WO2001062693A1

WO2001062693A1 - Method for selectively converting perfluoroisobutene contained in a halogenated hydrocarbon current

Info

Publication number: WO2001062693A1
Application number: PCT/FR2001/000491
Authority: WO
Inventors: Eric Jorda; Jean-Marc Sage; Eric Lacroix
Original assignee: Atofina
Priority date: 2000-02-21
Filing date: 2001-02-20
Publication date: 2001-08-30
Also published as: AU3572201A; FR2805257A1; FR2805257B1

Abstract

The invention concerns a method for selectively converting PFIB (perfluoroisobutene) contained in a halogenated hydrocarbon current which consists in: either contacting said halogenated hydrocarbon current containing PFIB with HF in the presence of a hydrofluorination catalyst to add HF on the PFIB, or in contacting said halogenated hydrocarbon current containing PFIB with HCl in the presence of a hydrochlorination catalyst to add HCl on the PFIB.

Description

PROCESS FOR THE SELECTIVE CONVERSION OF PERFLUOROISOBUTENE CONTAINED IN A STREAM OF HALOGENATED HYDROCARBONS

[Field of the invention]

The present invention relates to a process for the selective conversion of perfluoroisobutene contained in a stream of halogenated hydrocarbons.

Fluorinated polymers such as PTFE or certain grades of PVDF copolymer, which are highly valued for their performance, are made from fluorinated olefins such as C2F4, C2H2F2 and C3F6.

Most of the methods known in the literature for the synthesis of C2F4 and C3F6 consist of a pyrolysis of molecules such as CHCIF2

(HCFC-22), CHF3 (HFC-23) and other products from the recycling of this pyrolysis (C-C4F8, C2F4, "heavy" ...). One of the by-products of these pyrolysis, known under the abbreviation PFIB (perfluoroisobutene or CF2 = C (CF3) 2), is extremely toxic. It is obviously recommended to remove all traces of this impurity in the final product. It is also obvious, for safety reasons, that it is preferable to avoid or reduce the circulation or storage of streams containing significant quantities of PFIB.

The total separation of PFIB from other halogenated hydrocarbons by conventional means of chemical engineering (such as distillation for example) is very difficult and dangerous. The process of the invention consists in selectively converting the PFIB, that is to say without converting or altering the other halogenated hydrocarbons, into a product which is clearly less toxic and which separates more easily from the other halogenated hydrocarbons.

[The prior art and the technical problem]

The solutions usually recommended in the prior art for the elimination of PFIB use the reactivity of perfluorinated olefins with respect to alcohols to form ethers. The greater reactivity of the PFIB in comparison of C2F4 and C3F5 makes it possible to selectively remove the impurity without significant loss of yield in useful products.

US patent 5516946 in its introduction mentions without any detail that the PFIB can react with alcohols to form ethers. Patent EP 2098 describes the reaction of PFIB contained in a stream of fluorinated compounds with a solution of HF or HCl in methanol under conditions such that the methanol remains liquid. However, these methods have several drawbacks:

- The absorption columns necessary for this treatment are placed at the end of the process and the PFIB is therefore present in a large part of the installation.

- This technique is not practical to implement continuously.

- The recovery and development of mineral acids generated by pyrolysis are made difficult by the presence of this toxic "heavy" compound.

- A large quantity of soiled alcoholic effluents must be managed. As another purification method, mention may be made of the use of filter cartridges based on basic oxides described in US Pat. No. 5,300,714. This method deals with liquid flows (which is not very practical with C2F4 and

C3F6 because of the risk of explosion) and has a limited lifespan since based on the quantity of solid contained in the cartridge. This process cannot be used on an industrial scale.

Another solution proposed in US patent 5705719 allows the elimination of PFIB directly on the gas flow at the outlet of pyrolysis. This solution is of course much more advantageous for the treatment of flows in the rest of the process. It is based on the selective adsorption of PFIB on activated carbon with a large specific surface and large pore volume at temperatures below 100 ° C. This solution however requires a carbon regeneration phase which necessarily involves stopping production or using two adsorption columns in parallel.

[Brief description of the invention] We have now found a process for the selective conversion of PFIB (perfluoroisobutene) contained in a stream of halogenated hydrocarbons in which:

Either the said stream of halogenated hydrocarbons containing the PFIB is brought into contact with HF in the presence of a hydrofluorination catalyst to effect the addition of HF to the PFIB,

• either the said stream of halogenated hydrocarbons containing the PFIB is brought into contact with HCI in the presence of a hydrochlorination catalyst to effect the addition of HCI to the PFIB.

