Process for preparing pentafluoroethane by dismutation of tetrafluorochloroethane

Abstract: Pentafluoroethane (HFC 125) is selectively prepared by dismutation of tetrafluorochloroethane (HCFC 124) effected by contacting the latter, in the gas phase, with a Cr₂O₃ catalyst at temperatures ranging from 150° to 330°C. Along with HFC 125, also HCFC 123 substantially free from isomers 123a and 123b is obtained.

Description: The present invention relates to a process for preparing pentafluoroethane (hereinafter referred to as HFC 125). In particular, it relates to the preparation of pentafluoroethane by means of dismutation reaction, in the presence of catalysts, of tetrafluorochloroethane CF 3 CHClF (hereinafter referred to as HCFC 124). As is known, it is useful to have available processes for the industrial production of HFC 125, which is a fluorocarbon fully harmless towards atmospheric ozone and therefore an excellent potential substitute for the present CFC. There are known processes for preparing HCFC 125 by fluorination, with HF, of proper chlorinated precursors such as HCFC 124 and 123 (CF 3 CHCl 2 ). However, in order to obtain, according to such processes, appreciable conversions into the product to be obtained, severe reaction conditions are required, which generally lead to the formation of appreciable amounts of undesirable products such as, for example, CFC 114 (CF 3 CFCl 2 ), which, as is known, is one of the compounds, which are harmful for ozone. Furthermore, such processes involve the feeding of anhydrous HF at high temperature, its separation from the reaction products and the subsequent recycle or discharge, all these steps requiring corrosion-proof materials and being potential sources of both hazard and pollution. US-A-4 766 260 discloses a gas-phase process for the manufacture of CF 3 -CHCl 2 and CF 3 -CHFCl by contacting a tetrahaloethylene with HF in the presence of a metal selected from Cr, Mn, Ni and Co on a high fluorine content alumina support; US-A-4 145 368 discloses catalytic comproportionation of CF 3 -CCl 3 and CF 3 -CH 2 Cl in the gaseous phase to CF 3 -CHCl 2 in the presence of chromium oxides; US-A-3 787 331 discloses catalytic disproportionation of CF 2 Cl-CFCl 2 in the gaseous phase into CF 2 Cl-CF 2 Cl and C 2 F 2 Cl 4 using a supported catalyst containing manganese chromium and nickel; US-A-3 651 156 exemplifies the disproportionation reaction of C 2 F 3 Cl 3 to C 2 F 4 Cl 2 and C 2 F 2 Cl 4 over a catalyst obtained by fluorination of chromium hydroxide. WO-A 92/02476 discloses the interconversion of HCFC-124 to HFC-125 and HCFC-123 at 50°-500°C in the presence of a Cr 2 O 3 catalyst among others, optionally supported on alumina or carbon. There is obtained a poor selectivity in HFC-125 and relatively high amounts of undesirable by-products. It has now been found by the Applicant that it is possible to obtain HFC 125 with high selectivity and efficiency, without using HF, by means of a dismutation reaction of HCFC 124 in the gas phase, in the presence of a proper catalyst. In particular, said dismutation is conducted by contacting gaseous HCFC 124 with a catalyst based on chrome oxide (Cr 2 O 3 ) carried on AlF 3 , at temperatures ranging from 220° to 280°C. It is possible to use temperatures exceeding 330°C, however they are of little interest because they give rise to too high amounts of by-products. The reaction is accompanied by the simultaneous formation of CF 3 CHCl 2 (HCFC 123) with minor amounts (less than 1%) of isomer CF 2 Cl-CHFCl (HCFC 123a) and traces of isomer CF 2 H-CFCl 2 (HCFC 123b). In consideration of the well-known usefulness of preparing HCFC 123 as much as possible free from isomers 123a and 123b, there are evident, therefore, the further advantages resulting from the preparation of HFC 125 by means of the dismutation reaction of HFC 124, as described hereinbefore. Thus, object of the present invention is a process for preparing HFC 125 which comprises contacting HCFC 124 in the gas phase with a Cr 2 O 3 catalyst carried on AlF 3 having a fluorine content corresponding to at least 90% of AlF 3, at temperatures ranging from 220 to 280°C. In practice, it is operated by causing a gaseous HCFC 124 flow to pass through a catalyst bed at temperatures within said range. Preferably, but not necessarily, it is operated with a time of contact between HCFC 124 and the catalyst, considered as the ratio between the volume of the reagent under the reaction conditions and the volume of the catalytic bed, ranging from 1 second to 20 seconds, preferably from 8 to 10 seconds. The