The present invention provides a process for preparation of tetrafluoropropene from chloro substituted difluoropropanes.

BACKGROUND OF THE INVENTION
Hydrofluoroolefins, particularly, tetrafluoropropenes are important compounds and possess several applications as refrigerants, blowing agent, solvents etc.
There are several methods are known in the literature for the preparation of hydrofluoroolefins.
U.S. Pat. No. 2,996,555 provides a process for preparation of 2,3,3,3-tetrafluoropropene using trichlorodifluoropropane and hydrogen fluoride. The process is carried out in presence of chromium oxyfluoride as catalyst. The catalyst is less stable at room temperature and degrades readily giving the desired product in low yield and is therefore a non-economical.
U.S. Pat. No. 9,102,579 provides a process for preparation of tetrafluoropropene, by two-step process using pentafluoropropane as an intermediate. It involves fluorination of 1,1,1-trichloro-2,2-difluoropropane to give pentafluoropropane followed by its dehydrohalogenation to tetrafluoropropene. The process gives variety of by-products that needs multiple purification steps. It also discloses the preparation of 1,1,1-trichloro-2,2-difluoropropane via photo chlorination of 1-chloro-2,2-difluoropropane in a glass reactor. It is observed that during chlorination, some decomposition of starting material releases hydrogen fluoride that reacts with and corrodes the glass reactor. Thus this process is industrially not viable and expensive.
Therefore, there is a need in the art to develop a process for preparing tetrafluoropropene and intermediates, which is economical and industrially satisfactory.
OBJECT OF THE INVENTION
The main object of present invention is to provide an economical and improved process for preparation of tetrafluoropropene and intermediate thereof. The process for preparation of tetrafluoropropene from chlorodifluoropropane is a two-step process involving in-situ generation of an active catalyst from chromia.

SUMMARY OF THE INVENTION
In an aspect, the present invention provides a process for preparation of 2,3,3,3-tetrafluoropropene, comprising the steps of:
a) chlorinating 1-chloro-2,2-difluoropropane in presence of a catalyst to obtain 1,1,1-trichloro-2,2-difluoropropane; and
b) simultaneously fluorinating and dehydrohalogenating 1,1,1-trichloro-2,2-difluoropropane to obtain 2,3,3,3-tetrafluoropropene.

DETAILED DESCRIPTION OF THE INVENTION
As used herein, “chlorinating” refers to reaction of 1-chloro-2,2-difluoropropane with a chlorinating agent. The chlorinating agent for present invention is chlorine.
The reactor for present invention may be comprised of materials, which are resistant to corrosion such as Hastelloy, Inconel, Monel and/or fluoropolymers linings.
As used herein, fluoropolymer refers to fluorinated ethylene-propylene (FEP), polytetrafluoroethylene (PTFE), polychlorotrifluoro-ethylene (PCTFE) and polyvinylidene fluoride (PVDF) or like.
The glass reactor are not compatible during reaction of fluorinated compounds. The reaction of fluorinated or chlorinated compounds produces hydrogen fluoride in the reaction mixture due to small decomposition. The hydrogen fluoride has very high reactivity with glass or silica and corrodes the reactor. This side reaction will reduce life cycle of reactor and increase equipment cost.
SiO2 + 6HF ? H2SiF6 + 2H2O [1]
In an embodiment, the chlorination of 1-chloro-2,2-difluoropropane to obtain 1,1,1-trichloro-2,2-difluoropropane is carried out in a non-glass reactor selected from Hastelloy, Inconel, Monel and/or fluoropolymers linings. This process modification will greatly improve reactor life and reduce process cost.
In another embodiment of present invention, the chlorination of 1-chloro-2,2-difluoropropane to obtain 1,1,1-trichloro-2,2-difluoropropane is carried out in presence of a catalyst.
As used herein, the term “catalyst” refers to a compound that speed up reaction process. The catalyst is used in the form liquid, powder, granules, pellets or sticks. The catalyst may be supported or unsupported. The catalyst is used in fixed bed catalyst or fluid bed catalyst. The catalyst may be supported on carbon, silica or mixture thereof.