[Detailed description of the invention]

As regards the stream of halogenated hydrocarbons, these can be saturated or unsaturated. By way of example of unsaturated halogenated hydrocarbons, mention may be made of tetrafluoroethylene C2F4 and hexafluoropropene C3F5. Examples of saturated halogenated hydrocarbons that may be mentioned are octafluorocyclobutane C-C4F8. The conditions which cause the presence of PFIB in such streams are generally pyrolyzes and therefore these streams also contain heavy fluorinated products, ie compounds having 4 or more carbon atoms. It would not be departing from the scope of the invention if the stream contained inert products for a hydrofluorination or hydrochlorination reaction such as CO 2 or nitrogen. The invention is particularly useful for streams containing up to 10 mol% of PFIB, advantageously up to 5% and preferably up to 2%. The production of C2F4 and C3F5 generally comes from the pyrolysis of HCFC-22 or HFC-23.

The stream of halogenated hydrocarbons at the outlet of the pyrolysis of HCFC-22 therefore contains unconverted HCFC-22, HCl at a rate of one mole per mole of converted HCFC-22, C2F4, C3F6, 0.01 to 5 % molar of PFIB and heavy fluorinated products. The stream of halogenated hydrocarbons at the outlet of the HFC-23 pyrolysis therefore contains unconverted HFC-23, HF at the rate of one mole per mole of converted HFC-23, C2F4, C3F6, 0.5 to 5 Molar% of PFIB and heavy fluorinated products As regards contacting with HF or HCI in the presence of a catalyst, the temperature can be between 100 ° C. and 200 ° C. and preferably between 140 ° C and 180 ° C The molar ratio of HF or HCI to PFIB must be greater than 1 if we want to convert all the PFIB, higher ratios favor the reaction If the stream of halogenated hydrocarbons containing the PFIB comes from a pyrolysis as mentioned above so there is no need to add HF or HCI, it is enough to use the HF or HCI present. In addition, the quantity is largely sufficient. , the ratio of HF or HCI present on the PFIB is generally between 10 and 100 Advantageously the stream of halogenated hydrocarbons thus that the HF or the HCI are in the gaseous state and are brought into contact with the catalyst in the solid state. Any device known from the technique of so-called "fixed ht" reactors can be used. depends on the concentration of PFIB in the stream of halogenated hydrocarbons, in fact this conversion leads to a reduction in the number of moles and therefore according to thermodynamics the conversion is favored by a high pressure As regards quantities of PFIB less than 10 mol%, the pressure is not very significant, that of the halogenated hydrocarbon stream containing the PFIB is used, in fact a lower pressure because of the pressure drops in the reactor supply pipes and in the catalyst bed. performs this conversion of the PFIB between 1 × 10 ^** 5 Pa and 10 × 10 ^** 5 Pa absolute

As regards the hydrofluorination reaction, it is possible to use catalysts known to those skilled in the art such as bulk or supported catalysts (on fluorinated alumina or on carbon for example) based on Cr, Zn, Ni oxide , Mg alone or as a mixture For this invention, for example, mixed catalysts composed of oxides, halides and / or oxyhalides of nickel and chromium deposited on a support consisting of aluminum fluoride or a mixture of fluoride d aluminum and alumina such as described in patent FR 2669022. However, chromium (III) oxides with large specific surface areas (> 100 m ² / g measured by BET with nitrogen) are doped with transition metals or not, with these catalysts , such as for example the 1/8 "chromium oxide (E 410) tablets sold by the company Engelhard.

More advantageously, it is a Cr (III) / carbon type catalyst obtained by impregnating an active carbon with a solution of chromium III salt or a solution of CrC> 3 subsequently reduced. Preferably the activated carbon has a large surface area and a large pore volume. By large area activated carbon and large pore volume is meant a surface area carbon greater than 500 m ² / g measured by BET with nitrogen and whose total pore volume is greater than

1 cm ³ / g. By way of example of such coals, mention may be made of Norit®Rox 0.8.

Impregnation with a chromium III salt solution or a Crθ3 solution followed by a reduction is a known technique, it is described for example in patent EP 0456552. The catalyst is then conditioned to HF between 180 ° C. and 380 ° C and advantageously between 180 ° C and 300 ° C. The amount of chromium expressed as a mass percentage of chromium metal (in the impregnation solution) relative to the weight of coal is between

2 and 50% and advantageously between 5 and 35%. The reaction temperature can be between 100 ° C and 200 ° C and preferably between 140 ° C and 180 ° C. The contact time is advantageously between 5 and 30 s and preferably between 10 and 30 s. The contact time is defined according to the practice of reactions in a fixed bed, namely as the ratio between the volume of the part of the reactor which contains the catalyst (which corresponds to the apparent volume of the catalyst) over the volume flow rate of the gases under the conditions of reaction temperature and pressure. Very good results were obtained at 150 ° C with a contact time of 15 s.

