FR2549039A1 - PROCESS FOR THE PREPARATION OF 3,3,3-TRIFLUORO-2-TRIFLUORO-MEHYLPROPENE - Google Patents

Abstract

THE INVENTION RELATES TO A PROCESS FOR THE PREPARATION OF 3,3,3-TRIFLUORO-2-TRIFLUOROMETHYLPROPENE. ACCORDING TO THE INVENTION, 1,1,1,3,3,3-HEXAFLUOROPROPANE-2,2-DIOL DIHYDRATE AND AN ORGANIC COMPOUND WHICH FORM CETENE BY THERMAL DECOMPOSITION IN A REACTIONAL TUBE AT TEMPERATURES BETWEEN 450 AND 650C. THE INVENTION APPLIES IN PARTICULAR TO THE CHEMICAL INDUSTRY.

Description

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  The present invention relates to a process for preparing 3,3,3-trifluoro-2-trifluoromethylpropene

(which will be called HFIB).

  HFIB is a useful compound as a material of certain polymers and also as intermediates for various organic fluorine compounds. Several types of processes are known for the preparation of HFIB A popular process involves the reaction between hexafluoroacetone and an organic compound which forms ketene by thermal decomposition, as described in the preliminary publication of the Japanese patent application. 50-142504 (1975) In practice, this method has a drawback which is the need to handle hexafluoroacetone which is a harmful gas. As another method, it is known to obtain HFIB from a lower alkyl ether of alcohol. octafluoroisobutylique in carrying out in succession a hydrolysis, a reduction and dehydration However, this process does not seem favorable for an industrial application because of the difficulty of obtaining the

  raw material and the complexity of the process.

  The subject of the present invention is a new industrially favorable method of preparation

of HFIB.

  A method according to the invention comprises the step

  heating 1,1,1,3,3,3-hexafluoropropane2,2-diol dihydrate (which will be called HFPDW) and an organic compound which forms a ketene by thermal decomposition in a reaction tube at temperatures between 450 ° C and 30 ° C. and 650 C.

  Preferably, the organic compound as a source of ketene is selected from acetic acid, acetic anhydride, acetone and a diketene. Each of these compounds decomposes to form ketene when heated in a reactor maintained at room temperature. 450-650 C If desired, two or more of these ketene-forming compounds may be used together As is known, ketene tends to react with water to form acetic acid. However, it has been found that in the reaction system according to the invention, the ketene reacted preferentially with the organic component of HFPDW although HFP Ei contains water molecules and that

  water is formed while the reaction is taking place.

  The mechanisms of the reaction in this process are assumed to be the following although this is not

not totally clarified.

o _ H Og

211/4 15 5

He o O H l 1, 3 (CF o-o H

At 2:20

H

U 1 CF H X -1

O; go CH 2 3 C il 0

2 H 20

HFPDW ketene

F 3 C \ / CF 3

C LH 2

+ C 02 + 3 H 20

  HFIB HFPDW (boiling point 106 ° C.) is easy to obtain and does not pose any problem for industrial handling. The vapor phase reaction according to the invention can easily be carried out by simple heating substantially at atmospheric pressure, without using any catalyst. By the process according to the invention, we obtain

HFIB at pretty good yields.

  The reaction gas leaving the reaction tube contains water and carbon dioxide as can be seen from the equation above. Water and any potential high boiling by-products, such as acetic acid, can be removed by passing the reaction gas through an air-cooled trap and then washing the gas with water Subsequently, the carbon dioxide can be removed by washing with alkali Preceded by these treatments, we obtain a HFIB crude liquid by condensation in a sufficiently cooled trap The crude HFIB contains 10 to 10 kinds of low-boiling organic compounds (such as 3,3,3-trifluoropropene)

  as unavoidable byproducts or impurities.

  Through theoretical analysis and experiments it has been confirmed that low boiling point impurities can be separated from HFIB by fractionation.

  at low temperature or pressure distillation.

  However, if a separation of many kinds of impurities with various boiling points is completely accomplished by distillation in industrial practice, there is a considerable loss of HFIB during the distillation operations and the loss of HFIB increases while the raw product contains larger quantities

  low boiling impurities.

  It has been discovered and confirmed that the low boiling point organic impurities present in the crude HFIB obtained by the synthesis process according to the invention can easily and effectively be removed by washing crude HFIB in the gaseous state with water. Concentrated Sulfuric Acid This sulfuric acid treatment is completely feasible because removal of impurities is accomplished without loss of HFIB. Thus, it has become possible to greatly improve the purity of HFIB before distillation for final refining. Another advantage of concentrated sulfuric acid is that water is introduced into crude HFIB gas during the previous washing operations using sequentially water and an alkali is removed together with the low boiling impurities. sulfuric acid can be achieved by a conventional gas scrubbing process The temperature of the concentrated sulfuric acid is It is not specified and the purpose of the washing is completely achieved by using the concentrated sulfuric acid left at room temperature. Fractional distillation of the washed product to obtain HFIB at very high purity can easily be

  accomplished by low temperature distillation or pressure distillation.