As used herein, catalyst for chlorination may be selected from a group consisting of azo compounds, organic peroxides, metal halides or combination thereof. The azo compounds are azo nitriles and azo esters selected from a group consisting of azobisisobutyronitrile, azodicarbonamide, 4,4'-azobis(4-cyanopentanoic acid), 2,2-azobis(2,4-dimethylvaleronitrile, azobis(methylisobutyrate), azobis(ethylisobutyrate) or like and mixture thereof, organic peroxides is selected from a group consisting of di-tert-butylperoxides, benzoyl peroxide, 2,5-bis(tert-butylperoxy)-2,5-dimethylhexane or like, metal halides is selected from a group consisting of stannic dichloride, stannic tetrachloride, iron chloride, chromium chloride, copper chloride, nickel chloride, antimony chloride, antimony fluoride, tantalum pentachloride and aluminium chloride or combination thereof.
In preferred embodiment, chlorination of 1-chloro-2,2-difluoropropane to obtain 1,1,1-trichloro-2,2-difluoropropane is carried out in presence of an azo compound.
In another embodiment, chlorination of 1-chloro-2,2-difluoropropane to obtain 1,1,1-trichloro-2,2-difluoropropane is carried out in absence of organic solvent.
In another embodiment, chlorination of 1-chloro-2,2-difluoropropane to obtain 1,1,1-trichloro-2,2-difluoropropane may also be carried out in presence of organic solvent. The organic solvent may be selected from tetrachloromethane, perchloroethane and trichloromethane or like. The molar ratio of solvent with respect to 1-chloro-2,2-difluoropropane is selected in the range from 4 to 10.
In another embodiment, the chlorination of 1-chloro-2,2-difluoropropane is carried out at a temperature between 50-180°C.
In preferred embodiment, chlorination of 1-chloro-2,2-difluoropropane to obtain 1,1,1-trichloro-2,2-difluoropropane is carried out using Hastelloy, Inconel or Monel reactor.
The flow rate of chlorine is maintained according to batch size and suitably provided to reaction mixture at constant flow. The chlorine is maintained at a flow rate of 10-100 g/hour.
In an embodiment, inert gas is purged in the reaction mixture after chlorination to remove unreacted chlorine and hydrogen chloride. The reaction mixture refers to a mixture comprising 1,1,1-trichloro-2,2-difluoropropane, hydrogen chloride, 1,1-dichloro-2,2-fluoropropane isomers, 1-chloro-2,2-difluoropropane and chlorine or like.
In an embodiment, 1-chloro-2,2-difluoropropane is chlorinated in presence of azobisisobutyronitrile in a Hastelloy reactor to obtain a reaction mixture containing 1,1,1-trichloro-2,2-difluoropropane, hydrogen chloride, 1,1-dichloro-2,2-fluoropropane isomers and 1-chloro-2,2-difluoropropane or like. The selectivity of 1,1,1-trichloro-2,2-difluoropropane in the reaction is greater than 80%.
In another embodiment, 1,1-dichloro-2,2-difluoropropane by-product is recycled and used for preparation of 1,1,1-trichloro-2,2-difluoropropane.
In another embodiment, 1,1,1-trichloro-2,2-difluoropropane is used in situ for preparation of tetrafluoropropene.
In another embodiment, reaction mixture containing 1,1,1-trichloro-2,2-difluoropropane, hydrogen chloride, 1,1-dichloro-2,2-fluoropropane and 1-chloro-2,2-difluoropropane or like is distilled to separate out 1,1,1-trichloro-2,2-difluoropropane which is used for preparation of tetrafluoropropene.
The 1,1,1-trichloro-2,2-difluoropropane is isolated, having purity greater than 80 % and more preferably 90 % and most preferably between 95-99 % by distillation.
In an embodiment, 1-chloro-2,2-difluoropropane used for present invention may be prepared from fluorination of 1,2,2-trichloropropane.
In an embodiment, 1-chloro-2,2-difluoropropane may be prepared from fluorination of 2,3-dichloropropene.
In an embodiment, 1,2,2-trichloropropane may be prepared from chlorination of 1,2-dichloropropane.
In an embodiment, 2,3-dichloropropene may be prepared from dehydrochlorination of 1,2,3-trichloropropane.
In an embodiment, 1,2,3-trichloropropane may be prepared by chlorination of allyl chloride.