In a conventional pyrolysis experiment of HFC-23 (700 at 1100 ° C) intended to produce C2F4 and C3F5, we find in the output products, in moles, from 0.5 to 2% of PFIB and one mole of HF per mole of converted reagent. We have found that if this stream is cooled and passed over a type catalyst. Cr (III) / carbon, the PFIB is more than 95% transformed into C ^ FgH. If the temperature remains below 200 ° C, only the PFIB is transformed and C2F4 and

C3F5 are not affected. It is recommended to stay above 140 ° C to maintain a total conversion of the PFIB. It is obvious, for reasons of reactor size, that the shorter the contact time, the more attractive the process. We were able to observe conversions greater than 95% up to 10 s of contact time. Below, the conversion drops quickly. For reasons of economic interest linked to the productivity of the process, we have not studied contact times greater than 60 s. However, it would not depart from the scope of the invention if we used longer contact times. It will be noted that for temperatures above 175 ° C, it is recommended to limit the contact time to 20 s to avoid touching the C2F4 and C3F6.

As regards the hydrochlorination reaction, the same catalysts and the same operating conditions are used as for the hydrofluorination reaction. Only HF is replaced by HCI each time HF is used in the process.

[Examples]

1 / Tests with addition of HF

In a 140 cm ^ Inconel reactor, 80 crrι3 of Cr catalyst (28% by weight) / charcoal prepared as indicated below are introduced: 250 cm ³ of Norit®Rox 0.8 charcoal (SBET≈ 900 m ² / g, density apparent = 0.39, total pore volume = 1.2 cm ³ / g) are impregnated with stirring with a solution composed of 52 g of Crθ3 per 70 ml of water. The impregnation is carried out in one hour and the catalyst is left under stirring for an additional hour. The catalyst is then dried under a stream of nitrogen for 3 hours at 120 ° C. Prior to the reaction, the catalyst is dried in the reactor under nitrogen flow for 2 hours at 150 ° C., then treated for four hours at 200 ° C. with 1 mol / h of anhydrous HF. After this treatment, the supply of HF is stopped and the temperature of the catalytic bed is brought back to 150 ° C. The reactor is then supplied with 0.23 mol / h of HF and 0.33 mol / h of a gaseous mixture containing approximately 60% of HFC-23, 20% of CF ₄ , 15% of C ₃ F ₆ and 1.2% of PFIB . This organic mixture comes from an HFC-23 pyrolysis experiment. The contact time (te) calculated at the reaction temperature according to the formula tc = V / D (V is the volume of catalyst and D the gas volume flow) is equal to 14.8 s. After 1 h and 3 h of reaction, the composition of the outlet gas is identical to the inlet composition, with the exception of the PFIB which has been entirely transformed into C4F9H.

We have studied the influence of critical parameters such as te and temperature. The results are summarized in the following table:

2 / Lifespan

In order to check the behavior of the catalyst, we carried out a longer test under conditions identical to the previous example. The results are summarized in the following table:

No deactivation of the catalyst is visible after 144 h of reaction. 3 / Continuous tests with HFC-23

In order to validate this experience under conditions close to an industrial process, we installed the fluorination reactor at the outlet of the pyrolysis reactor. Pyrolysis is carried out in an Inconel tube of 2.5 cm ³ at 1000 ° C with a contact time at the reaction temperature of 0.06 s. The flow rate of HFC-23 supplied is 1.5 mol / h. The composition of the outlet gas in organic products is 66% of HFC-23, 19% of C2F4, 11% of

C3F6 and 0.7% of PFIB. This gas as well as the co-produced HF directly feeds a 500 cm ³ Inconel reactor containing 256 cm ³ of catalyst. This catalyst, the composition and treatment of which have been described in the preceding examples, is maintained at 150 ° C. After 2 and 5 hours of operation, the output composition is identical to the input, with the exception of the PFIB which, apart from the experimental errors, has been entirely transformed into C4F9H.

4 / Continuous comparative test with coal only Pyrolysis is carried out in an Inconel tube of 2.5 cm ³ at 1000 ° C with a contact time at the reaction temperature of 0.06 s. The flow rate of HFC-23 supplied is 1.5 mol / h. The organic product composition of the outlet gas is 60% HFC-23, 21% C-2F4, 14% C ₃ F ₆ and 1.2% PFIB. This gas, as well as the co-produced HF, directly feeds an Inconel reactor

500 cm ³ containing 256 cm ³ of activated carbon Norit® RX3 extra (S _B E _T > 800 m ² / g, apparent density = 0.42, total pore volume = 1cm ³ / g) at 150 ° C, treated at 200 ° C under HF as described in the previous examples. The molar composition at the outlet after 1 h and 3 h of reaction is 60.2% of F23, 21.3% of CF ₄ , 14.1% of C ₃ F ₆ , 0% of PFIB and 0.2% of C4F9H.

