FR2690686A1 - Vinylidene fluoride mfr. - by co-pyrolysis of the fluorination prod. stream obtd. by gas phase reaction of vinylidene chloride and hydrogen fluoride - Google Patents

Abstract

Mfr. of vinyldene fluoride (I) comprises the stages of (a) continuously feeding gaseous HF and gaseous vinylidene chloride (VDC) into a first reactor maintained at a temp. of 22 - 300 deg.C. and contg. a Lewis acid catalyst, to give a prod. stream of fluorination prods. contg. 1-chloro-1,1-difluoroethane (142b), 1,1-dichloro-1-fluoroethane (141b), 1,1,1-trifluoroethane (143a) and HCl, (b) continuously feeding the prod. stream from (a) into a second reactor maintained at 200-700 deg.C. and (c) continuously withdrawing (I) from the second reactor. The catalyst in step (a) is pref. of SnF4 on an active carbon support. The reaction prods. from stage (a) pref. have a residence time in the second reactor of 1-30 sec.. An O2-contg. gas in an amt. of 4-7 mole % or chlorine gas in an amt. of 0.1-4 mole % based on the amt. of vinylidene chloride introduced may be fed into the second reactor to increase the yield of (I). USE/ADVANTAGE - The copyrolysis of the fluorination prods. from the first stage in the prsence of HCl exhibits good selectivity to the prodn. of vinylidene fluoride.

Description

 The present invention relates to the production of vinylidene fluoride (VDF) from vinylidene chloride (VDC), more particularly a process in which the fluorination products generated in the catalytic hydrofluorination of VDC in the gas phase are converted into VDF by co-pyrolysis .

 Vinylidene fluoride (VDF) is a monomer useful in the preparation of fluorocarbon polymers which are endowed with excellent properties of resistance to atmospheric factors and chemical agents. VDF is usually produced on an industrial scale by fluorinating 1,1,1-trichloroethane to form crude 1,1-difluoro-l-chloroethane ("142b"), collecting the crude 142b by distillation, then by dehydrochlorinating the crude 142b to form VDH. Attempts have been made to directly fluorinate the VDC into VDF in the presence of a catalyst consisting of anhydrous A1F3 or of the association of AlF3 and a transition metal fluoride (patent No. 4,827,055), but the short catalytic life of the preferred associated catalyst (a few hours) makes the process economically unattractive.

Other attempts to convert VDC directly to VDF have been reported, with the use of HF (in the gas phase) when the catalyst is a chromium salt or a combination of A1F3 and an "activator" chosen from La (NO3) 3, NH4V03, and SnCl2 (United States patent 3,600,450), or with the use of aqueous HF and an alumina catalyst coated with Cr2O3 and Nio (United States patent America No. 4,147,733).

Several processes have also been reported for the conversion of pure 142b into monomeric VDF [for example US Pat. Nos. 3,246,041 and 3,118,005
G. Huybrechts et al., Intl. J. Chem. Kinet., 17, 157-165 (1985); and H. Muller et al., Chem. Ind.

Tech. 56 (8), 626-628 (1984)].

The present invention is based on the pyrolysis of impure 142b together with the other fluorination products which include l, l-aichloro-l-fluorethane ("141b"), 1,1,1-trifluoroethane ("143a") and hydrogen chloride ("HCl"). The presence of HCl makes the result of this process unpredictable. For example, it is known that HC1 can be easily oxidized with air to form chlorine gas [CN Satterfield, Heterogeneous Catalysis in
Practice, McGraw Hill (1980), 206-208], after which the generated chlorine can be used to chlorinate the CH3 group of 142b to form l, l-difluoro-1,2-dichloroethane [M.

Hudlicky, Chemistry of Organic Fluorine, Compounds, Ellis
Horwood Limited (1976) 218]. Furthermore, the pyrolysis of 141b in the presence of HCl produces mainly, as is known, 1-chloro-1-fluorethylene (United States Patent No. 2,894,043) and it has been found that pyrolysis of 143a in the presence of HCl gave only modest yields of monomeric VDF (patent application simultaneously pending in the name of the same Applicant, filed on the same date as this application, registration number IR 3247). could not expect that pyrolysis of impure 142b would be accomplished with good selectivity toward the monomer.