In a specific embodiment, chlorination of 1-chloro-2,2-difluoropropane to obtain 1,1,1-trichloro-2,2-difluoropropane is carried out using chlorine in presence of azodicarbonamide at 170°C.
In a specific embodiment, chlorination of 1-chloro-2,2-difluoropropane to obtain 1,1,1-trichloro-2,2-difluoropropane is carried out in presence of azobisisobutyronitrile at 80°C.
In specific embodiment, chlorination of 1-chloro-2,2-difluoropropane to obtain 1,1,1-trichloro-2,2-difluoropropane is carried out in presence of azobisisobutyronitrile at 80°C in stainless steel reactor.
As used herein, the “fluorinating” refers to reacting 1,1,1-trichloro-2,2-difluoropropane or an organic feed containing 1,1,1-trichloro-2,2-difluoropropane with fluorinating agent. The fluorinating agent for present invention is anhydrous hydrogen fluoride.
As used herein, anhydrous refers to moisture content of less than 1000ppm, preferably less than 100ppm and most preferably between 10-100ppm. The anhydrous hydrogen fluoride is very important for selectivity towards 1234yf. The moisture of more than 100ppm resulted impacted yield and selectivity of 1234yf.
The molar ratio of hydrogen fluoride with respect to 1,1,1-trichloro-2,2-difluoropropane is in a range from 2-10 and more preferably 2-5.
In an embodiment, initially chromia is dehydrated with inert gas at 100-200°C. The inert gas refers to nitrogen, helium, xenon, argon or mixture thereof.
In an embodiment, simultaneous fluorination and dehydrohalogenation is carried out in a reactor selected from Hastelloy, Inconel and Monel reactor.
In another embodiment of the present invention, the chromia catalyst is activated using hydrogen fluoride prior to its use in fluorination step.
In another embodiment of the present invention, the activation of the catalyst is carried out in-situ.
As used herein, “active catalyst” refers to a catalyst formed by reaction of chromia and hydrogen fluoride. The active catalyst for present invention is chromium oxyfluoride. Chromium oxyfluoride refers to a compound or mixture of formula CrOxFy, wherein x and y is 1 or 2, provided x+y is 3. It is observed that chromium oxyfluoride catalyst is not stable at room temperature.
In an embodiment, the chromia catalyst is activated in a reactor before fluorination and dehydrohalogenation. The chromia catalyst used for present invention has surface area from 90-150m2/g and pore volume of greater than 0.2cc/g.
In preferred embodiment, chromia catalyst is amorphous in nature and used in the pellet form.
In an embodiment, chromia catalyst used for present invention is supported on carbon. The chromia catalyst is present as a fixed bed catalyst.
In an embodiment, chromium oxyfluoride is prepared in-situ for preparation of tetrafluoropropene.
In specific embodiment, activation and simultaneous fluorination and dehydrohalogenation is carried out in an Inconel reactor.
In an embodiment, fluorination and dehydrohalogenation is carried out in presence of active catalyst.
In an embodiment of present invention, active catalyst is prepared in-situ in fluorination and dehydrohalogenation reaction.
As used herein, in-situ refers to a process of preparing an intermediate in reaction and using it, without isolation.
The contact time of hydrogen fluoride and 1,1,1-trichloro-2,2-difluoropropane in reactor is 5-50 seconds.
In an embodiment, after dehydration, chromia is fluorinated using hydrogen fluoride or a mixture of nitrogen and hydrogen fluoride to form chromium oxyfluoride in-situ, which is the active catalyst for present invention. The concentration of hydrogen fluoride may vary from 1-100 % and more preferably 50-90 % in a mixture of nitrogen and hydrogen fluoride.
The chromia may additionally contain an elemental metal and may be selected from a group consisting of zinc, tin, iron, chromium, copper and nickel or like. The mass percentage elemental metal may vary from 0.1 % to 1 % of chromia.
In another embodiment, a simultaneous fluorination and dehydrohalogenation of 1,1,1-trichloro-2,2-difluoropropane to obtain tetrafluoropropene is carried out using an organic feed containing 1,1,1-trichloro-2,2-difluoropropane.
