|Title of Invention||
A PROCESS FOR THE PREPARATION OF 2-CHLORO-1,1,1-TRIFLUOROETHANE
|Abstract||A process for the preparation of 2-chloro-1,1,1-trifluoroethane by contacting in the gas phase a feed comprising trichloroethylene and HF with a co-precipitated chromia alumina catalyst impregnated with zinc compound at a temperature of 275° to 400°C optionally under pressure and recovering of 2-chloro-1,1,1-trifluoroethane in a conventional manner from the product stream.|
|Full Text||This invention relates to A process for the preparation of 2-chloro-1,1,1-trifluoroethane (HCFC-133a). HCFC-133a is prepared by using a novel catalyst. HCFC-133a is an intermediate for the preparation of 1,1,1,2-tetrafluoroethane which is used as a replacement for some chlorofluorocarbons in the refrigeration and airconditioning industry.
It is known in the art that the catalytic vapor phase fluorination of haloalkanes with hydrogen fluoride(HF) results in the formation of fluorine rich haloalkanes. Aluminium fluoride is one of the catalysts known in the art for the halogen exchange. However a suitable catalyst is required for the fluorination of haloalkenes to give fluorine rich haloalkanes.
A US Patent 2,885,427 (1959) has found CrF33H2O as a suitable catalyst for the fluorination of haloalkanes and haloalkenes. CrF3.3H2O is only a precatalyst which is oxygenated at 6OOoC to obtain an active catalyst whose empirical formula was found to be Cr03F2. The reaction of trichloroethylene (TCE) with HF in vapor phase at 350oC using the above catalyst gave HCFC-133a as the major component and 1,1,1,2-tetrafluoroethane as a minor component.
The formation of HCFC-133a from TCE involves several steps. The first step is the addition of HF as per Markovnikov's rule to give 1 -fluoro-1,1,2-trichloroethane (HCFC-131 a). Subsequently the chlorines present in HCFC-131a will be successively replaced by fluorine via the intermediates 1,2-dichloro-1,1,difluoroethane (HCFC-132b), to give finally HCFC-133a. It is known in the art that the ease of replacement of chlorine bound to a carbon, by fluorine
follows the order trihalide (-CX3)> dihalide (-CHX2) > primary halide (-CH2X) where X = CI. In the specific case of the catalytical fluorination of TCE a very high yield of HCFC-133a is obtained.
The discovery of oxygenated CrF3.3H20 as a precatalyst lead to the development of several new catalysts based on the oxides of chromium, nickel, cobalt, aluminium etc. The patents US 3,752,850 (1973), US 3,859,424 (1975), described the use of Cr(OH)3 or Cr203. XH20 as a precatalyst, which is activated by a process of calcination, followed by fluorination with HF. The fluorination of TCE to give HCFC-133a was carried out at atmospheric pressure using HF.TCE in mole ratio 6:1. The best conversion and selectivity were obtained at temperatures in the range 300°-340°C. The yield of HCFC-133awas93%. The patents US 3.7554ZZ41973) US 4,129,603 (1978) and US 4,158,675 (1979) report a fluorination catalyst prepared by the sequence of precipitation of Cr(OH)3 from Cr 3+ salts using a base, steam treatment at 95° C, dehydration, calcination and HF treatment. The US patent 3,755,477 reports an yield of 85% HCFC-133a using HF:TCE in mole ratio 6:1 at 300° C and atmospheric pressure.
The EP 0641598 A2 (1994) discloses a process for the fluorination catalyst by firing Cr(lll) hydroxide in hydrogen atmosphere. The catalyst obtained was crystalline Cr2O3. HF: TCE using in mole ratio 15:1, a conversion of 91.2% TCE and 95.3% selectivity for HCFC-133a was obtained.
The US patent 5,155,082 (1992) disclosed a catalyst prepared by blending AI(OH)3 and chromium oxide in the presence of a solvent. Very high selectivity for HCFC-133a was reported although no values were given.
