IMPROVED PROCESS FOR THE PREPARATION OF 1,1,1,2-TETRAFLUOROETHANE

Abstract: Improved process for the preparation of 1,1,1,2-tetrafluoroethane by the steps of chlorofluorination of selected organic compounds using Cl₂ and HF followed by hydrogenolysis of the resulting products, the improvement residing in the use of excess HF and recycle of the unreacted comopunds and byproducts to the chlorofluorination step.

Description: Improved process for the preparation of 1,1,1,2-tetrafluoroethane (FC-134a) by chlorofluorination of selected organic compounds in the presence of excess HF followed by hydrogenolysis of the resulting products. Background of the Invention DD 117,580 discloses a process for preparing a fluorinated alumina catalyst in the presence of promoter metals from an activated alumina, and its use for fluorinating two-carbon compounds in the gas phase. Asymmetrical C₂ fluorocarbons and pentafluorochloroethane (FC-115) are claimed to be prepared by this process using a molar ratio of tetrachloroethylene/chlorine/hydrogen fluoride of 1/1/3 to 1/1/5.1. Pentafluorochloroethane concentrations ranged from 6% to 57% of the products at 350°C and 8.5 s contact time, and at 450°C and 7.3 s contact time, respectively. Vechhio, M. et. al. , J. Fluorine Chem., 4 , 117-39 (1974) describe the chlorofluorination of tetrachloroethylene using Cl₂ and HF at ca . 400°C in the presence of an aluminum fluoride catalyst to yield the asymmetrical isomers of dichlorotetrafluoroethane and trichlorotrifluoroethane as the major products. In another experiment a 1/1 molar mixture of CCl₂FCClF₂(FC-113)/HF was passed over an AlF₃ catalyst at 400°C, and 10.3 mol% FC-115 and 2.5 mol% C₂Cl₄F₂ (FC-112 and FC-112a) were obtained. In a comparable experiment using a 1/1 FC-113/N₂ molar mixture, 6.8 mol% FC-115 and 8.8 mol% C₂Cl₄F₂ (FC-112 and FC-112a) were observed. The FC-115 concentration is dependent on both the HF concentration and disproportionation reactions, while the FC-112s concentration is dependent only on disproportionation reactions. The FC-112s concentration when HF was in the feed suggest that disproportionation reactions are occurring to a significant extent when the HF/FC-113 molar ratio is 1/1; additionally, more FC-115 was observed with HF in the feed. U.S. 4,605,798 discloses a process for the preparation of trichlorotrifluoroethane, dichlorotetrafluoroethane, and monochloropentafluoroethane by the reaction of chlorine, HF, and tetrachloroethylene over a variety of catalysts, some of which are claimed to be oxides or halides of aluminum. FC-115 is disclosed to be produced in a fluorination-dismutation (i.e., disproportionation) zone by the reactions of HF and dichlorotetrafluoroethane. Furthermore, the amount of asymmetrical isomer CF₃CCl₂F (FC-114a) produced is claimed to comprise less than 7% of the dichlorotetrafluoroethane products GB 1,578,933 claims a process for the preparation of 1,1,1,2-tetrafluoroethane (FC-134a) or 1,1,2,2-tetrafluoroethane (FC-134) by hydrogenating a haloethane having four or five fluorine atoms of formula CF₂XCFYZ where X is fluorine or chlorine; and when X is fluorine, Y is chlorine or fluorine; and when Y is chlorine Z is chlorine, fluorine or hydrogen; and when Y is fluorine Z is hydrogen; and when X is chlorine Y is fluorine and Z is either chlorine or hydrogen, over a supported Pd catalyst at a temperature of 200°C to 450°C. In example 7 it is seen that in order to convert all of the intermediate 1,1,1,2-tetrafluoro-2- chloroethane (FC-124) to FC-134a a hydrogenolysis temperature of greater than 350°C is required. Gervasutti, C. et . al ., J. Fluorine Chem., 19 , 1-20 (1981) disclose a process for the preparation of FC-134a from isomeric mixtures of dichlorotetrafluoroethanes through selective hydrogenolysis of 1,1,1,2-tetrafluoro-2,2-dichloroethane (FC-114a) catalyzed by Pd/C. The 