PROCESS FOR PREPARING 1,1,1-TRIFLUORO-2,2-DICHLOROETHANE BY HYDROFLUORINATION IN THE PRESENCE OF CATALYSTS

Abstract: A process for preparing 1,1,1-trifluoro-2,2-dichloroethane by hydrofluorination, in the gas phase, of perchloroethylene in the presence of a catalyst comprising chromium oxides suppor­ ted on AlF₃ in the gamma and/or beta form.

Description: This invention relates to a process for preparing 1,1,1-trifluoro-2,2-dichloroethane by hydrofluorination, in the gas phase, of perchloroethylene in the presence of a catalyst comprising Cr₂O₃ supported on AlF₃. Well known is the need of having available industrial processes for preparing 1,1,1-trifluoro-2,2-dichloroethane with good yields. It is well known how to prepare 1,1,1-trifluoro-2,2-dichloroethane starting from perchloroethylene in the presence of catalysts based on Cr₂O₃ and precursors thereof. See for example, IT-A-24689 A/78 US-A-3,755,477 and DE-B-1246703. The latter document discloses the fluorination with HF, in the gas phase, of, among others, perchlorethylene in the presence of a catalyst consisting of γ-Al₂O₃ which has been activated by halides of polyvalent metals, such as Cr, then heated in air at 500 to 650°C, and finally partially (70-80%) fluorinated with HF. However, said catalyst is of no use for an industrial scale process, as it becomes inactive within rather short periods of time, owing to its fluorination during the reaction. The process described in US-A-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 (US-A-4,145,368 and US-A-4,192,822) for preparing the compound prepared according to the invention comprises reacting 1,1,1-trifluoro-2-chloroethane with chlorine. The yields of the compound also prepared according to the invention are very low, however, in order 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 Cr₂O₃, 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 Cr₂O₃ or the ones cited above are employed 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 TaF₅, BF₃, SbCl₃F₂. When perchloroethylene is utilized as a starting material, it is not possible to obtain the product prepared according to 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 TaF₅. See, for example, Journal of Fluorine Chemistry 13 (1979), p. 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. In this context US-A-2744148, 3787331 and 3793229 may be referred to. There are known, in fact, fluorination catalysts based on chromium oxides, oxyfluorides, fluorides, or copper, iron, nickel, manganese, cobalt fluorides, etc. carried on Al₂O₃, AlF₃, activated carbon, CaF₂. 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. US-A-3,258,500 describes various fluorination catalysts, among others Cr₂O₃. The 1,1,1-trifluoro-2,2-dichloroethane yields are of about 16% if it is operated at about 300°C and they drop to about 3.5% if it is operated at 400°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 in order 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 yield. Furthermore, this patent indicates, as a possible alternative, the possibility of supporting chromium oxide on various inert supports; for example Al₂O₃ (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 starting material. 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 and in the presence of catalysts which comprise Cr₂O₃ supported on AlF₃, in the gamma and/or beta form, said AlF₃ being in the preformed state on supporting thereon the chromium composed. Generally, the amount of Cr₂O₃ ranges from 1 to 15% by weight, calculated as Cr on the catalyst. The percentage of Cr₂O₃ is a function of the surface area of AlF₃ in the gamma form. Carriers having a high surface area, of the order of 15-30m²/g, and particularly 25-30 m²/g, are generally preferred. In this case it is possible to use Cr₂O₃ amounts also lower than 5%. If the surface area is lower, higher Cr₂O₃ amounts are generally utilized. The carrier can be in the form of powders having a particle size generally ranging from 20 to 200 µm; if necessary, it can be also in the form of pellets. AlF₃ may contain, besides the gamma and/or beta form, also the delta form, generally in percentages of up to 30% by weight. AlF₃ in the alpha form can be present too, although it should be preferably limited since it was observed that this form is little active. The catalyst employed according to the invention can be prepared in various manners, one of the preferred methods being the following: the AlF₃ 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 CrCl₃.6H₂O. 