WO2011157907A1

WO2011157907A1 - Method for preparing trifluoroethylene

Info

Publication number: WO2011157907A1
Application number: PCT/FR2011/000314
Authority: WO
Inventors: Dominique Guillet; Michel Devic; Gérard Guilpain; Thierry Lannuzel
Original assignee: Arkema France
Priority date: 2010-06-15
Filing date: 2011-05-27
Publication date: 2011-12-22
Also published as: FR2961203B1; FR2961203A1

Abstract

The invention relates to a method for manufacturing trifluoroethylene F1123 (CF₂=CHF), including at least one step consisting of reacting R-134a (CF₃-CFH₂) with a metal hydroxide, preferably an alkaline or alkaline-earth hydroxide. The reaction is carried out either in the liquid phase, preferably with an alkaline hydroxide, or in the gaseous phase with a solid reactant, preferably an alkaline-earth hydroxide, at a temperature of between 120 and 450°C and at a pressure of between 1 and 10 bar absolute.

Description

Process for the preparation of trifluoroethylene

FIELD OF THE INVENTION

The invention relates to a process for the preparation of trifluoroethylene F1 123 (CF ₂ = CHF), generally called TrFE or VF3 from HFC-134a, tetrafluoroethane of formula CF ₃ -CFH ₂ , also called R-134a in the following of text.

BACKGROUND

Hydrofluorocarbons (HFCs) and in particular hydrofluoroolefins (HFOs) are compounds known for their properties as coolants and heat transfer fluids, fire extinguishers, propellants, foaming agents, blowing agents, gaseous dielectrics, polymerization medium or monomer, carrier fluids, agents for abrasives, drying agents and fluids for power generation unit. Unlike CFCs (chlorofluorocarbons) and HCFCs (hydrochlorofluorocarbons), which are potentially harmful to the ozone layer, HFOs do not contain chlorine and therefore do not pose a problem for the ozone layer.

Trifluoroethylene is particularly known as a monomer or comonomer for the manufacture of fluoropolymers having remarkable characteristics, excellent chemical resistance, good thermal resistance and electrical properties, electroactive polymers (piezoelectric, pyroelectric, optoelectric, electrostrictive)

There is a whole range of processes for the synthesis of trifluoroethylene using as raw materials CFCs. For example, the reaction of CFC 113 (CCI ₂ F-CCIF ₂ ) with hydrogen in the presence of a catalyst can be mentioned. The synthesis is based on a gas phase process at a temperature between 250 and 500Ό. Nickel-bearing catalysts, nickel-chromium mixed oxides, and supported rhodium may be mentioned as catalysts. This path has been widely explored and developed, in particular by the companies DuPont and Solvay. The synthesis of trifluoroethylene can be compared by hydrogenolysis of CFC 113 (chlorotrifluoroethylene) generally referred to as CFTE, in the presence of catalysts based on Group VIII metals of the periodic table such as platinum or palladium.

To a large extent, the processes for producing hydrotrifluoroethylene use a dehydrohalogenation reaction of HCFCs and HFCs. In this regard, mention may be made of DuPont US Pat. No. 5,750,808, which aims to dehydrohalogenate HCFC-133 (CFH ₂ -CF ₂ CI) on sodium-cesium zeolite catalysts. However, the most important work in this area has been the dehydrohalogenation of R-134a (CF ₃ - CFH ₂ ).

French patent BF 2710 054 (1993) in the name of the applicant describes a process for the manufacture of TrFE in which R-134a is passed, diluted in nitrogen, at a high temperature of between 400 and 600 ° C., on a catalyst. based on aluminum fluoride.

PCT Patent Application WO 97/29065 (ICI) describes the synthesis of TrFE by pyrolysis of R-134a at high temperature, 900 to 1200Ό, in the presence of steam and in the absence of any catalyst.

PCT Patent Application WO 2009/010472 A1 (Solvay) describes the synthesis of TrFE from R-134a by passing the latter, diluted in nitrogen, onto a type AIF ₃ catalyst deposited with a support having a high specific surface area. (100 to 300 m ² / g) at a temperature between 300 and 500 ^* 0.

Other patents, such as, for example, US Pat. No. 6,031,141 or EP 0406748, which implement catalysts comprising Lewis acids, may also be mentioned.

It is essential for the industrial developments to be able to have low-cost TrFE. The cost of access to the raw material is an important item. The choice of the latter is therefore essential. For the manufacture of TrFE, R-134a has the advantage over other compounds of either CFC or HCFC, the advantage of being widely available and therefore offered at a reasonable price.