 The invention provides a process for the production of vinylidene fluoride from vinylidene chloride, in which (a) VDC and hydrogen fluoride ("HF") are continuously charged in the gas phase in a first reactor maintained at a temperature of about 22 to about 300 "C in the presence of a catalyst of the type of a Lewis acid fixed or not on a support to form a stream of fluorination products containing 142b, 141b, 143a and HC1, (b) the product stream from (a) is continuously charged in a second (pyrolysis) reactor maintained at a temperature of about 200 to about 700 "C and (c) VDF is continuously discharged from the second reactor. A preferred Lewis acid catalyst is SnF4 attached to activated carbon. Selectivity to monomeric VDF can be improved by the simultaneous supply of oxygen and chlorine gas to the second reactor.

The Applicant has just highlighted the unexpected fact that the impure fluorinated products of the hydrofluorination in gas phase of VDC catalyzed by an acid of
Lewis could be co-pyrolyzed giving good yields of monomeric VDF. In addition, since the preferred catalyst based on SnF4 on activated carbon can be used continuously for many days, there is now created an economical route for the continuous production of VDF from
VDC.

In the hydrofluorination stage, the HF and the VDC are continuously charged in the gas state in a first reactor charged with a Lewis acid catalyst fixed or not on a support and generally maintained at a temperature ranging approximately from the room temperature (22 "C) to about 300" F, preferably about 50-150 "C in order to selectively obtain fluorination products containing mainly 142b and HCl. In general, HF and VDC contact in a molar ratio of from about 1: 1 to about 10: 1, preferably from about 1: 1 to about 3: 1, especially from about 2: 1 to 3: 1. Normally, the reactants are maintained in contact for about 1 to about 150 seconds, preferably for about 5 to about 60 seconds. The reaction is generally carried out at atmospheric pressure in the presence of a suitable catalyst based on Lewis acid fixed or not on a support. The catalyst can include, for example, salts (preferably chlorides) of the following: bismuth (III), tin (IV), antimony (V) and arsenic (III). The chlorides are converted to the corresponding fluorides by activation with hydrogen fluoride. It is also possible to use boron trifluoride, KBF4, HBF4, B203, B (OH) 3, aluminum fluoride (for example in the form of fluorinated gamma aluminum oxide), etc. The catalyst can be used directly or it can be fixed on a suitable support such as activated carbon or gamma aluminum oxide. Such catalysts fixed on a support can be used for example in the form of pellets or granules . Tin (IV) salts, preferably SnCl4, are particularly useful. During the activation by HF of activated SnCl4 fixed on a support (for example by heating in a stream of nitrogen at 50 "C to expel the air from the reactor, then passage of a charge of HF and nitrogen gas on the catalyst until it is fluorinated), the catalyst formed from
Resistant solid SnF4 remains strongly adherent to the carbon support without it being leached from the catalyst bed. The catalyst remains active for several days of continuous implementation of the process. Boron trifluoride deposited on fluorinated gamma aluminum oxide also constitutes a particularly useful catalyst for this process. The hydrofluorination process can be implemented in the presence or in the absence of a gaseous vehicle. If a gaseous vehicle is used, the preferred gas is nitrogen, in amounts of about 1 to 100% by volume based on the volume of the VDC feed.

Although a continuous process has been described, the hydrofluorination process can also be carried out on a discontinuous or semi-continuous basis. In a continuous operation, HF and VDC are normally passed through a tubular reactor and the stream of fluorination products (comprising 142b, 141b, 143a, and HCl) is transferred to a second tubular reactor for co-pyrolysis . Reactor construction materials are not critical, except that they must have the structural and physical characteristics necessary to withstand the reaction conditions. The percentage of conversion of VDC to fluorine products is normally equal to or greater than 99.9 t.