1,1,2,2-tetrafluoro-1,2-dichloroethane (FC-114) was more stable toward hydrogenolysis. The concentration of FC-134a, from FC-114a hydrogenolysis, in the reaction products is shown to increase up to a maximum temperature of about 200°C above which its concentration decreases. This invention provides a multi-step process with recycle for preparing 1,1,1,2-tetrafluorethane (FC-134a), useful as a refrigerant, in very high yield by minimizing the formation of CF₃CClF₂ (FC-115) via disproportionation of C₂Cl₂F₄ (FC-114 and FC-114a), without suppressing the isomerization of CClF₂CClF₂ (FC-114) to CF₃CCl₂F (FC-114a). SUMMARY OF THE INVENTION The present invention provides an improvement in the process for the preparation of 1,1,1,2-tetrafluorethane by (a) reacting at least one compound of the formula C₂Cl 4-x Z x or C₂Cl 6-y Z y wherein x=0 to 4, y=0 to 6, and Z is at least one of hydrogen or fluorine and wherein the total number of fluorine atoms in the compound is ≦ 3, in the gaseous phase with chlorine in at least the stoichiometric amount needed to convert C₂Cl 4-x Z x and/or C₂Cl 6-y Z y to C₂Cl₂F₄, and with HF, at a temperature of about 300°C to about 450°C, in the presence of at least one catalyst selected from fluorinated alumina or AlF₃ to produce 1,1,1,2-tetrafluorodichloroethane (FC-114a), 1,1,2,2-tetrafluorodichloroethane (FC-114) and pentafluorochloroethane (FC-115); and (b) contacting the FC-114a and FC-114 produced in (a) in the gaseous phase with H₂ at a temperature of about 100°C to about 350°C in the presence of a hydrogenolysis catalyst to produce a gaseous mixture comprising 1,1,1,2-tetrafluoroethane (FC-134a), unreacted FC-114 and FC-114a, 1,1,1,2-tetrafluorochloroethane (FC-124), 1,1,1-trifluoroethane (FC-143a), and unreacted pentafluorochloroethane (FC-115). One aspect of the improvement resides in utilizing an amount of HF in (a) in at least 0.5 moles in excess of the stoichiometric amount needed to convert C₂Cl 4-x Z x and/or C₂Cl 6-y Z y to C₂Cl₂F₄. The excess HF has the beneficial effect of suppressing the disproportionation of FC-114a and/or FC-114, which results in the formation of unwanted FC-115. In addition the excess HF does not inhibit the isomerization of FC-114 to FC-114a, which is necessary to high yield production of FC-134a from the hydrogenolysis in (b). Because the presence of excess HF eliminates any substantial formation of FC-115, which if recycled to (a) could represent a yield loss by reacting to form hexafluoroethane, a further improvement resides in the additional step (c) of recycling the unreacted FC-114 and FC-114a, FC-124 and FC-143a from (b) to step (a) for the production of additional FC-114a. Note that in the practice of this invention because of the excess HF, isomerization of FC-114 to FC-114a, which is converted to FC-134a, is not inhibited. When the reaction temperature of (b) is less than about 350°C, the FC-124 byproduct cannot be recycled to (b) because the complete hydrogenolysis of FC-124 to FC-134a requires a temperature higher than about 350°C. The direct hydrogenolysis of FC-124 in the above process would require an additional hydrogenator, thereby adding to the cost and the complexity of the process. Another penalty of operating the hydrogenolysis reactor at temperatures above about 200°C is the decrease in yield of the desired product FC-134a. With control of disproportionation and recycle in accordance with this invention, the overall yield of FC-134a is over 95%. Using conventional methods which lack the control of disproportionation and recycle, the overall yield of FC-134a is less than 80%. DETAILS OF THE INVENTION The starting compounds useful in the process of this invention defined by the formula C₂Cl 4-x Z x wherein x=0 to 4 and Z is hydrogen, fluorine, or a combination of hydrogen and fluorine with the proviso that the total number of fluorine atoms in