  In the synthesis process according to the invention, the raw materials are usually vaporized before introduction into the reaction tube. It is optional to use a vaporizer or several vaporizers in addition to the reaction tube or to use a section on the upstream side of the reaction tube. As a vaporizer The mixture of HFPDW with a compound employed as a ketene source can be accomplished by any of the following three methods, although these are not limiting in any way and that the synthesis of HFIB is not not

  influenced by the mixing mode of the reagents.

  (1) Initially, HFPDW and the ketene-forming compound are mixed in the liquid state. The liquid mixture is introduced into a vaporizer and the vaporized mixture is introduced into the reaction tube. This process is convenient for maintaining the proportion of the mixture. ketene-forming compound with constant HFPDW during a continuous process

of synthesis.

  (2) Using two vaporizers, HFPDW and the ketene-forming compound are vaporized separately.

  Then, the two vaporized compounds are mixed and

  introduced into the reaction tube.

  (3) Two reaction tubes are connected in series The ketene-forming compound is introduced into the first reaction tube which is maintained at a sufficiently high temperature for vaporization and optionally decomposition of this compound to form ketene,

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  example at a temperature between 450 and 650 C

  while HFPDW is vaporized in a separate vaporizer.

  At the outlet of the first reaction tube, the gas produced in the first reaction tube is mixed with vaporized HFPDW and the resulting gas mixture is introduced into the second reaction tube which is maintained at

450-6500C.

  In a process according to the invention, the proportion of the compound employed as a source of ketene to HFPDW is determined so that the molar ratio of ketene theoretically derived from this compound to HFPDW is understood.

  between 1: 1 and 5: 1, and preferably between 2: 1 and 4: 1.

  The reaction gas (containing the HFIB vapor) leaving the reaction tube is guided to a cooling trap, which is kept at a sufficiently low temperature, such as about -78 ° C. for the condensation of HFIB, by an air-cooled trap. A device for washing with water and a device for washing with alkali. Preferably, crude HFIB obtained in this way is washed with concentrated sulfuric acid while crude HIFIB is in the gaseous state, almost at room temperature, either before or after condensation in the cooling trap Washing can be carried out in a conventional gas washing tower such as a packed tower or a bubble tower Concentrated sulfuric acid concentration used for washing 'is not

strictly limited.

  During the washing operation, the acid concentration gradually decreases as the concentrated sulfuric acid absorbs organic impurities and water present in crude HFIB. In an experiment using 98 wt.% Concentrated industrial sulfuric acid, a decrease in the removal capacity of organic impurities from crude HFIB became appreciable at a sulfuric acid concentration lowered to about 70%

in weight.

  The material of the reaction tube must be

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  sufficiently resistant to heat so that the reaction tube can retain sufficient physical strength when heated to 650 C. Examples of suitable materials include glass, alumina, silica-based ceramics, copper and aluminum. brass. Steels comprising stainless steels and nickel and nickel alloys are not suitable for the reaction tube in the process according to the invention because these materials tend to promote the decomposition of HFPDW and / or the undesirable decomposition of the compound. used as a source of ketene, which becomes a cause of lowering performance and

purity of HFIB.

  The preparation of HFIB according to the invention will be better illustrated by the nonlimiting examples which follow.

EXAMPLE 1

  Using a dosing pump, a mixture of 220 g (1 mole) of HFPDW and 224 g (2 moles) of anhydride-acetic was introduced into a copper tube of 31 mm internal diameter and 450 mm in length at a constant flow rate of 13 g / h The space velocity was h-1. In the reaction tube, an upstream end section forming a third of the entire length was used as a vaporizer, so that the temperature in this section is maintained at 200-250 C In the remaining section used as reactor, the temperature

  The reaction tube was packed with ceramic Raschig rings having a diameter of 7 mm so that the fractional vacuum was 70%.

  A reaction gas was passed out of the reaction tube through an air-cooled trap, washed with water and then with alkali and then with water.

  concentrated sulfuric acid and then guided to a cooling trap maintained at -78.degree.