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, optionally in the presence of steam. The activation treatment is generally carried out at temperatures from 200 to 600°C, preferably from 330°C to 500°C and particularly from 350°C to 500°C, in order to convert chromium to the oxide form. The above-mentioned allotropic structures of AlF₃ are known and are characterized by the X-ray diffraction pattern, as is reported in J.C.P.D.S. 1981 and in FR-A-1,383,927 to Du Pont. The above-considered gamma c , delta c and beta c phases are the ones described in FR-A-1,383,927. 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 to 500°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-AlF₃ inactive phase. The molar ratio between reagents HF and C₂Cl₄ is at least of 3:1, in particular of 5:1. It was ascertained that lower molar ratios lead to low conversions of C₂Cl₄ 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 to 500°C and preferably from 300 to 400°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 material, of partially fluorinated intermediates, while only about 20% by moles consist 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 in order to obtain the product prepared according to the invention. To the feeding mixture in the recycle there is added 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 used herein, the term "net yield" is defined by the following expression: In the present invention, the net yield of by-products is very low, generally it is of the order of 10%. The utilization of the compound prepared according to the process of the invention is well known in the art; in particular it is utilized as fluid for aerosol or as propellant. The following examples are given merely to illustrate this invention, without being however a limitation thereof. Example 1 A) 240 g of aluminium trifluoride, consisting of 30% of delta-AlF₃, 60% of gamma-AlF₃ and 10% of beta-AlF₃ and alpha-AlF₃, were impregnated with a solution prepared by dissolving 52.3 g of CrCl₃.6H₂O in 58 ml of distilled water. The aluminium trifluoride was composed of particles having diameters ranging from 20 to 200 µm, the average diameter being 80 µm. The aluminium trifluoride was impregnated by dropping of the Cr₂Cl₃.6H₂O under stirring; subsequently the catalyst was partially dried in an oven at 110°C for 1.5 hours, whereafter it was placed into a tubular reactor made of Inconel 600, having an inside diameter of 5 cm, a height of 80 cm and being equipped on the bottom with a porous septum made of sinterized Inconel for uniformly distributing the gases entering from the bottom and for supporting the inactive catalytic bed. The thermocouples for the temperature measurements were placed in a sheath arranged in the middle of the reactor. The catalyst placed in the above reactor was heated up to 200°C in a nitrogen flow of 100 l/h; the N₂ flow was stopped and 100 l/h of air and 80 l/h of HF were introduced into the reactor after having been mixed with each other in a preheater arranged upstream of the reactor. Heating was resumed in an air and HF stream until a temperature of 400°C was reached; this temperature was maintained for 2 hours, whereafter the catalyst was cooled in a N₂ flow. B) Using the catalyst so prepared and the same apparatus, 0.855 moles/h of C₂Cl₄ and 3.542 moles/h of HF were introduced into the reactor. The reaction temperature was 360°C and the pressure was slightly higher than the atmospheric pressure. The contact time was 5 seconds, calculated as the ratio between non-fluidized catalyst volume and volumetric flow of the reagents fed at the reaction temperature and pressure. The gases leaving the reactor were collected for 1 hour; after absorption of HCl and HF in water and after washing of the