The present invention provides a process for preparation of 1,1-difluoroethane.

BACKGROUND OF THE INVENTION
Mechanical refrigeration systems, and related heat transfer devices such as heat pumps and air conditioners, using refrigerant liquids are well known in the art for industrial, commercial and domestic uses. Chlorofluorocarbons (CFCs) were developed in the 1930s as refrigerants for such systems. However, since 1980s the effect of CFCs on the stratospheric ozone layer has received much attention. In 1987 a number of governments signed the Montreal Protocol to protect the global environment setting forth a timetable for phasing out the CFC products.
Thus, there is a requirement for a non-flammable, non-toxic alternative to replace these CFCs. In response to such demand industry has developed a number of hydrofluorocarbons (HFCs), which have a zero ozone depletion potential.
Hydrofluorocarbons such as 1,1-difluoroethane (HFC-152a) have essentially no ozone depletion potential (ODP) and has low global warming potential (GWP), and therefore, they have been found to be acceptable refrigerants and, in some cases, as potential blowing agents in the production of plastic foams.
Various methods are known in the art for preparation of HFC-152a such as European Patent No. 2336101 provides a process for the production of 1,1-difluoroethane (HFC-152a), using strong Lewis acid fluorination catalyst such as halides of antimony impregnated on activated carbon.
The use of activated carbon impregnated with a strong Lewis acid fluorination catalyst makes the process costly at industrial scale.
The said EP patent also discloses that the pressure employed in the reaction zone is generally not critical and may be at atmospheric, super-atmospheric or under vacuum.
Similarly, U.S. Patent No. 5208395 also discloses a heterogeneously catalyzed gas phase process for producing 1,1-difluoroethane comprising contacting hydrogen fluoride and 1,1-dichloroethane in vapour phase in the presence of a costly solid catalyst consisting of tin tetrafluoride supported on activated carbon at atmospheric pressure to obtain a maximum of 74% 1,1-difluoroethane.
Thus there is a need to develop an alternative process for the preparation of 1,1-difluoroethane.
Surprisingly, the inventors of the present invention found that reaction pressure has a great role in the preparation of 1,1-difluoroethane from 1,2-dichloroethane and at a particular range of pressure, the compound of the present invention can be prepared with a very good selectivity.

OBJECT OF THE INVENTION
The object of the present invention is to provide an alternative and cost effective process for preparation of 1,1-difluoroethane.

SUMMARY OF THE INVENTION
The present invention provides a continuous vapour phase process for preparation of 1,1-difluoroethane, comprising a step of contacting 1,2-dichloroethane with an anhydrous hydrogen fluoride in presence of a catalyst at a pressure of 4 to 20 kg/cm2 to obtain 1,1-difluoroethane.