Only a small part of the PFIB is transformed into C-4FgH, the rest being adsorbed on carbon. A regeneration step by vacuum treatment and / or temperature inert scanning is therefore necessary to desorb the PFIB. This regeneration penalizes the productivity of the installation and generates a flow containing PFIB, therefore toxic, which requires additional treatment.

5 / Tests with addition of HCI

80 cm ³ of Cr (28% by weight) / carbon catalyst prepared as indicated below are introduced into a 140 cm ³ Inconel reactor.

250 cm ³ of Norit®Rox 0.8 carbon (S _B ET = 900 m ² / g, apparent density = 0.39, total pore volume = 1.2 cm ³ / g) are impregnated with stirring with a solution composed of 52 g of Crθ3 for 70 ml of water. The impregnation is carried out in one hour and the catalyst is left under stirring for an additional hour. The catalyst is then dried under a stream of nitrogen for 3 hours at 120 ° C.

Prior to the reaction, the catalyst is dried in the reactor under a stream of nitrogen for 2 hours at 150 ° C., then treated for four hours at 175 ° C. with 1 mol / h of anhydrous HCl. After this treatment, the supply of HCI is reduced to 0.17 mol / h and 0.16 mol / h is also supplied with a gaseous mixture containing inter alia about 50% of HFC-22, 40% of C-2F4, 3% C3F6 and 0.5% of FBIP. This organic mixture comes from an HFC-22 pyrolysis experiment. The contact time (te) calculated at the reaction temperature according to the formula tc = V / D (V is the volume of catalyst and D the gas volume flow) equals 23.7 s. After 3 h of reaction, the composition of the outlet gas is identical to the inlet composition, except for the PFIB which has been transformed into C4F ₈ HCI at 80%.

Claims

1 Process for the selective conversion of PFIB (perfluoroisobutene) contained in a stream of halogenated hydrocarbons in which: "either the said stream of halogenated hydrocarbons containing the PFIB is brought into contact with HF in the presence of a catalyst of hydrofluorination to effect the addition of HF to the PFIB,

^B or the said stream of halogenated hydrocarbons containing the PFIB is brought into contact with HCI in the presence of a hydrochlorination catalyst to effect the addition of HCI to the PFIB.

2 The method of claim 1 wherein the stream of halogenated hydrocarbons containing PFIB also contains tetrafluoroethylene C-2F4 hexafluoropropene C3F6 and heavy fluorinated products.

3 Method according to any one of the preceding claims, in which the proportion of PFIB in the stream of halogenated hydrocarbons is at most 10% by moles and more advantageously less than 5%.

4 Process according to any one of the preceding claims, in which the stream of halogenated hydrocarbons as well as the HF or the HCl are in the gaseous state and are brought into contact with the catalyst in the solid state.

5 A method according to any preceding claim wherein the temperature of the selective conversion reaction of PFIB is between 100 ° C and 200 ° C.

6 The method of claim 5 wherein the temperature is between 140 ° C and 180 ° C. 7 Process according to any one of the preceding claims, in which the conversion of PFIB is carried out by hydrofluorination in the presence of a catalyst of the Cr (III) / carbon type obtained by impregnating an active carbon with a solution of chromium III salt. or a solution of C D3 reduced thereafter.

8 The method of claim 7 wherein the contact time is advantageously between 5 and 30 s and preferably between 10 and 30 s.

9 The method of claim 7 or 8 wherein (i) the stream of halogenated hydrocarbons comes from a pyrolysis of HFC-23 and contains unconverted HFC-23, THF at a rate of one mole per mole of HFC- 23 converted, C2F4, C3F6, 0.5 to 5 mol% of PFIB and heavy fluorinated products and (ii) that the HF which it contains is used for the PFIB conversion reaction.

10 Process according to any one of claims 1 to 6 in which the conversion of PFIB is carried out by hydrochlorination in the presence of a Cr (III) / carbon type catalyst obtained by impregnating an active carbon with a salt solution. chromium III or a solution of Crθ3 reduced thereafter.

11 The method of claim 10 wherein (i) The stream of halogenated hydrocarbons comes from a pyrolysis of HCFC-22 and contains unconverted HCFC-22, HCl at a rate of one mole per mole of HCFC- 22 converted, C2F4, C3F6, 0.01 to 5 mol% of PFIB and heavy fluorinated products and (ii) that the HCI which it contains is used for the PFIB conversion reaction.