of C.sub.2 Cl.sub.4 and, in any case, shorten the catalyst life as they favour the deposition of carbon and organic pitches onto the catalyst surface, thereby reducing the catalytic activity.
The reaction temperature ranges from 250.degree. to 500.degree. C. and preferably from 300.degree. to 400.degree. C.
The contact time between reagents and catalyst mass ranges from 1 second to 30 seconds, preferably from 3 to 15 seconds.
The 1,1,1-trifluoro-2,2-dichloroethane yields obtained through the process of the invention are of the order of 15-20% by moles if it is operated under the above-mentioned conditions.
However, contrary to expectation, in the process of the invention the other reaction products consist, besides for the most part of the unreacted starting product, of partially fluorinated intermediates, while only about 20% by moles consists of highly fluorinated low-boiling products.
By simple stripping it is possible to separate the unreacted perchloroethylene and the partially fluorinated intermediates and to recycle them again to obtain the product of the invention. The feeding mixture in the recycle is added with fresh perchloroethylene.
In this manner, the yields of 1,1,1-trifluoro-2,2-dichloroethane product can reach 90% by moles.
This is a very important advantage of the process of the present invention with respect to the above-cited known processes for preparing 1,1,1-trifluoro-2,2-dichloroethane. As in the above-illustrated patents, the reaction by-products were so highly fluorinated that they could not be recycled.
For the purpose of a better comprehension of the present invention it is therefore useful to consider the net yield of 1,1,1-trifluoro-2,2-dichloroethane, intermediates and by-products.
As regards this invention, the term "net yield" means the following expression: ##EQU1##
In the present invention, the net yield of by-products is very low, generally it is of 10%.
The utilization of the compound forming the object of the invention are well known in the art. In particular it is utilized as a fluid for aerosol or as a propellant.
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Christoph et al. |
Apr 1965 |
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Date |
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DEX |
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GBX |
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GBX |
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GBX |
Continuations (1)
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Number |
Date |
Country |
| Parent |
163659 |
Mar 1988 |
|
Patent Grant
4967023