The processes for synthesizing TrFE from R-134a described in the prior art have drawbacks. In fact, the processes using very high temperature pyrolysis, 1000 and more, besides the fact that they are expensive in energy, can be conducted only under difficult conditions and lead to low conversion yields while driving. the formation of many impurities (low selectivity). Catalytic dehydrohalogenation processes have the disadvantage of using expensive noble metals whose regeneration is also very expensive for relatively low conversion rates.

The object of the present invention is to overcome the above disadvantages by proposing a method for synthesizing TrFE from R-134a that avoids the use of catalysts.

The subject of the present invention is therefore a method for producing trifluoroethylene F1123 (CF ₂ = CHF) comprising at least one step consisting of a dehydrofluorination reaction of R-134a (CF ₃ -CFH ₂ ) with a metal hydroxide.

This metal hydroxide will preferably be based on alkali or alkaline earth metals.

The alkali or alkaline earth hydroxides may generally react on R-134a according to the reaction scheme below. Among the alkali hydroxides that can be used in the process, mention may in particular be made of LiOH, NaOH and KOH. NaOH and KOH, and more preferably KOH, are preferably selected. Among the alkaline earth hydroxides that can be used in the process, mention may especially be made of Mg (OH) ₂ and Ca (OH) ₂ . We will preferably choose Ca (OH) ₂ .

The overall reaction is as follows.

CF ₃ -CFH + 1 / n M (OH) _n - * CF ₂ = CFH + 1 / n MF _n

where M represents the metal of the hydroxide used and n is the number representing its valence, ie 1 for the alkalis and 2 for the alkaline earths.

The process of the invention may be conducted in "liquid phase" or "gaseous" phase. By reaction in liquid phase it should be understood that the reaction is conducted in the presence of a metal hydroxide, preferably alkali or alkaline earth, contained in a liquid medium even if the fluorinated reagent is in the reaction medium in the gas phase. This hydroxide will be either i) as an aqueous solution, or ii) in the molten state. By reaction in the gas phase it should be understood that the reaction is conducted in the presence of a metal hydroxide, preferably alkali or alkaline earth metal, in solid form.

The choice of the implementation route, liquid phase or gaseous phase, depends on the nature of the hydroxide used and thermodynamic conditions, including temperature, used for the implementation of the process. The choice of type of hydroxide and operating conditions will be dictated by the economic conditions and by a conduct as easy as possible in terms of process.

The liquid phase reaction comprises contacting the R-134a gas with the liquid containing the hydroxide. The hydroxide content of the liquid medium, which will be either a solution or molten hydroxide in the case where the reaction temperature is substantially higher than its melting point, is generally between 20 and 99% by weight. and preferably between 50 and 96%. The temperature of the mixture is maintained between 120 and 400 ° C and preferably between 250 and 350 ° C. The reaction is conducted at a pressure between 1 and 10 bars absolute.

For the liquid phase reaction, the use of the alkali metal hydroxides is preferred because of their high solubility in water. Among the alkali hydroxides the choice is made for cost reasons on soda or potash, potash being particularly preferred because it gives rise to the formation of hydrates which have a relatively low melting point.

According to one embodiment of the method of the invention in the liquid phase, the mixture of water and hydroxide can be obtained from the hydrates of the hydroxides. In the case of potash, it corresponds to the formula KOH, xH ₂ 0, x being between 1 and 2 inclusive.

As with any chemical reaction involving kinetic factors and thermodynamic factors, the contact time between the reagents is an important parameter. Indeed, the contact time must be sufficient to allow a good conversion (converted R-134a content) to be obtained without being too long to avoid a degradation of the olefin that, like any olefin, is sensitive to high temperatures. which can cause its decomposition and the formation of tars (heavy) and light ones.

For processes in the liquid phase with potassium hydroxide in particular, the contact time parameter between the gas molecules and the reagent within the liquid will be related to the dispersion of the gas bubbles in the liquid reagent, their average size and their reaction time. stay in the liquid. This residence time and the size of the bubbles can be adjusted by the gas injection speed, hourly flow per unit volume, and the stirring force applied to the liquid medium. 00314

5

The reaction with an alkaline hydroxide produces, in addition to the TrFE, an alkaline fluoride (such as KF, NaF, LiF, etc.). In an alternative embodiment of the process, the alkaline fluoride produced may, in a subsequent step, be contacted with calcium hydroxide to regenerate the alkaline hydroxide, then reusable, and precipitate calcium fluoride, the latter product having the advantage of being easily recoverable as a raw material for the synthesis of HF for example. The dehydrofluorination reaction in the liquid phase can be carried out in any type of reactor known to those skilled in the art. A stirred reactor, a static mixer, a reactive column, a nozzle or simply bubbling R-134a into the liquid, for example the mixture of water and hydroxide, can be used.