pressure is not critical; usually it is operated at atmospheric pressure or above the atmospheric pressure. The HCFC 124 flow at the catalytic bed inlet can be diluted with inert gases, for example nitrogen. The chrome oxide carried on AlF 3 has characteristics suitable for the use in fluid bed reactors. The AlF 3 is preferably in the gamma and/or beta form, but it can contain also the delta form, generally up to 30% by weight. The carried catalyst can be prepared according to various procedures; a preferred procedure comprises the steps of impregnating the carrier with an aqueous solution of a trivalent chrome salt, drying and then subjecting the so impregnated carrier to an activation treatment with air or nitrogen in the presence of water vapour, at temperatures from 200° to 600°C, but preferably from 350° to 500°C. The Cr 2 O 3 content in the carried catalyst generally ranges from 1 to 15% by weight, calculated as Cr on the catalyst. The AlF 3 carrier has a fluorine content corresponding to at least 90% of AlF 3 calculated on the total weight of the carrier. The following examples are given to illustrate, but not to limit the scope of the present invention. EXAMPLE 1 Preparation of carried Cr 2 O 3 A catalyst suited to be used in a fluid bed was prepared by impregnating a granulated AlF 3 carrier (mixture of β, γ and/or δ phases: 25-30 mq/g, fluorine content: about 95% of the theoretical value) with an aqueous solution of CrCl 3 , in a ratio of 492 g of CrCl 3 .6H 2 O per Kg of AlF 3 , according to one of the methods of the art. The catalyst so obtained was dried in oven at 120°C for a few hours, then it was introduced into an Inconel 600 tubular reactor having a diameter of 50 mm, equipped with a fritted bottom and electrically heated; it was heated to 400°C and treated for 10 hours with a 100 Nl/h air flow. The chrome content of the catalyst was equal to 8% by wt. Dismutation reaction 250 cc (306 g) of the catalyst so prepared were introduced into the above said reactor. At 280°C and at a slightly higher pressure than the atmospheric pressure, about 250 g/h of HCFC 124 were fed. The product leaving the reactor was washed in water, dried, sampled and analyzed by means of GLC, while water was titrated to determine the acidity; the following products were obtained (mols %): CF 3 -CF 2 H 11.1 C 2 F 4 HCl 76.6 CF 2 Cl-CHFCl negligible CF 3 -CHCl 2 11.5 Other products 0.8 0.010 moles/h of acidity were titrated in the washing water. EXAMPLE 2 (comparative) Into the reactor of the preceding example there were charged 250 cc of AlF 3 utilized as a carrier for the catalyst prepared in example 1, and 150 g/h (1 mol/h) of HCFC 124 were fed along with 25 l/h of N 2 . Operating in like manner as in the preceding example, at 280°C the following products (mols %) were obtained: CF 3 -CF 2 H 1.2 C 2 F 4 HCl 97.4 C 2 F 3 HCl 2 1.3 Other products 0.1 Therefore, the catalytic activity of the carrier only is extremely low. EXMAPLE 3 A catalyst suitable for the use in a fluid bed was prepared as in example 1, the only exception being the final calcination, which was carried out in nitrogen instead of in air. 250 cc of the catalyst so prepared were charged into the reactor utilized in the preceding examples. At 280°C and at a slightly higher pressure than the atmospheric pressure, 180 g/h of HCFC 124 diluted with 25 Nl/h of nitrogen were fed. Operating in like manner as in the preceding examples, the following products were obtained (in mols %): CF 3 -CF 2 H 36.7 C 2 F 4 HCl 32.5 CF 2 Cl-CFHCl <0.05 CF 3 -CHCl 2 27.8 Other products 2.8 Free acidity = 0.065 mols/h. EXAMPLE 4 (Comparative) The reaction temperature was brought to 320°C, all the other conditions being the ones of example 1. The following products (in mols %) were obtained: CF 3 -CF 2 H 17.43 C 2 F 4 HCl 61.48 CF 2 Cl-CFHCl 0.10 CF 3 -CHCl 2 15.64 Other products 5.35 Free acidity = 0.035 mols/h.

Claims: A process for preparing pentafluoroethane, which consists of contacting tetrafluorochloroethane in the gas phase with a catalyst consisting of chrome oxide (Cr 2 O 3 ) supported on AlF 3 having a fluorine content corresponding to at least 90% of AlF 3 , at temperatures ranging from 220°C to 280°C. Process according to claim 1, wherein the support consists of AlF 3 . Process according to claims 1 and 2, wherein the AlF 3 is in gamma or beta form, optionally in mixture with up to 30% by weight of delta form.

Inventors: CUZZATO PAOLO, CUZZATO, PAOLO
Applicants: AUSIMONT SPA, AUSIMONT S.P.A.
Application year: 1993
Main classification: C07C17
IPC codes: C07C17, B01J23, C07B61, C07C19