the molecule is ≦ 3, preferably include x=4 and Z=H, i.e., CH₂=CH₂; x=4 and Z=F and H, i.e., CHF=CH₂, CF₂=CH₂, CHF=CHF, and CF₂=CHF; x=3 and Z=H, i.e., CHCl=CH₂; x=3 and Z=H and F, e.g., CFCl=CH₂, CHCl=CHF, CFCl=CHF; x=3 and Z=F, i.e., CFCl=CF₂; x=2 and Z=H, i.e., CCl₂=CH₂, and CHCl=CHCl; x=2 and Z=F and H, i.e., CCl₂=CHF and CFCl=CHCl; x=2 and Z=F, i.e., CCl₂=CF₂ and CFCl=CFCl; x=1 and Z=H, i.e., CCl₂-CHCl; x=1 and Z=F, i.e., CCl₂=CFCl; and x=0, i.e., CCl₂=CCl₂. The starting compounds useful in the process of this invention defined by the formula C₂Cl 6-y Z y wherein y=0 to 6 and Z is hydrogen, fluorine or a combination of hydrogen and fluorine with the proviso that the total number of fluorine atoms in the molecule is ≦ 3, preferably include y=6 and Z=H, i.e., CH₃CH₃; y=6 and Z=F and H, i.e., CH₂FCH₃, CHF₂CH₃, CH₂FCH₂F, CHF₂CH₂F, CF₃CH₃; y=5 and Z=H, i.e., CH₃CHClF, CH₂FCH₂Cl, CH₃CClF₂, CHF₂CH₂Cl, CH₂FCHClF, CF₃CH₂Cl, CHF₂CHClF, and CH₂FCF₂Cl; y=4 and Z=H, i.e., CH₃CHCl₂; y=4 and Z=F and H, i.e., CH₂FCHCl₂, CH₂FCCl₂F, CHClFCH₂Cl, CClF₂CH₂Cl, CHF₂CHCl₂, CF₃CHCl₂, CHF₂CFCl₂, CH₂ClCH₂Cl, CHClFCHClF, and CClF₂CHClF; y=3 and Z=H, i.e., CH₃CCl₃ and CHCl₂CH₂Cl; y=3 and Z=F and H, i.e., CCl₃CH₂F, CCl₃CHF₂, CFCl₂CH₂Cl, CHCl₂CHClF, CHCl₂CF₂Cl, CFCl₂CHClF, and CFCl₂CF₂Cl; y=3 and Z=F, i.e., CCl₃CF₃; y=2 and Z=H, e.g., CCl₃CH₂Cl; y=2, Z=F and H, e.g., CCl₃CHClF; y=2 and Z=F, i.e., CCl₃CClF₂; y=1 and Z=H, i.e., CCl₃CHCl₂; y=1 and Z=F, i.e., CCl₃CCl₂F; and y=0, i.e., CCl₃CCl₃. Mixtures of any of the above compounds can be used. Tetrachloroethylene is especially preferred. The reaction of the starting compound or compounds with HF and Cl₂ in the presence of fluorinated alumina or AlF₃, is conducted at about 300°C to 450°C, preferably about 325°C to 400°C, and most preferably about 350°C to 375°C. The contact time can vary widely depending on the degree of conversion desired and generally will be about 1 to 60 seconds, preferably about 15 to 30 seconds at atmospheric pressure. The amount of HF should be at least 0.5, preferably at least 1.0 moles, and more preferably at least 2.0 moles, in excess of the stoichiometric amount needed to convert C₂Cl 4-x Z x or C₂Cl 6-y Z y to C₂Cl₂F₄ (FC-114 and FC-114a), in order to effectively control disproportionation reactions. When the starting compound is CCl₂=CCl₂, the molar ratio of HF to CCl₂=CCl₂ can range from about 4.5/1 to 10/1, preferably about 5/1 to 10/1 and more preferably about 6/1 to 7/1. The HF may be diluted with inert materials if so desired. Such inert materials which are suitable for the process of this invention include nitrogen, helium or argon. The amount of Cl₂ should be at least the stoichiometric amount needed to convert C₂Cl 4-x Z x or C₂Cl 6-y Z y to C₂Cl₂F₄. Generally, the molar ratio of Cl₂ to CCl₂=CHCl₂ can range from about 1/1 to 10/1. The reaction of the starting compound or compounds with HF/Cl₂ may be conducted in any suitable reactor, including fixed and fluidized bed reactors, The reaction vessel should be constructed from materials which are resistant to the corrosive effects of hydrogen fluoride and chlorine such as Hastelloy® and Inconel®. The reaction mixture comprising FC-114a and FC-114, prepared as described above, is contacted with hydrogen in the presence of palladium on carbon, at about 100°C to 230°C, in any suitable reactor, including fixed and fluidized bed reactors. The reaction vessel should be constructed from materials which are resistant to the corrosive effects of hydrogen halides. The contact time can vary widely depending on the degree of conversion desired and generally will be about 1 to 60 seconds, preferably about 10 to 30 seconds. Generally the molar ratio of H₂ to 1,1,1,2-tetrafluorodichloroethane can range from about 0.5/1 to 20/1, preferably about 0.5/1 to 10/1 and more preferably about 1/1 to 3/1. During the course of the