reaction product with an aqueous solution of NaOH, 141 g of product were recovered, the molar composition of which being as follows: CF₃CHClF 1.2% CF₃-CHCl₂ 5.1% CCl₂=CClF + CClF₂-CHCl₂ 5.0% C₂Cl₄ 88.0% The conversion of C₂Cl₄ was 12%, while the net yield of CF₃-CHCl₂ was 42.9% and the net yield of by-products was 10%. Example 1A (comparative test) Into the apparatus of example 1 there were charged 345 g of the aluminium trifluoride used for preparing the catalyst of example 1. At a reaction temperature of 360°C and at a pressure slightly higher than the atmospheric pressure, 0.886 moles/h of C₂Cl₄ and 3.44 moles/h of HF were fed. After a process analogous to the one described in example 1, 147 g of a reaction product having the following molar composition were recovered: CF₃-CHCl₂ traces CCl₂=CClF + CClF₂-CHCl₂ 1.1% C₂Cl₄ 98.5% Therefore, aluminium trifluoride as such is not suitable for the production of CF₃-CHCl₂. Example 2 A) 340 g of an aluminium trifluoride essentially consisting of gamma-AlF₃ having a surface area of 28 m²/g was impregnated with a solution obtained by dissolving 52.3 g of CrCl₃.6H₂O in 58 ml of distilled water. The impregnation and the drying process were the same as in example 1; the dried catalyst was charged into the reactor of example 1 and was heated up to 450°C with air at a flow rate of 100 l/h; it was maintained at this temperature until only traces of chloride ions were present in the washing solution of the air leaving the reactor. Subsequently it was cooled with nitrogen. The Cr amount referred to the catalyst was 3% by weight. B) The catalyst of example 2A (345 g) was charged into the apparatus of example 1. At a reaction temperature of 360°C and at a pressure slightly higher than the atmospheric pressure, 0.925 moles/h of C₂Cl₄ and 3.755 moles/h of HF were fed; within one hour there were collected 148 g of a reaction product, the molar composition of which was as follows: CF₃-CHClF 9% CF₃-CHCl₂ 15.8 % CCl₂=CClF+CClF₂-CHCl₂ 5.7% C₂Cl₄ 64.0% The C₂Cl₄ conversion was 36%. The CF₃CHCl₂ net yield was 44%, the by-products net yield was 25%. Example 3 Employed were the catalyst of example 2, the apparatus of example 1, a reaction temperature of 340°C and a pressure slightly higher than the atmospheric pressure. 0.949 moles/h of C₂Cl₄ and 9.725 moles/h of HF were fed. The reaction product was withdrawn for one hour, thereby collecting 153 g of a reaction product having the following molar composition: CF₃-CHClF 4.8 % CF₃-CHCl₂ 14.8% CCl₂-CClF + CClF₂-CHCl₂ 7.4% C₂Cl₄ 71.0% The C₂Cl₄ conversion was 29%, the CF₃CHCl₂ net yield was 51% and the by-products net yield was 16%. Example 4 A) 340 g of an aluminium trifluoride containing 80% of gamma-AlF₃ having a surface area of 18 m²/g was impregnated with a solution obtained by dissolving 52.3 g of CrCl₃.6H₂O in 58 ml of distilled water. The impregnation and drying process was the same as in example 1. After drying, the catalyst was impregnated a second time with the same amount of chromic solution having the same concentration, until a chromium content of 6% by weight was obtained. The catalyst was then dried again, was charged into the reactor of example 1 and was treated with a nitrogen flow (100 l/h) saturated with steam at 40°/50°C. The treatment was carried out at 360°C until only traces of chloride ions were present in the nitrogen washing solution leaving the reactor. Subsequently it was dried with dry nitrogen. B) The catalyst of example 4A (345 g) was placed into the apparatus of example 1. At a reaction temperature of 300°C and at a pressure slightly higher than the atmospheric pressure there were fed 0.801 moles/h of C₂Cl₄ and 4.171 moles/h of HF; within one hour there were collected 130 g of a reaction product, the molar composition of which was as follows: CF₃-CHClF 2.2% CF₃-CHCl₂ 11.2% CCl₂=CClF + CClF -CHCl₂ 8.1% C₂Cl₄ 76.7% The conversion of C₂Cl₄ was 23.3%. The CF₃CHCl₂ net yield was 48.1% and the net yield of by-products was 9.4%. Example 5 (comparative test) A) To a solution of 10 kg of dodecahydrated chromic potassium alum [ K Cr(SO₄)₂.12 H₂O] in 71.5 liters of water there was added, at