In another embodiment, present invention provides a process for preparation of tetrafluoropropene, comprising the steps of:
a) chlorinating 1-chloro-2,2-difluoropropane in presence of catalyst to obtain 1,1,1-trichloro-2,2-difluoropropane;
b) activating chromia catalyst using hydrogen fluoride; and
c) simultaneous fluorinating and dehydrohalogenating an organic feed containing 1,1,1-trichloro-2,2-difluoropropane using active catalyst of step b) to obtain 2,3,3,3-tetrafluoropropene, wherein active catalyst is prepared in-situ.
In another embodiment, the present invention provides a process for preparation of 2,3,3,3-tetrafluoropropene, comprising the steps of:
a) activating chromia catalyst in presence of hydrogen fluoride; and
b) simultaneous fluorination and dehydrohalogenation of an organic feed containing 1,1,1-trichloro-2,2-difluoropropane to obtain 2,3,3,3-tetrafluoropropene, wherein active catalyst is prepared in-situ.
The organic feed refers to a solution of 1,1,1-trichloro-2,2-difluoropropane in an organic solvent. The organic solvent is selected from chlorinated solvents such as dichloromethane.
The concentration of 1,1,1-trichloro-2,2-difluoropropane in organic feed is selected in the range between 20-90 % and more preferably 30-80 %.
In another embodiment, the simultaneous fluorination and dehydrohalogenation is carried out using anhydrous hydrogen fluoride at a temperature of 150 to 600°C and more preferably at a temperature of 300-450°C. The temperature of reactor is increased slowly or 20-50°C per hour. The simultaneous fluorination and dehydrohalogenation is continuous process.
In another embodiment, 1,1,1- trichloro-2,2-difluoropropane and hydrogen fluoride are vaporised before passing through active catalyst.
In another embodiment, 1,1,1- trichloro-2,2-difluoropropane and hydrogen fluoride are pre-vaporised and charged in the reactor from different inlets.
In another embodiment, hydrogen fluoride and 1,1,1- trichloro-2,2-difluoropropane are vaporised and mixed in a static mixer before passing through active catalyst.
In specific embodiment, 1,1,1- trichloro-2,2-difluoropropane and hydrogen fluoride are pre-vaporised and mixed in a static mixer at a temperature in the range 150-300°C before passing through active catalyst.
In another embodiment, organic feed and hydrogen fluoride are vaporised and mixed in a static mixer at a temperature in the range 150-300°C before passing through active catalyst.
The selectivity of formation of 2,3,3,3-tetrafluoropropane is greater than 75% and more preferably greater than 85% and most preferably between 85-95%.
In an embodiment, reactor outlet stream is quenched in a base solution or an aqueous scrubber. The base for present invention is selected from a group consisting of sodium carbonate, potassium carbonate, potassium hydroxide, sodium hydroxide or like. For present invention, quenching refers to neutralising acid content from a stream and acid content is due to presence of hydrogen fluoride, hydrogen chloride or mixture thereof.
In an embodiment, reaction outlet stream is quenched and distilled to obtain tetrafluoropropene.
In an embodiment, 2,3,3,3-tetrafluoropropene is isolated by distillation.
In an embodiment, 1-chloro-2,2-difluoropropane used for present invention is having purity greater than 80% and more preferably greater than 90 % and most preferably between 95-99%.
The catalyst and excess hydrogen fluoride is regenerated and recycled.
In another embodiment, process for preparation of 2,3,3,3-tetrafluoropropene does not involve preparing pentafluoropropane, intermediate.
In another embodiment, a simultaneous fluorination and dehydrohalogenation is carried out to obtain tetrafluoropropene from 1,1,1-trichloro-2,2-difluoropropane.
In specific embodiment, 1,1,1-trichloro-2,2-difluoropropane and hydrogen fluoride are vaporised and passed through active catalyst at a temperature of 300 to 450°C.
In specific embodiment, a solution of dichloromethane and 1,1,1-trichloro-2,2-difluoropropane is vaporised at 150-200°C and mixed with vaporised hydrogen fluoride and passed through active catalyst at a temperature of 300-450°C.
In a specific embodiment, a mixture of 1,1,1-trichloro-2,2-difluoropropane containing 1,1-dichloro-2,2-difluoropropane is vaporised at 150-200°C and mixed with vaporised hydrogen fluoride and passed through active catalyst at a temperature of 300-450°C.