reaction unreacted FC-114a, FC-114, FC-124 and FC-143a can be recycled to the initial contacting step, after separating the desired product FC-134a. Pressure is not critical in either the reaction with HF and Cl₂ or the subsequent hydrogenolysis. Atmospheric and superatmospheric pressures are the most convenient and are therefore preferred. EXAMPLES In the following illustrative examples of the invention, parts and percentages are by weight and temperatures are in degrees Celsius unless otherwise specified. All product compositions are given in area percent. General Procedure for Chlorofluorination The reactor (0.5 inch ID by 12 inch length Inconel® pipe) was charged with alumina or aluminum fluoride as described in the following examples and placed in a sand bath. The bath was gradually heated to 400°C while N₂ at 50 cc/min was passed through the reactor to remove traces of water. When the reactor charge was alumina, the temperature was lowered and maintained at about 200°C while HF and N₂ (1/4 molar ratio) was passed through the reactor, and then slowly decreasing the N₂ flow until only HF was being passed through the reactor. At this point, the temperature was gradually raised to 450°C and maintained there for 15 to 300 minutes. The HF treatment converts alumina to fluorinated alumina. When the reactor charge was aluminum fluoride, this last process step is omitted. The temperature was then adjusted to the indicated values followed by initiation of flow of the halogenated ethane or ethylene derivative. All flows were adjusted to give the indicated molar ratios and contact times in the Examples. The reactor effluent was sampled on-line with a Hewlett Packard HP 5890 gas chromatograph using a 20′ x 1/8″ column containing Krytox® on an inert support and helium flow of 35 cc/min. Analysis conditions were 70°C for 3 min. followed by temperature programming to 180°C at a rate of 6°/minute. General Procedure for Hydrogenolysis A tubular reactor of stainless steel or Hastelloy®, 1/4″ to 3/4″ OD, was charged with the desired catalyst. Hydrogen, 25 cc/min was passed through the reactor and the temperature was increased to 300°C at a rate of 0.5°/min and held at 300°C for 3 h. The reactor temperature was then reduced to the desired temperature and H₂/C₂F₄Cl₂ at the specified ratio was passed over the catalyst. EXAMPLE 1 Chlorofluorination of CCl₂=CCl₂ The general procedure for chlorofluorination was followed using AlF₃ (15.7 g, 25 mL, 12-20 mesh). The reaction temperature was 375°C, the mol ratio of HF/C₂Cl₄/Cl₂ was 5/1/1 and the contact time was 15 s. The composition of the product stream was 0.5% CF₃CClF₂, 0.8% CClF₂CClF₂, 49.2% CF₃CCl₂F, 0.7% CF₃CHCl₂, 3.2% CCl₂FCClF₂, 12.9% CF₃CCl₃, 0.1% CClF₂CHCl₂, 0.6% CClF=CCl₂, 1.1% CCl₂FCCl₂F, 0.3% CCl₂FCCl₃, and 30.5% CCl₂=CCl₂. The yield (based on CCl₂=FCCl₂ converted) of CF₃CCl₂F was 70.8%. EXAMPLE 2 Chlorofluorination of CCl₂=CCl₂ The procedure and the AlF₃ catalyst were the same as used in Example 1. The reaction temperature was 375°C, the mol ratio of HF/C₂Cl₄/Cl₂ was 5/1/1 and the contact time was 30 s. The composition of the product stream was 1.0% CF₃CClF₂, 1.2% CClF₂CClF₂, 58.8% CF₃CCl₂F, 0.9% CF₃CHCl₂, 1.6% CCl₂FCClF₂, 9.4% CF₃CCl₃, 0.1% CClF₂CHCl₂, 0.9% CClF=CCl₂, 0.5% CF₂ClCCl₃, 0.2% CCl₂FCCl₃, and 25.4% CCl₂=CCl₂. The yield (based on CCl₂=CCl₂ converted) of CF₃CCl₂F was 78.8%. EXAMPLE 3 Hydrogenolysis of CF₃CCl₂F (FC-114a) to CF₃CH₂F The general procedure for hydrogenolysis was followed using as catalyst 0.5% Pd/C (5.0 g) at a hydrogen pressure of 446 KPa. The other reaction conditions, and products are shown in Table 1. EXAMPLE 4 Chlorofluorination of CF₃CHClF The procedure and the AlF₃ catalyst were the same as used in Examples 1 and 2. The reaction temperature was 375°C, the mol ratio of HF/CF₃CHClF/Cl₂ was 1/1/1.5 and the contact time was 30 s. The composition of