The patents WO 92/16480 (1992) and WO 92/16481 (1992) disclosed a new catalyst prepared by impregnation of zinc compound on Al2O3 and optionally containing one or more other metal selected from the group with atomic number 75-71. This catalyst have high selectivity for HCFC-133a in the fluorination of TCE. However, very high contact times are required in the fluorination of TCE.
The use of compounds of zinc and/or magnesium as promoters on chromium based catalyst impregnated on Alumina or AIF3 was reported in the EP 0502605 (1992). In fluorination using HF:TCE in a mole ratio 10:1 a conversion of only 40.9% was reported at 310°C and contact time of 1 sec.
The main object of the present invention is to provide a process of preparation of HCFC-133a by fluorination of TCE using a novel catalyst. Another object of the invention is to reduce the relative percentage of the strong acid sites in catalyst in order to achieve high selectivity. Yet another objective is to provide enough crushing strength to the catalyst for use under pressure.
Accordingly, the present process provides a process for the preparation of 2-chloro-1,1,1-trifluoroethane which comprises contacting in the gas phase a feed comprising trichloroethylene and HF with a co-precipitated chromia alumina catalyst impregnated with zinc compound such as herein described at a temperature of 275° to 400°C optionally under pressure and recovering of 2-chloro-1,1,1-trifluoroethane in a conventional manner from the product stream.
The co-precipitated chromia-alumina catalyst may contain chromium:aluminium in the atomic ratios of 1:1 to 1:14.
The amount of zinc compound used for impregnation of co-precipitated chromia alumina catalyst may range from 2-12% by weight.
The mole ratio of anhydrous hydrogen fluoride and trichloroethylene may be in the range of 6:1to12:1.
The ratio of the catalyst to feed (w/F) may be in the range of 65 -150 g.h/mole. The contacting may be carried out in the pressure range 15-210 psig.
DETAILS OF THE INVENTION
A commercial process for HCFC-133a uses TCE and anhydrous hydrogen fluoride as raw materials. The addition of HF to TCE and the subsequent exchange of chlorine by fluorine requires the presence of a suitable catalyst to achieve maximum atom economy.
The process can be carried out both at atmospheric pressure and under pressure. The process under pressure has the advantage of directly feeding the product stream into distillation columns operating under pressure for the separation of the desired product and byproducts and to recover and recycle the un-reacted starting materials and intermediates.
The factors that influence the conversions and selectivity are given below:
1. The precatalyst and its activation with HF
2. Mole ratio of HFTCE
3. Reaction temperature
4. The ratio of the weight of the catalyst to the number of moles per hour in the feed expressed as w/F g.h/mole
The catalytical activity in the halogen exchange has been attributed to the Lewis acid sites. In the case of chromia based catalyst the activity was attributed to the number of reversibly oxidizable sites in the precatalyst. In the alumina based catalyst the formation of p AIF3 during activation is critical to the catalytical activity.
The catalyst based on chromia alone were found quite efficient in fluorination at atmospheric pressures. Under pressure this catalyst exhibited a fall in the conversions and selectivity. Also volatile compounds are generated that condense at the reactor exit causing blockage, a serious draw back for commercial operation. The use of graphite to increase the strength of the catalyst resulted in a loss in activity.
This invention takes advantage of the catalytical activity of both chromia and alumina and reports the preparation of a co-precipitated catalyst starting from salts of Cr3* and AI(NO3)3. The relative atomic ratios of Cr:AI may be in the range 1:1 to 1:14 preferably in the range 1:3 to 1:10 and most preferably in the range 1:3 to 1:5.The co-precipitation is done by using a base selected from NaOH, KOH and NH4OH, preferably with NH4OH. The precipitation is carried out at various dilutions using the base of strength 1 to 6 molar, preferably 4-6 molar. The quantity of water used to dissolve the combined quantity of chromium (III) salt and aluminium nitrate are in the weight ratio 38:1 to 4:1 preferably 19 : 1 to 4:1 and most preferably 10:1 to 4:1.The total acidity of the precatalyst is known to depend upon the pH at which hydroxides are formed. The precipitation is completed by adjusting the final pH in the range 7-8. The hydroxides are filtered washed with water dried to constant weight at a temperature in the range 70°-150 °C, preferably in the range 70 c - 120 ° C. The dried catalyst is powdered and shaped into tablets or extrudes and calcined in nitrogen atmosphere at a temperature in the range 350° C - 400 °C, preferably in the range 380° C - 400°Cfor 24 to 48 hours. The shape of the catalyst has no effect on its activity.