  While the operation was continued for about 22 hours, 283 g of the mixture of HFPDW and acetic anhydride was pumped into the reaction tube, and the total amount of the reaction products became 280 g (98.9%). The organic condensate in the cooling trap reached 93.2% and was found to be crude HFIB at 98.5% purity. At this stage, the HFIB yield was 87.8% based on of HFPDW introduced into the crude HFIB reaction tube was subjected to distillation to obtain 75 g of refined HFIB at a purity of 99.998%. EXAMPLE 2 Using a vaporizer maintained at 170 ° C., HFPDW was vaporized at a flow rate of 14.8 g / h. In a separate vaporizer maintained at 170 ° C., diketene was vaporized at a rate of 7.5 g / h. HFPDW and the vaporized diketene were directly introduced into a glass reaction tube 21.5 mm in internal diameter and 300 mm long, so that the molar ratio of

  diketene to HFPDW in the reaction tube of about 1.3: 1 The space velocity was 300 h -1 The temperature in the reaction tube was maintained at 550-570C.

  A reaction gas was passed from the reaction tube through an air-cooled trap, washed with water and then with alkali and then guided to a maintained cooling trap. C. The operation was continued for about 15 hours to introduce 220 g (1 mole) of HFPDW and 112 g (1.33 moles) of diketene into the reaction tube. The total amount of the reaction products became 322 g ( 97% recovery) The organic condensate in the cooling trap reached 172 g and was found to be crude HFIB at 80% purity. At this stage, the HFIB yield was 84% based on IHFPDW.

subjected to a reaction.

  The crude HFIB was allowed to return to gaseous state at 20 ° C and in that state it was passed through a wash bottle containing 300 g of concentrated sulfuric acid. 137 g of organic material containing 98.3% of HFIB Indeed, practically

  no loss of HFIB accompanied the washing of crude HFIB gas with washed crude HFIB sulfuric acid was subjected to pressure distillation at 2 bar.

  The temperature was 80 C. As a result 115 g were obtained.

  of refined HFIB at a purity of 99.9998%.

EXAMPLE 3

  HFIB was synthesized and purified generally according to Example 1 but using 4 moles of glacial acetic acid per mole of HFPDW in place of the acetic anhydride of Example 1 In this case,

  Spatial velocity in the reaction tube was 100 h -1.

  The operation was continued for about 6.6 hours to introduce 36.9 g of the mixture of HFPDW and acetic acid into the reaction tube. The total amount of the reaction products became 35.2 g (95% recovery). 3%) The organic condensate in the cooling trap reached 8.7 g and was found to be

  The HFIB yield was 98.7% pure. Therefore, the HFIB yield was 65.6%.

EXAMPLE 4

  The procedure of Example 2 was repeated until the condensation stage in the cooling trap (-78 ° C.), generally in the same manner except that 4 moles of carbon were used. acetone per mole of HFPDW in place of the diketene of Example 2 and that the space velocity -1 was 180 hrs-A. The operation was continued for about 5.3 hours to introduce 25.1 g of the HFPDW mixture. and acetone in the reaction tube The amount of reaction product collected was 17.5 g (69.8% recovery) because a considerable amount of the gaseous material remained uncondensed to the cooling trap and exited. recovery and purification circuit. The organic condensate in the cooling trap reached 9.6 g and was found to be crude HFIB with a purity of 71.9%. Therefore, the yield of HFIB

was 76%.

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Claims (9)

R E V E N D I C A T IO N S   Process for the preparation of 3,3,3-trifluoro-2-trifluoromethylpropene, characterized in that it comprises   the step of heating 1,1,1,3,3,3-hexafluoro-propane-2,2diol dihydrate and an organic compound which forms ketene by thermal decomposition in a reaction tube at temperatures between 450 and 650C.   2. Process according to claim 1, characterized in that the abovementioned organic compound is selected from the group consisting of acetic acid,   acetic anhydride, acetone and diketene.   3. Method according to claim 2, characterized in that the proportion of said organic compound to said dihydrate is such that the molar ratio of ketene theoretically derived from said compound   dihydrate is between 1: 1 and 5: 1.   4. Method according to claim 3, characterized in that the aforementioned molar ratio is between 2: 1 and 4: 1.   Process according to claim 1, characterized in that the heating in said reaction tube is carried out substantially at atmospheric pressure. 6. Method according to claim 1, characterized in that it further comprises the step of vaporizing the aforementioned dihydrate and the organic compound   above before the heating step.   7. Process according to claim 1, characterized in that it further comprises the step of washing a crude product obtained in the heating step with concentrated sulfuric acid while said raw product is in the state. gaseous to remove organic byproducts. 8. Process according to claim 7, characterized in that the above-mentioned washing step is 2549039   accomplished at room temperature.   9. The method of claim 8, characterized in that it further comprises the step of washing the crude product with water and then with an alkali before washing with concentrated sulfuric acid.