the reaction effluent was 7.3% CF₃CClF₂, 2.5% CClF₂ClF₂, 54.5% CF₃CCl₂F, 5.0% CF₃CHCl₂, 0.6% CCl₂FCClF₂, 6.4% CF₃CCl₃, 2.0% CF₃CHF₂, 0.5% CF₂HCF₂Cl, 0.1% CCl₂FCCl₂F, and 21.1% CF₃CHClF. The yield (based on CF₃CHClF converted) of CF₃CCl₂F was 69.1%. EXAMPLE 5 Chlorofluorination of CF₃CH₃ The procedure and the AlF₃ catalyst were the same as used in Examples 1, 2 and 4. The reaction temperature was 350°C, the mol ratio of HF/CF₃CH₃/Cl₂ was 2/1/4 and the contact time was 20 s. The composition of the product stream was 1.2% CF₃CClF₂, 2.1% CClF₂CClF₂, 59.5% CF₃CCl₂F, 12.8% CF₃CHCl₂, 2.5% CCl₂FCClF₂, 18.9% CF₃CCl₃, 1.2% CF₃CHClF, 0.8% CF₃CH₂Cl, and 0.7% CF₂ClCCl₃. The yield (based on CF₃CH₃ converted) of CF₃CCl₂F was 59.5%. EXAMPLE 6 Isomerization of CClF₂CClF₂ a. The procedure and the AlF₃ catalyst were the same as used in Examples 1, 2, 4 and 5. The CClF₂CClF₂ feed contained about 9% CF₃CCl₂F. The reaction temperature was 375°C, the mol ratio of HF/CClF₂CClF₂/N₂ was 0/1/2 and the contact time was 30 s. The composition of the product stream was 31.4% CF₃CClF₂, 2.2% CClF₂CClF₂, 18.2% CF₃CCl₂F, 3.0% CCl₂FCClF₂, 36% CF₃CCl₃, 4.4% CCl₃CClF₂, 0.8% CCl₃CCl₂F, and 3.9% CCl₂=CCl₂. The large concentrations of CF₃CClF₂, CF₃CCl₃, and CCl₂FCClF₂ indicate that a significant amount C₂Cl₂F₄ has disproportionated. b. The above experiment was repeated at 425°C with mol ratios of HF/CClF₂CClF₂/N₂ of 6/1/0 and a contact time of 15 s. The composition of the product stream was 10.9% CF₃CClF₂, 38.7% CClF₂CClF₂, 50.2% CF₃CCl₂F, and 0.2% CF₃CCl₃. These results show clearly the inhibiting effect of HF on C₂Cl₂F₄ disproportionation while simultaneously not affecting the isomerization of CClF₂CClF₂ to CF₃CCl₂F.

Claims: In the process for the preparation of 1,1,1,2-tetrafluoroethane by (a) reacting at least one compound of the formula C₂Cl 4-x Z x or C₂Cl 6-y Z y wherein x=0 to 4, y=0 to 6, and Z is at least one of hydrogen or fluorine and wherein the total number of fluorine atoms in the compound is ≦ 3, in the gaseous phase with chlorine in at least the stoichiometric amount needed to convert C₂Cl 4-x Z x and/or C₂Cl 6-y Z y to C₂Cl₂F₄, and with HF, at a temperature of about 300°C to about 450°C, in the presence of at least one catalyst selected from fluorinated alumina or AlF₃ to produce 1,1,1,2-tetrafluorodichloroethane (FC-114a), 1,1,2,2-tetrafluorodichloroethane (FC-114) and pentafluorochloroethane (FC-115); and (b) contacting the FC-114a and FC-114 produced in (a) in the gaseous phase with H₂ at a temperature of about 100°C to about 350°C in the presence of a hydrogenolysis catalyst to produce a gaseous mixture comprising 1,1,1,2-tetrafluoroethane (FC-134a), unreacted FC-114 and FC-114a, 1,1,1,2-tetrafluorochloroethane (FC-124), 1,1,1-trifluoroethane (FC-143a), and unreacted pentafluorochloroethane (FC-115); the improvement comprising utilizing an amount of HF in (a) in at least 0.5 moles in excess of the stoichiometric amount needed to convert C₂Cl 4-x Z x and/or C₂Cl 6-y Z y to C₂Cl₂F₄. The process of Claim 1 comprising the additional step (c) recycling the unreacted FC-114 and FC-114a, FC-124, and FC-143a from (b) to step (a). The process of Claim 1 or 2 wherein the compound of (a) is tetrachloroethylene. The process of Claim 1, 2 or 3 wherein the amount of HF is at least 1.0 mole in excess of the stoichiometric amount. The process of any one of Claims 1 to 4 wherein the amount of HF is at least 2.0 moles in excess of the stoichiometric amount. The process of any one of Claims 1 to 5 wherein the temperature in (a) is from about 325°C to about 400°C. The process of Claim 6 wherein the temperature in (a) is from about 350°C to about 375°C.

Inventors: MANZER, LEO ERNEST, RAO, VELLIYUR NOTT MALLIKARJUNA
Applicants: E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1989
Main classification: C07C19
IPC codes: C07C17, B01J27, C07C19, C07B61