The resulting halocarbon products are mainly formed from 142b, with smaller amounts of 143a and 141b. The molar ratio of hydrogen chloride to 142b which usually exists is between 1: 1 and 5: 1, preferably between 1: 1 and 2: 1 (one mole of HCl being generated for each mole of 142b and two moles of HCl being generated for each mole of 143a).

 In the co-pyrolysis step, the stream of fluorination product leaving the first reactor is continuously charged in the form of gas in a second reactor generally maintained at a temperature of about 200 to about 700 "C, preferably about 400 to about 600 "C and for a contact time of about 0.1 0 100 seconds, preferably about 1 to about 30 seconds to reach the maximum selectivity with respect to the monomeric VDF (as represented by the example 3 below). Selectivity towards VDF is also improved by the arrival in the second reactor of small catalytic quantities of oxygen or of chlorine gas in conjunction with the product stream coming from the first reactor. If chlorine is used, the latter is generally present in an amount ranging from about 0.1 to about 4 mole% based on the amount of VDC loaded, preferably from about 1 to about 3 t and especially from about 1-2 t. When oxygen is used, it is generally present in an amount ranging from about 1 to about 10 mole% (based on the charged VDC), preferably from about 4 to about 7 t. Again, although a continuous operation has been described, co-pyrolysis could be accomplished as a batch or semi-continuous process.

Using the optimal conditions, the process can be made highly selective with respect to VDF.

The monomer can be collected at the outlet of the second reactor by known techniques. For example, the
HCl and HF formed as by-products can be eliminated by passing the products at the outlet of the second reactor in a backwash washing tower with an alkaline current consisting for example of an aqueous solution of a hydroxide such as l potassium hydroxide, sodium or calcium, normally a 20% KOH solution. The washed product can then be sent to a drying tower, packed with a suitable dehydrating agent such as anhydrous calcium sulfate, to remove water, and the organic materials are separated by distillation under high pressure. Incompletely fluorinated by-products can be recycled to the fluorination reactor. Another variant consists in eliminating the HF before washing for recycling to the fluorination reactor.

 In the examples, the stream of pyrolysis products was purified and dehydrated and then analyzed using a gas chromatograph. The optimal conditions are not used in all the examples, which are mainly intended to illustrate the effects produced on the co-pyrolysis reaction by the presence or absence of oxygen and chlorine gas, and by the temperature variations. and duration of contact.

EXAMPLE 1 Effect of Aqueous Oxygen
HF and VDC were charged in gaseous form in a molar ratio of 2.2: 1 in a first reactor at 75 "C for 52 seconds on a catalyst consisting of SnF4 fixed on activated carbon (prepared by activation with HF of SnCl4 on activated carbon). After washing and dehydration, the cg analysis (gas chromatograph) showed a conversion rate of more than 99.6% of the
VDC in fluorinated products and a composition comprising (in mole%), 92.6% of 142b, 1.4% of 141b and 6% of 143a; the molar ratio of HCl to 142b was therefore about 1.1: 1. The product stream leaving the first reactor was passed through the pyrolysis reactor for 53 seconds at 575oC using various proportions of oxygen between 0 and 5.7 mole% (based on the loaded VDC). Analysis of the products by gas chromatograph gave the following results
2% VDF 142b 141b 143a Others
0 14.4 0.8 1.2 69 14.7
2.8 46.9 0.6 2.0 32 19.4
5.7 55.3 0.5 2.7 21 20.1
Example 2 Effect of the temperature in the reactor
pyrolysis
In this example, the conditions were the same as in Example 1, except that the oxygen charge in the second reactor was maintained at 5.7 mole% and that the contact time in the second reactor was 40.6 seconds), but that the temperature was varied in the two tests. The results of gas chromatography are shown below and demonstrate that the selectivity with respect to VDF increases with temperature.

