VAPOUR PHASE PREPARATION OF 1,1,1,2,3,3,3-HEPTAFLUOROPROPANE

Abstract: A process for the preparation of 1,1,1,2,3,3,3-heptafluoropropane (HFC 227ea) by reacting hydrogen fluoride with hexafluoropropane (HFP) in the vapour phase over a fluorination catalyst using a molar ratio of HF:HFP is at least 3.5:1. The reaction can be carried out under adiabatic conditions with less catalyst fouling. The use of HFC 227ea as diluent allows the use of smaller plant.

Description: VAPOUR PHASE PREPARATION OF 1,1,1,2,3,3,3-HEPTAFLUOROPROPANE The present invention relates to a process for the preparation of 1,1,1, 2,3,3, 3-heptafluoropropane from hexafluoropropene, particularly to the process therefor carried out in the vapour phase. Hydrofluorocarbons, referred to hereinafter for convenience as HFC 's, are widely used as replacements for chlorofluorocarbon compounds. Such uses include use in medical applications, for example as an aerosol propellant, use as a fire suppressant, use in refrigeration applications and in other applications. 1,1,1,2,3,3,3-heptafluoropropane, which will be referred to hereinafter for convenience as HFC 227ea, has zero ozone depletion potential and is particularly beneficial in medical propellant applications in the light of its combination of properties including low toxicity, non-flammability, solvent properties and boiling point. It is known in the art to produce HFC 227ea by the hydrofluorination of hexafluoropropene, optionally in the presence of a catalyst, in the liquid phase or vapour phase. Hydrogen fluoride is known for use as a hydrofluorination agent in such a hydrofluorination reaction. A variety of materials may be employed as catalyst in such a hydrofluorination reaction. The liquid phase reaction of hexafluoropropene, hereinafter referred to for convenience as HFP, with hydrogen fluoride for the preparation of HFC 227ea is well known in the art. For example, as catalysts in the liquid phase reaction, WO 97/11042 and EP 0,634,383 disclose the use of an organic amine complexed with hydrogen fluoride; WO 96/0243 discloses the use of certain antimony catalysts; and EP 0,634,384 discloses the use of a weak ion-exchange resin containing tertiary amine groups. The vapour phase reaction of HFP with hydrogen fluoride for the preparation of HFC 227ea has been less widely studied. However, as catalyst in the aforementioned vapour phase reaction, GB 902590 discloses the use of an activated carbon catalyst with an at least equimolar gaseous mixture of hydrogen fluoride and HFP, at atmospheric pressure and high temperature; for high yield, the HF.HFP molar ratio is preferably between 1.5:1 and 3:1. The use of activated carbon as catalyst in the vapour phase reaction has the drawback that it rapidly becomes deactivated. EP 0,562,509 describes a process for reactivating such a carbon catalyst by heating it at 450-900°C in a stream of inert gas or under reduced pressure. US 4,158,023 describes the preparation of HFC 227ea as a non-isolated intermediate in the vapour-phase preparation of octafluoropropane from HFP. It discloses the use of a chromium oxyfluoride catalyst in the vapour phase reaction of HFP with hydrogen fluoride for the preparation of HFC 227ea, at 100-350°C, at generally normal pressure and with a low molar ratio of HF:HFP, namely from 1:1 to 3:1. We have now found surprisingly that using a high HF:HFP molar ratio for the conversion of HFP into HFC 227ea, namely at least 3.5:1, we can obtain the same conversion at lower temperature, with less catalyst fouling, than using a HF:HFP molar ratio of 1 : 1. at the same pressure and with the same contact time. According to the first aspect of the present invention there is provided a process for the preparation of HFC 227ea by reacting hydrogen fluoride with HFP in the vapour phase over a fluorination catalyst characterised in that the molar ratio of HF:HFP is at least 3.5: 1. In the process according to the first aspect of the present invention, the molar ratio of HF:HFP is preferably in the range 3.5:1 to 100:1, more preferably in the range 4:1 to 50:1. In the process according to the first