In another embodiment, a mixture of 1,1,1-trichloro-2,2-difluoropropane and hydrogen fluoride are vaporised and passed through catalyst from separate inlets at a temperature of 300-450°C.
In a specific embodiment, present invention provides a process for preparation of tetrafluoropropene, comprising the steps of:
a) chlorinating 1-chloro-2,2-difluoropropane in presence of azobisisobutyronitrile at 80°C to obtain 1,1,1-trichloro-2,2-difluoropropane;
b) activating chromia catalyst in presence of hydrogen fluoride;
c) simultaneous fluorinating and dehydrohalogenating 1,1,1-trichloro-2,2-difluoropropane using active catalyst of step b) at a temperature of 300 to 450°C to obtain 2,3,3,3-tetrafluoropropene, wherein active catalyst is prepared in-situ.
Tetrafluoropropene is isolated by any method known in the art, for example, chemical separation, extraction, acid-base neutralization, distillation, evaporation, and filtration or a mixture thereof.
Unless stated to the contrary, any of the words “comprising”, “comprises” and includes mean “including without limitation” and shall not be construed to limit any general statement that it follows to the specific or similar items or matters immediately following it.
Embodiments of the invention are not mutually exclusive, but may be implemented in various combinations. The described embodiments of the invention and the disclosed examples are given for the purpose of illustration rather than limitation of the invention as set forth in the appended claims.
The following example is given by way of illustration and therefore should not be construed to limit the scope of the present invention.

EXAMPLES
Example 1: Preparation of 1,1,1-trichloro-2,2-difluoropropane
1-chloro-2,2-difluoropropane (1000g) and azobisisobutyronitrile (14g) were charged into an round bottom flask equipped with condenser assembly. The reaction mass was heated to 80°C and chlorine (775g) was purged at the rate of 30-45 g/hour. The reaction mass temperature was maintained at 80-90°C during chlorine addition. The reaction progress was analysed on gas chromatography. After reaction, nitrogen was purged in the reaction mass to remove excess chlorine and gaseous by-product. The reaction mass was purified by distillation at 100-120°C to isolate pure 1,1,1-trichloro-2,2-difluoropropane. Or the reaction mass was continuously distilled and the product was preheated and directly used in the next step without isolation.
Purity: 98 %; Yield: 74 %.
Example 2: Preparation of 1,1,1-trichloro-2,2-difluoropropane
1-chloro-2,2-difluoropropane (500g) and azobisisobutyronitrile (10 g) were charged into a glass reactor equipped with condenser and mechanical stirrer. The reaction mass was heated to 70°C and chlorine (390g) was purged the reaction mass. The temperature was maintained at 70-80°C during chlorine addition. The reaction progress was analysed by gas chromatography. After reaction, nitrogen was purged into reaction mass to removed excess chlorine and gaseous by-product. The reaction mass was distilled to isolate pure 1,1,1-trichloro-2,2-difluoropropane. Or the reaction mass was continuously distilled and the product was preheated and directly used in the next step without isolation.
Purity: 97.5 %; Yield: 76 %.
Example 3: Preparation of 2,3,3,3-tetrafluoropropene
Chromia catalyst was filled in an Inconel reactor and reactor was heated to 200°C. The nitrogen was passed through the reactor at 200°C for 10-15 hrs. Then, temperature was reduced to 100°C and hydrogen fluoride was passed at a constant flow rate to activate the chromia catalyst. Along with hydrogen fluoride flow, the reactor heating was started and slowly increased to 350°C. 1,1,1-trichloro-2,2-difluoropropane(1 mole) and hydrogen fluoride (10 moles) were charged into vaporiser at the rate of 1.5 and 1.65 g/min respectively. The pre-vaporised hydrogen fluoride and 1,1,1-trichloro-2,2-difluoropropane were mixed and passed through active catalyst bed at 350°C in the reactor. The reactor outlet was purged in water scrubber and condensed at -50°C. The condensed mixture was distilled to isolate 2,3,3,3-tetrafluoropropene.
Purity: 96%; Yield: 80 %.