T (C) VDF 142b 141b 143a Others 550 29.3 0.4 6.0 37.3 24.1 600 47.6 0.3 1.6 37.1 13.5
Example 3 Effect of the contact time in the pvrolvse reactor
The procedure of Example 1 was repeated (without oxygen; nitrogen gas was charged to the pyrolysis reactor in an amount equal to about 26.6% by volume of the entire charge) except that varied the duration of contact. The CG results are shown below and indicate that a contact time of less than 30 seconds greatly increases the selectivity towards VDF.

Time (s) VDF 142b 141b 143a Others
57.6 17.4 0.8 1.2 69 11.7
42.3 25.3 2.4 10.2 42.8 19.3
28.2 84.4 8.2 0 6.1 1.3
Example 4 Effect of Chlorine on the Pyrolysis Reaction
The hydrofluorination was carried out as in Example 1, with the difference that the molar ratio of HF to VDC was increased 2.8: 1 and that the contact time was 48 seconds. The GC analysis showed a 99.9% conversion rate of the VDC into fluorine products consisting of 62.8% of 142b, 32.6% of 141b and 4.6% of 143a; the molar ratio of HCl to 142b was approximately 1.1: 1. By pyrolysis for 35 seconds at 550 ° C. in the presence and in the absence of chlorine gas, the following results were obtained
Cl% VDF 142b 141b 143a Others
0 41.4 24.6 7.8 6.1 20.1 2.87 62.5 3.8 1.5 2.6 29.6
Example 5 - Effect of temperature and residence time
in the first reactor
The fluorination reaction of Example 4 was repeated except that the temperature was raised to 135 ° C and the contact time was lowered to 41 seconds. Analysis showed a conversion rate of 99, 9% and that the fluorinated products comprised 65.1% of 142b, 19.8% of 141b and 15.1% of 143a; the molar ratio of HCl to 142b was approximately 1.5: 1. By pyrolysis for 35 seconds at 550 "C in the presence of chlorine gas, the following results were obtained
Cl% VDF 142b 141b 143a Others 2.87 73.8 4.8 1.5 8.9 11.0
EXAMPLE 6 Lifespan of the Catalyst
The procedure of Example 5 was repeated with the difference that the operation was carried out continuously for one week. The analyzes of the pyrolysis products obtained at the start and at the end of the week are reproduced below and indicate that the catalyst Sn / C remained active for several days.

Time VDF% 142b 141b 143a Others
Start 73.8 4.8 1.5 8.9 11.0
Weekend 51.0 8.3 1.8 8.4 30.6

Claims (6)

 1. A process for producing vinylidene fluoride, characterized in that it comprises the stages consisting (a) of continuously charging gaseous hydrogen fluoride and gaseous vinylidene chloride in a first reactor maintained at a temperature of approximately 22 "C at around 300" C in the presence of a catalyst based on Lewis acid to obtain a stream of fluorination products containing 1 chloro-1,1-difluoroethane, 1,1-dichloro-1- fluorethane, 1,1,1-trifluoroethane and hydrogen chloride, (b) continuously loading the product stream from step (a) into a second reactor maintained at a temperature of about 200 to about 700 "C and (c) continuously discharging vinylidene fluoride from the second reactor.  2. Method according to claim 1, characterized in that the catalyst of step (a) comprises SnF4 attached to activated carbon.  3. The method of claim 1, characterized in that the product stream from step (a) passes through the second reactor in a period of about 1 to about 30 seconds.  4. Method according to claim 1, characterized in that oxygen-containing gas is continuously charged into the second reactor in an amount equal to about 4 to about 7 mole%, based on the amount charged with vinylidene.  5. The method of claim 1, charac terse in that chlorine gas is continuously charged into the second reactor in an amount equal to about 0.1 to about 4 mole% based on the charged amount of vinylidene chloride.  6. A method of producing VDF, characterized in that it comprises the steps consisting (a) of continuously charging a gas mixture of 142b, 141b, 143a and HCl in a reactor maintained at a temperature of about 200 C at about 700 "C for about 0.1 to about 100 seconds and (b) continuously discharging the VDF from the reactor.