aspect of the present invention, a gaseous mixture of HFP and hydrogen fluoride is passed through a heatable reactor tube containing a hydrofluorination catalyst. The reactor tube may be of conventional design made of appropriate material resistant to hydrogen fluoride. As examples of fluorination catalysts which may be used in the process according to the present invention may be mentioned inter alia (i) fluorination catalysts based on metal oxides/halides or oxyhalides or mixed metal oxides/halides/oxyhalides, for example chromia or alumina; (ii) other metallic oxides/halides/oxyhalides supported on chromia or alumina, for example oxides of zinc, iron, magnesium or nickel; and (iii) metal oxides/halides/oxyhalides, or mixed metal oxides/halides/oxyhalides supported on carbon. Preferably, the fluorination catalyst is dried and subjected to a pretreatment, eg with hydrogen fluoride, prior to use. Preferred fluorination catalysts are disclosed in our EP 0,502,605 and WO 93/25508, the disclosures in which are incorporated herein by way of reference. The metal on which the fluorination catalyst used in the present invention is based is preferably a d- or p-block metal, oxide, fluoride or oxyfluoride thereof, and more preferably chromium, aluminium or a Group Villa metal. The catalyst may be compressed into pellets and used in a fixed bed or, alternatively, catalysts of appropriate particle size may be used in a fluidised bed. The reaction of HFP with hydrogen fluoride is not equilibrium limited, therefore it is possible to obtain very high conversions per pass, eg 50-100%. The reaction of HFP with hydrogen fluoride is significantly exothermic. The reaction can be carried out in a cooled reactor, eg a salt-cooled reactor, or in an adiabatic reactor under conditions which moderate the heat generated through formation of HFC 227ea. For example, the heat generated can be removed between reactors in a series thereof and/or by use of a diluent in the reaction mixture. The reaction according to the present invention is preferably carried out in the presence of a diluent. Where the process according to the present invention is carried out in the presence of a diluent the exotherm generated by the addition of hydrogen fluoride to HFP is moderated and we have found that the reaction can be carried out in an adiabatic reactor or series of adiabatic reactors, that is a reactor to which none, or little thermal regulation, ie heating or cooling is required. The diluent is preferably a HFC or hydrogen fluoride, more preferably a HFC. However, we do not exclude the possibility that it may be an alternative diluent, eg nitrogen, etc. As examples of preferred HFC diluents may be mentioned inter alia 1,1,1,2-tetrafluoroethane, pentafluoroethane or preferably HFC 227ea. The use of HFC 227ea or hydrogen fluoride as diluent in the reaction facilitates separation of the products at the end of the reaction and allows the use of a simpler separation train and smaller plant. For example, where the maximum acceptable exotherm across an adiabatic reactor is 50°C and the HF:HFP molar feed ratio is 10:1 we have found that for 50% HFP conversion, 70% HFP conversion and 100% HFP conversion using recycled HFC 227ea as diluent, the relative size of the reactor(s) compared with the use of excess hydrogen fluoride as diluent is about 30%, ie a reduction in reactor size of about 70%. Furthermore, the presence of recycled HFC 227ea in the feed to the adiabatic reactor(s) does not adversely affect the conversion of HFP into HFC 227ea since, as mentioned above, the reaction of HFP with hydrogen fluoride is not equilibrium limited. It will be appreciated that use of a low cost adiabatic reactor provides significant capital cost advantage over a salt-cooled reactor employing an internal cooling surface. In the process according to the present invention, the contact time is typically less than 5 minutes and preferably is 1-5 seconds. The process according to the present invention is typically carried out in the temperature range 100-500°C. The process according to the present invention is preferably carried