The dehydrofluorination reaction in the gaseous phase in the presence of a solid reagent is generally carried out at a temperature such that the water formed during the reaction is removed, partly or wholly, from the reaction medium by entrainment with the flow. gas from the reactor comprising TrFE. This temperature is between 150 and 450 ° C, and preferably between 300 and 420.

The dehydrofluorination reaction in the gas phase can be carried out at atmospheric pressure, but it is also possible to work at a pressure greater than atmospheric pressure, generally between 1 and 10 bar absolute and preferably between 1 and 4 bar.

The solid hydroxide will be used in powder form or preferably in the form of granules whose granulometry will be chosen to give the solid reagent a high porosity.

The gas-solid reactions are favored by a high porosity of the solid reagent which makes it possible to increase the contact surface between the gaseous reagent and the solid hydroxide reagent and thus increase the conversion for the same contact time. However, the search for porosity that is too high can lead to problems with the process, for example due to the attrition of the granules leading to an increase in pressure drops.

The preferred solid reactant in this embodiment of the process is formed from calcium hydroxide. Its content of Ca (OH) ₂ is at least 67% by weight. In addition, this solid reagent may also contain water in substantial proportion by weight. T FR2011 / 000314

6 example of 1 to 20% by weight, this water content being particularly related to the strong hygroscopic nature of the hydroxide.

In this preferred variant of the process according to the invention in the gaseous phase, the solid reagent contains, for the most part, calcium hydroxide and will advantageously also comprise other alkali metal and / or alkaline earth metal hydroxides, such as NaOH, KOH and g (OH). ₂ and Ba (OH) ₂ .

Soda lime (Soda lime), which is calcium hydroxide at a content generally of between 80 and 95% by weight and containing a few percent by weight of NaOH and KOH, can advantageously be used as solid reagent.

The KOH content in the solid reactant is preferably between 0.1 and 5% by weight and advantageously between 2 and 4% by weight.

The NaOH content in the solid reactant is preferably between 0.1 and 5% by weight and advantageously between 1 and 3% by weight.

The amount of water (moisture content) present in the solid reagent is preferably between 1 and 20% by weight and advantageously between 7 and 20% by weight and more particularly between 10 and 15% by weight.

The solid reagent, comprising calcium hydroxide, may be used in powder form or preferably in the form of granules giving the reagent a high porosity. A granulometry of granules of between 1 and 10 mm is preferred.

As an example of solid reagent, mention may be made of soda lime, in particular that marketed by the company GRACE under the trademark SODASORB HP. This preferred variant of the process of the invention is preferably carried out at a temperature of between 150 and 450 ° C., before advantageously between 300 and 420 ° C. The dehydrofluorination reaction, according to this variant, can be carried out at atmospheric pressure, but it is also possible to work at a pressure greater than atmospheric pressure, for example between 1 and 10 bar absolute.

The reaction in the gaseous phase can be carried out according to the usual means used in the industry to carry out a reaction between a gas and a solid, for example in a fluidized bed reactor with the solid reagent in fine powder or in a fixed bed reactor or in a rotary reactor of the cement kiln type with the solid reagent in the form of a porous granule. The process is preferably carried out semi-continuously in one or more fixed bed tubular reactors operating alternately, for example according to the technology used for the treatment of gases by molecular sieves. After reaction, the solid reagent partially or completely converted into calcium fluoride is discharged from the reactor. Calcium fluoride, after possible separation of the unreacted calcium hydroxide, can be used as raw material in the manufacture of hydrofluoric acid.

The process according to the present invention may comprise the recycling of the gaseous reactants from the unreacted reactor to the dehydrofluorination stage, the separation being able to be carried out during a distillation stage, or by simple preferential condensation, facilitated by the large difference in boiling point between R-134a and TrFE. This recycling makes it possible at the same time to increase the overall yield of the conversion of R-134a into TrFE while allowing the reaction medium to be maintained at the temperature level defined for the reaction by "absorbing" the heat produced during the exothermic synthesis reactions. .

The process according to the present invention can be carried out in the presence of a reaction-inert gas, for example nitrogen or helium.

It may be advantageous to carry out the reaction by introducing into the reaction zone a stream of water vapor independently of the water produced by the reaction or which may come from the solid reagent.

One of the advantages of the method of the invention is that it can be conducted in a continuous or semi-continuous mode.