DETAILED DESCRIPTION OF THE INVENTION
In an embodiment, 1,2-dichloroethane is contacted with anhydrous hydrogen fluoride at a pressure ranging from 4 to 20 kg/cm2.
In another embodiment, the fluorination reaction is carried out at a pressure of 7 kg/cm2.
In still another embodiment, the fluorination reaction is carried out at a pressure of 5 kg/cm2.
The process of the present invention is a continuous process where reactant is continuously supplied at a constant feed rate in a fixed bed reactor and product is continuously separated.
In an embodiment, the product mixture of 1,1-difluoroethane (HFC-152a) contains additional components selected from unreacted 1,2 dichloroethane, unreacted hydrogen fluoride, vinyl chloride and 1-chloro-1-fluoroethane which may be recycled back to the reactor.
In an embodiment, the fluorination reaction of the present invention is carried out at a temperature selected in the range of 220 °C to 400 °C.
In a specific embodiment, the fluorination reaction is carried out at a temperature selected in the range of 220°C to 280°C.
In an embodiment, the molar ratio of anhydrous hydrogen fluoride to that of 1,2-dichloroethane is selected in the range of 2:1 to 10:1.
The vinyl chloride impurity formed along with desired product i.e., 1,1-difluoroethane has an azeotropic behaviour with 1,1-difluoroethane, and thus it is difficult to separate it from the product. In addition, this increases the net cost of production and hence makes it less economical for scale-ups.
The presence of vinyl chloride may further lead to the formation of tar by decomposition of vinyl chloride at high temperature. The formation of tar leads to catalyst deactivation.
So it is very important to reduce the amount of vinyl chloride formation during the preparation of 1,1-difluoroethane.
In an embodiment, the present invention provides a process for preparation of 1,1- difluoroethane having vinyl chloride impurity less than 5%, preferably less than 2% and most preferably less than 1%.
As used herein, the term “less than 1%” refers to an impurity in a range of 0 to 1%, wherein, zero impurity means that the impurity is not detectable in the final product.
As used herein, the term “less than 2%” refers to an impurity in a range of 0 to 2%, wherein, zero impurity means that the impurity is not detectable in the final product.
As used herein, the term “less than 5%” refers to an impurity in a range of 0 to 5%, wherein, zero impurity means that the impurity is not detectable in the final product.
In an embodiment, the process of the present invention is carried out in a vapour phase wherein both the reactants i.e., 1,2-dichloroethane and anhydrous hydrogen fluoride are supplied in vapour form.
The catalyst used in present invention is selected from a group consisting of chromia, activated carbon, chromium fluoride, nickel fluoride, aluminium fluoride and copper fluoride.
In a preferred embodiment, the catalyst used is activated chromia.
The catalyst can be used as such or is used after activating by passing hydrogen fluoride over it.
In an embodiment, the process of the present invention is carried out at a constant catalyst weight (W) to feed rate (F) ratio i.e., W/F of 100.
In an embodiment, 1,2-dichloroethane and anhydrous hydrogen fluoride are fed into the reactor at the rate of 30 to 50 gram/hour and 115 to 140 gram/hour respectively.
In a particular embodiment, 1,2-dichloroethane and anhydrous hydrogen fluoride are fed into the reactor at the rate of 40 gram/hour and 122 gram/hour respectively.
The process of the present invention is carried out using reactors made up of Inconel or Monel alloys.
In another embodiment, 1,1-difluoroethane is obtained with a selectivity of greater than 95% and vinyl chloride impurity less than 1%.
In specific embodiment, 1,1-difluoroethane is obtained with a selectivity of 97.39% and 0.84% of vinyl chloride.
As used herein, the term “isolating” refers to the method used to isolate the compound from the reaction mixture. The isolation is carried out using any of the process consisting of extraction, distillation, filtration, decantation, washing, dryings or combination thereof.
In a preferred embodiment, the compound of formula 1 is isolated using distillation technique.
The completion of the reaction may be monitored by gas chromatography (GC).
The 1,2-dichloroethane which is used herein as starting material can be prepared by any of the methods known in the art or can be obtained commercially.
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-difluoroethane at 5.0 kg/cm2 pressure and 270°C.
Activated chromia catalyst (650 gram) was charged in an Inconel reactor and the reaction temperature was gradually increased to 270°C. 1,2-dichloroethane and anhydrous hydrogen fluoride were fed into the reactor continuously at the rate of 40 gram/hr and 122 gram/hr respectively at 5.0 kg/cm2 pressure. The desired product was collected continuously and analysed by GC.
Selectivity: HFC152a- 91.44%; Vinyl chloride- 3.40%
Example: 2- Preparation of 1,1-difluoroethane at 7.0 kg/cm2 pressure and 270°C.
Activated chromia catalyst (650 gram) was charged in an Inconel reactor and the reaction temperature was gradually increased to 270°C. 1,2-dichloroethane and anhydrous hydrogen fluoride were fed into the reactor continuously at the rate of 40 gram/hr and 122 gram/hr respectively at 7.0 kg/cm2 pressure. The desired product was collected continuously and analysed by GC.
Selectivity: HFC152a- 94.81%; Vinyl chloride- 1.19%
Example: 3- Preparation of 1,1-difluoroethane at 10.0 kg/cm2 pressure and 270°C.
Activated chromia catalyst (650 gram) was charged in an Inconel reactor and the reaction temperature was gradually increased to 270°C. 1,2-dichloroethane and anhydrous hydrogen fluoride were fed into the reactor continuously at the rate of 40 gram/hr and 122 gram/hr respectively at 10.0 kg/cm2 pressure. The desired product was collected continuously and analysed by GC.
Selectivity: HFC152a- 94.69%; Vinyl chloride- 0.36%
Example: 4- Preparation of 1,1-difluoroethane at 5.0 kg/cm2 pressure and 225°C.
Activated chromia catalyst (650 gram) was charged in an Inconel reactor and the reaction temperature was gradually increased to 225°C. 1,2-dichloroethane and anhydrous hydrogen fluoride were fed into the reactor continuously at the rate of 40 gram/hr and 122 gram/hr respectively at 5.0 kg/cm2 pressure. The desired product was collected continuously and analysed by GC.
Selectivity: HFC152a- 92.85%; Vinyl chloride- 1.93%