out at superatmospheric pressure, for example 2-50 barg, more preferably 5-25 barg. It will be appreciated that where the process is carried out at superatmospheric pressure the productivity of the reactor is increased, smaller-scale plant can be used for the same throughput and less expensive refrigeration can be used in the primary separation stage following the reactor. The product stream from the reactor in which HFP is reacted with hydrogen fluoride to prepare HFC 227ea typically contains HFC 227ea, hydrogen fluoride and hexafluoropropene. After completion of the reaction, the HFC 227ea may be isolated from unchanged starting materials and by-products using conventional procedures, for example distillation. The present invention is further illustrated by reference to the following Examples. Example 1 This Example illustrates the advantage of carrying out the reaction of HF with HFP at a high HF:HFP molar ratio, namely 10:1. A mixed zinc/chromium oxide fluorination catalyst was charged to a reactor, dried under nitrogen at 200°C for 16 hours and treated with a vapour stream of hydrogen fluoride in nitrogen at 300-380°C for 16 hours. The temperature in the reactor was adjusted to give the desired reaction temperature and vapour comprising HFP, hydrogen fluoride and nitrogen in the molar ratio shown in Table 1 was charged to the reactor under atmospheric pressure. In a Comparative Test, Example 1 was repeated except that a HF:HFP molar ratio of 1 : 1 was used instead of 10: 1. The results are shown in Table 1 from which it can be seen that with a 10:1 molar ratio of HF:HFP and a contact time of 1 second: a) 50% conversion of HFP into HFC 227ea occurs at a lower temperature (320°C) than with a 1 :1 molar ratio (334°C); and b) 90% conversion of HFP into HFC 227ea can be achieved (at 354°C) whereas 90% conversion cannot be achieved with a 1 :1 mole ratio. Table 1 CT1 : Comparative Test Examples 2-3 These Examples illustrate the use of smaller adiabatic reactors when HFC 227ea or HF is used as diluent. The per-centage conversions of HFP into HFC 227ea were calculated to give a 50°C adiabatic temperature rise across the reactor for the feeds shown in Table 2. Table 2 CT2 is a Comparative Test. From the results in Table 2: a) it can be calculated that the relative size of an adiabatic reactor using HFC 227ea as diluent compared to using HF as diluent in the preparation of HFC 227ea by the reaction ofHFP with HF is: 50% HFP Conversion: 0.37 70% HFP Conversion: 0.32 100% HFP Conversion: 0.26; and b) it can be seen that in the absence of a diluent and using a 10:1 HF:HFP molar feed ratio the conversion of HFP into HFC 227ea by reaction with HF is only 15%.

Claims: Claims 1. A process for the preparation of 1,1,1,2,3,3,3-heptaflouropropane (HFC 227ea) by reacting hydrogen fluoride with hexafluoφropene (HFP) in the vapour phase over a fluorination catalyst characterised in that the molar ratio of HF:HFP is at least 3.5:1. 2. A process as claimed in Claim 1 wherein the fluorination catalyst is dried and subjected to a pretreatment prior to use in the process. 3. A process as claimed in Claim 1 wherein the fluorination catalyst comprises an oxide, fluoride or oxyfluoride of a d- or p-block metal. 4. A process as claimed in Claim 1 carried out at superatmospheric pressure. 5. A process as claimed in Claim 1 carried out in the presence of a diluent. 6. A process as claimed in Claim 5 wherein the diluent is a hydrofluorocarbon (HFC). 7. A process as claimed in Claim 6 wherein the HFC is HFC 227ea. 8. A process as claimed in any one of Claims 4-7 carried out under adiabatic conditions. 9. A process as claimed in Claim 8 wherein the contact time is less than 5 minutes. 10. A process as claimed in Claim 8 carried out in the temperature range 100-500°C.

Inventors: EWING PAUL NICHOLAS, STEWART PAUL HENDRY, EWING, PAUL, NICHOLAS, STEWART, PAUL, HENDRY
Applicants: EWING PAUL NICHOLAS, ICI PLC, STEWART PAUL HENDRY, EWING, PAUL, NICHOLAS, IMPERIAL CHEMICAL INDUSTRIES PLC, STEWART, PAUL, HENDRY
Application year: 1998
Main classification: C07C17
IPC codes: B01J23, C07B61, C07C17, C07C19