EXPERIMENTAL PART Example 1 In a tubular reactor 800 mm long and 43 mm in diameter heated to 280 ° C. by an electrical resistance, 645 g of a soda lime of the following composition by weight of 3.5% NaOH are charged; KOH 0.09%; H ₂ O 1, 2%; Ca (OH) ₂ 95.2% in the form of granules with a particle size of between 2.4 and 4.8 mm. The reactor is passed through a gaseous stream of 0.7 mol / h of HFC-134a. The temperature under reactants in the reactor reaches 300 ° C. At the outlet of the reactor, the Water vapor is condensed in a water-cooled condenser and the reaction gases are dried on molecular sieve. At the outlet of the drying column, a sampling makes it possible to analyze the composition of the gases resulting from the reaction.

Their content analyzed by GPC on an SP1000 column in isotherm at 70 ° C. is 3 mol% in TrFE and 96.9% in R-134a.

Example 2

In the same apparatus as Example 1, 672 g of soda lime of the following composition are charged by weight 1% NaOH; 2% KOH; H ₂ O 13.5%; Ca (OH) ₂ 83.5% in the form of granules with a particle size of between 2.4 and 4.8 mm

The reactant flow rate at the inlet of the reactor is 0.7 mole / h of HFC-134a and 0.24 mole / h of an inert gas (nitrogen).

The tube is heated to 330 ° C. The reactor reagent temperature reaches 383 ° C and the reaction gases are analyzed at the dryer outlet. Their content is 9.4 mol% in TrFE and 61.1% in R-134a.

EXAMPLE 3 In the same apparatus as in Example 1, but limiting the reaction zone by loading the ends of the tube with an inert and refractory solid body such as corundum, 385 g of sodium hydroxide of the following composition by weight are charged NaOH 1% ; 2% KOH; H ₂ O 13.5%; Ca (OH) ₂ 83.5%, in the form of granules with a particle size of between 2.4 and 4.8 mm.

The tube is heated to 365 ° C. and a flow rate of 0.66 mol / h of HFC-134a and a flow rate of 2.4 mol / h of water are introduced. The temperature under reactants in the reactor reaches 410 ° C. The gases leaving the tube are analyzed. Their content is 19.2 mol% in TrFE and 74.6% in R-134a, which gives an overall conversion rate of R-134a of 25.3%.

Example 4

In a stainless steel cylindrical reactor 41 equipped with a turbine-type stirring, surrounded by a heating cloth and placed on a hot plate, is prepared 2.4 I of 75% potassium hydroxide solution. The reactor is heated to 150 ° C. and the stirring is adjusted to 800 ° C. rev / min. The introduction of R-134a is then started at the nominal flow rate of 1 mol / h. The gases from the reactor are passed through a dryer and then sampled for analysis as in Examples 1 to 3.

After 180 min of reaction, their content is 0.9% in TrFE and 99.1% in R-134a.

Claims

claims

1) Process for producing trifluoroethylene F1 123 (CF ₂ = CHF) comprising at least one step of reacting R-134a (CF ₃ -CFH ₂ ) with a metal hydroxide.

2) Process according to claim 1 characterized in that the reaction is carried out in the liquid phase with an alkaline hydroxide at a temperature between

120 and 400 and preferably between 250 and 350 ° and under a pressure of between 1 and 10 bars absolute.

3) Process according to claim 2 characterized in that the reaction is carried out in the presence of a liquid phase having an alkali hydroxide content of between 20 and 99% by weight, preferably between 50 and 96%.

4) Process according to claim 3 characterized in that the reaction is carried out with a solution of potassium hydroxide in water.

5) Process according to claim 1, characterized in that the reaction is carried out in the gaseous phase with a solid reagent consisting of an alkaline earth hydroxide at a temperature of between 150 and 450 ° C. and preferably between 300 and 420 ^* 0, at a pressure of between 1 and 10 bar absolute and preferably between 1 and 4 bar.

6) Process according to claim 5 characterized in that the alkaline earth hydroxide is calcium hydroxide. 7) Process according to claim 6 characterized in that the solid reagent has a content of Ca (OH) ₂ at least equal to 67% by weight.

8) Process according to claim 7 characterized in that the solid reagent has a KOH content of between 0.1 and 5% by weight and advantageously between 2 and 4% by weight, 9) Method according to one of claims 7 and 8 characterized in that the solid reagent has a NaOH content between 0.1 and 5% and preferably between 1 and 3% by weight. 10) Method according to one of claims 7 to 9 characterized in that the solid reagent has a water content of between 1 and 20% by weight.

11) Method according to one of claims 7 to 10 characterized in that the solid reagent is used in the form of powder or granules whose particle size is between 1 and 10 mm.

12) Method according to one of claims 5 to 11 characterized in that the reaction is carried out under a stream of water vapor.