Example: 5- Preparation of 1,1-difluoroethane at 7.0 kg/cm2 pressure and 225°C.
Activated chromia catalyst (650 gram) was charged in an Inconel reactor and the reaction temperature was gradually increased to 225°C. 1,2-dichloroethane and anhydrous hydrogen fluoride were fed into the reactor continuously at the rate of 40 gram/hr and 122 gram/hr respectively at 7.0 kg/cm2 pressure. The desired product was collected continuously and analysed by GC.
Selectivity: HFC152a- 97.39%; Vinyl chloride- 0.84%
Example: 6- Preparation of 1,1-difluoroethane at 10.0 kg/cm2 pressure and 225°C.
Activated chromia catalyst (650 gram) was charged in an Inconel reactor and the reaction temperature was gradually increased to 225°C. 1,2-dichloroethane and anhydrous hydrogen fluoride were fed into the reactor continuously at the rate of 40 gram/hr and 122 gram/hr respectively at 10.0 kg/cm2 pressure. The desired product was collected continuously and analysed by GC.
Selectivity: HFC152a- 95.51%; Vinyl chloride- 0.91%
Comparative example: 1- Preparation of 1,1-difluoroethane at 0 kg/cm2 pressure and 270°C.
Activated chromia catalyst (650 gram) was charged in an Inconel reactor and the reaction temperature was gradually increased to 270°C. 1,2-dichloroethane and anhydrous hydrogen fluoride were fed into the reactor continuously at the rate of 40 gram/hr and 122 gram/hr respectively at 0 kg/cm2 pressure. The desired product was collected continuously and analysed by GC.
Selectivity: HFC152a- 46.92%; Vinyl chloride- 42.4%

Comparative example: 2- Preparation of 1,1-difluoroethane at 3 kg/cm2 pressure and 270°C.
Activated chromia catalyst (650 gram) was charged in an Inconel reactor and the reaction temperature was gradually increased to 270°C. 1,2-dichloroethane and anhydrous hydrogen fluoride were fed into the reactor continuously at the rate of 40 gram/hr and 122 gram/hr respectively at 3 kg/cm2 pressure. The desired product was collected continuously and analysed by GC.
Selectivity: HFC152a- 73.48%; Vinyl chloride- 20.27%

WE CLAIM:

1. A continuous vapour phase process for preparation of 1,1-difluoroethane, comprising a step of contacting 1,2-dichloroethane with an anhydrous hydrogen fluoride in presence of a catalyst at a pressure of 4 to 20 kg/cm2 to obtain 1,1-difluoroethane.
2. The process as claimed in claim 1, wherein the fluorination reaction is carried out at a pressure range of 5 to 10 kg/cm2.
3. The process as claimed in claim 1, wherein 1,2-dichloroethane and anhydrous hydrogen fluoride are continuously supplied at a constant feed rate in a fixed bed reactor and product is continuously separated.
4. The process as claimed in claim 1, wherein 1,2-dichloroethane and anhydrous hydrogen fluoride are supplied in vapour phase.
5. The process as claimed in claim 1, wherein the molar ratio of anhydrous hydrogen fluoride to that of 1,2-dichloroethane is selected in the range of 2:1 to 10:1.
6. The process as claimed in claim 1, wherein 1,1-difluoroethane is obtained with vinyl chloride impurity less than 5%.
7. The process as claimed in claim 1, wherein 1,1-difluoroethane is obtained with a selectivity of greater than 95% and vinyl chloride impurity less than 1%.
8. The process as claimed in claim 1, wherein the catalyst used is selected from a group consisting of chromia, activated carbon, chromium fluoride, nickel fluoride, aluminium fluoride and copper fluoride.
9. The process as claimed in claim 1, wherein the reaction is carried out at a constant catalyst weight (W) to feed rate (F) ratio i.e., W/F of 100.

10. The process as claimed in claim 1, wherein 1,2-dichloroethane and anhydrous hydrogen fluoride are fed into the reactor at the rate of 30 to 50 gram/hour and 115 to 140 gram/hour respectively.