room temperature and under stirring, an aqueous solution of ammonium hydroxide, containing about 10% by weight of NH₃, until the pH reached a value of 8.8; the hydrated chromium oxide suspension was further stirred for about 30 minutes at room temperature, taking care that the pH remained between 8.8 and 9.0 by adding, if necessary, further amounts of NH₄OH. The suspension was then diluted with deionized water to bring the volume to about 160 liters, then it was allowed to settle for 24 hours, optionally accelerating settlement by means of flocculating agents (for example it is possible to use from 30 to 40 ppm of ECOCLAR® 2001, calculated on the slurry). The highest possible amount of aqueous phase was then syphoned off and the operation was repeated (dilution with washing effect) a further three times. After filtration, the hydrated chromium oxide cake was suspended in hot deionized water (85-90°C; 1,750 grams containing 15% of solid product suspended in three liters of water); hot washing was repeated several times until the sulphate ion (SO₄²⁻) concentration was lower than 0.04% by weight, referred to the calcined dry product. At last, the hydrated chromium oxide cake was suspended again in three liters of acetone at room temperature, thereby obtaining, by filtration, a hydrated chromium oxide, which was activated by calcination in a muffle furnace according to the following schedule: 5 hours at 100°C 2 hours at 400°C from 4 to 5 hours at a temperature between 500 and 550°C. Obtained were dark-green pellets having a content of SO₄ 2- ions lower than 0.04% by weight and a specific surface area of about 88 m²/g. B) Into the apparatus of example 1 there were charged 460 g of the chromium oxide catalyst of example 5A in pellet form, and, at a reaction temperature of 320°C and at a pressure slightly higher than the atmospheric pressure, 1.163 moles/h of C₂Cl₄ and 4.341 moles/h of HF were fed. After 1 hour, 143 g of a reaction product having the following molar composition were recovered: CF₃CHF₂ 10 % CF₃CHClF 6,1 % CF₃CCl₂F 4,6 % CF₃CH₂Cl 11,2 % CF₃CCl₂H 19,9 % CCl₂=CClF + CClF₂-CHC₁₂ 2 % C₂Cl₄ 32,1% The C₂Cl₄ conversion was 66,9%, the net yield of CF₃CHCT₂ was 29.7% and the non-recyclable by-products amounted to 67%.

Claims: A continuous process for preparing 1,1,1-trifluoro-2,2-dichloroethane which comprises reacting perchloroethylene with HF in the gas phase and in the presence of catalysts comprising Cr₂O₃ carried on AlF₃, in the gamma and/or beta form, said AlF₃ being in the preformed state on supporting thereon the chromium compound. The process according to claim 1, wherein the Cr₂O₃ amount ranges from 1 to 15% by weight calculated as chromium on the catalyst. The process according to claim 1 or 2, wherein the surface area of AlF₃ ranges from 15 to 30 m²/g. The process according to any one of claims 1 to 3, wherein AlF₃ is in the form of powders having a particle size ranging from 20 to 200 µm. The process according to any one of claims 1 to 4, wherein the AlF₃ carrier also contains the delta phase in an amount of up to 30% by weight. The process according to any one of claims 1 to 5, wherein the reaction temperature ranges from 300°C to 400°C. The process according to any one of claims 1 to 6, wherein the contact time between reagents and catalytic mass ranges from 1 second to 30 seconds. The process according to any one of claims 1 to 7, wherein the molar ratio between reagents HF and C₂Cl₄ is at least 3:1. The process according to any one of claims 1 to 8 wherein said catalyst is prepared by the steps of: a) impregnating AlF₃ with a solution of a soluble chromium salt; b) drying the catalyst; c) activating the catalyst with air or nitrogen at temperatures ranging from 200°C to 600°C. The process according to claim 9, wherein the activation step is conducted in the presence of steam. The process according to claim 9 or 10, wherein the activation temperature ranges from 330°C to 500°C.

Inventors: CARMELLO, DIEGO, GUGLIELMO, GIORGIO
Applicants: AUSIMONT S.P.A.
Application year: 1988
Main classification: C07C19
IPC codes: C07C17, B01J27, C07B61, C07C67, C07C19, B01J23