Example 4: Preparation of 2,3,3,3-tetrafluoropropene
An Inconel reactor filled with chromia catalyst was activated as per example-3. The reactor temperature was slowly increased to 375°C along with hydrogen fluoride flow. 1,1,1-trichloro-2,2-difluoropropane (1 mole) and hydrogen fluoride (5 moles) were vaporised at the rate of 1.2 and 0.7g/ min respectively in separate reactor zones and then passed through the reactor at 375°C. The reactor outlet stream was purged in sodium hydroxide solution and washed gas was condensed at -50°C. The condensed mixture was distilled to isolate 2,3,3,3-tetrafluoropropene.
Purity: 96 %; Yield: 76 %.

Example 5: Preparation of 2,3,3,3-tetrafluoropropene
An Inconel reactor filled with activated chromia catalyst was slowly heated to 350°C along with hydrogen fluoride flow. A solution of 1,1,1-trichloro-2,2-difluoropropane (90g) and dichloromethane (10g) and hydrogen fluoride (123g) were vaporised at the rate of separately and mixed in a static mixer. The mixed stream was passed through the active catalyst bed at 320°C. The molar ratio of 1,1,1-trichloro-2,2-difluoropropane and hydrogen fluoride in the feed were maintained at 1:10. The reactor outlet stream was purged into water followed by the sodium hydroxide solution and washed gas was condensed at -60°C. The condensed mixture was distilled to isolate 2,3,3,3-tetrafluoropropene.
Purity: 96 %; Yield: 78 %.
Example 6: Preparation of 2,3,3,3-tetrafluoropropene
An Inconel reactor filled with activated chromia catalyst was slowly heated to 350°C along with hydrogen fluoride flow. A mixture of 1,1,1-trichloro-2,2-difluoropropane containing 1,1-dichloro-2,2-difluoropropane was vaporised and mixed with vaporised hydrogen fluoride stream. The mixed stream was passed active catalyst bed at 350°C. The molar ratio of 1,1,1-trichloro-2,2-difluoropropane and hydrogen fluoride in the feed were maintained at 1:10. The reactor outlet stream was purged into water followed by sodium hydroxide solution and condensed at -50°C. The condensed mixture was distilled and isolated 2,3,3,3-tetrafluoropropene.
Purity: 95 %; Yield: 73 %.

WE CLAIM:

1. A process for preparation of 2,3,3,3-tetrafluoropropene, comprising the steps of:
a) chlorinating 1-chloro-2,2-difluoropropane in presence of a catalyst to obtain 1,1,1-trichloro-2,2-difluoropropane; and
b) simultaneously fluorinating and dehydrohalogenating 1,1,1-trichloro-2,2-difluoropropane in presence of a catalyst to obtain 2,3,3,3-tetrafluoropropene.
2. The process as claimed in claim 1, wherein the catalyst for chlorination is an azo compound selected from a group selected from azobisisobutyronitrile, azodicarbonamide, 4,4'-azobis(4-cyanopentanoic acid), 2,2-azobis(2,4-dimethylvaleronitrile, azobis(methylisobutyrate), azobis(ethylisobutyrate) or like and mixture thereof.
3. The process as claimed in claim 1, wherein the chlorination is followed by purging of inert gas selected from nitrogen, helium, argon or mixture thereof.
4. The process as claimed in claim 1, wherein the fluorination is carried out using an activated catalyst generated in situ by passing hydrogen fluoride through chromia.
5. The process as claimed in claim 1, wherein 1,1,1- trichloro-2,2-difluoropropane and hydrogen fluoride are vaporised before contacting with a catalyst.
6. The process as claimed in claim 1, wherein the hydrogen fluoride used in fluorination and dehydrohalogenation step is anhydrous having moisture content of 10-100ppm.
7. The process as claimed in claim 1, wherein 1,1,1- trichloro-2,2-difluoropropane and hydrogen fluoride are prevaporised at a temperature of 150 to 300°C and passed through active catalyst at a temperature of 300-450°C.
8. The process as claimed in claim 1, wherein 1,1,1- trichloro-2,2-difluoropropane is not isolated.
9. The process as claimed in claim 1, wherein conversion of 1-chloro-2,2-difluoropropane to 2,3,3,3-tetrafluoropropene is carried out without isolation of pentafluoropropane.
10. The process as claimed in claim 1, wherein the selectivity of formation of 2,3,3,3-tetrafluoropropane is greater than 75%.