The calcined catalyst was activated by treating sequentially with N2 at 400 °C for 24 hours followed by fluorination in the temperature range 150 ° to 400 °C till the exit stream of HF contains less than 1% of moisture.
The process also economises on the use of the Cr compound as raw material for the preparation of the catalyst thus minimising the cost and problems related to effluent disposal of the spent catalyst.
It was found that the performance of the co-precipitated Cr2O3/AI2O3 catalyst can be further improved by reducing the total acidity by impregnation or deposition with a compound of zinc. The addition of zinc compound results in suppressing the formation of 2-chloro-1,1,1,2-tetrafluoroethane (HCFC-124), pentafluoroethane (HFC-125) and 1,1,1-trifluoroethane (HFC-143a) in the fluorination of TCE. The addition of a zinc compound on Cr2O3/Al2O3 reduced the percentage of strong acid sites relative to the weak and medium acid sites as revealed by TPD of ammonia. The quantity of zinc compound taken is to give a zinc content in the range 2-12%, preferably in the range 3-7% by weight of co-precipitated Cr2O3/Al2O3 catalyst.
The stoichiometric ratio of HF:TCE required to give HCFC-133a is 3:1. It is found that excess of HF is required to obtain maximum conversions and selectivity. The mole ratio of HF:TCE should be in the range 6:1 to 12:1 preferably in the range 7:1 to 10:1.
The fluorination of TCE to give HCFC-133a is a multi step reaction. The degree of conversion and selectivity depends on the residence time which determines the W/F value. It was found that the preferred W/F value is in the range 65-150 and most preferably in the range 70-100.
It was found advantageous to carry out fluorination under pressure keeping in view the separation of different components in the product mixture. The required pressure was found to be in the range 70-210 psig.
It was found that the fluorination of TCE may be carried out in the temperature range 275°-400° C and preferably in the range of 275° -320° to obtain high conversions and selectivity for HCFC-133a.
The preparation of the pre-catalyst, its activation and use in the fluorination of TCE to give HCFC-133a is illustrated in the examples given below:
Examples: Catalyst Preparation.
All chemicals used are of commercial grade. Demineralised water was used throughout.
Catalyst A : Cr2O3/AI2O3:
341 g Cr (NO3)3.9H20 and 1440 g AI(NO3)3.9H20 were dissolved in 8600 g water at room temperature. The solution is kept under stirring and 10% ammonia solution is added at a uniform rate of 1300g/h till the pH attains 7.5. The slurry obtained is charged into an autoclave and heated at 90° C for 2 h and cooled to 50° C. The resulting slurry was filtered and washed with water. The cake obtained was divided into two portions in weight ratio 3:1. The major portion was dried for 2 h at 70° C and then at 120° C till constant weight. The dried cake was powdered to a particle size >125 mesh. The second portion was partially dried at 70° and mixed with the powder and then extruded into 2.5 mm dia pellets using standard procedures. The extrudes were calcinated at 400° C for 24 h in N2 atmosphere to get 262 g of co-precipitated catalyst designated as catalyst - A. The catalyst is x-ray amorphous.
Catalyst B: ZnCl2/Cr2O3/AI2O3:
100 g of extrudes of catalyst A were suspended for 1 h in a solution obtained by dissolving 15.4 g ZnCI2 in 89.0 g water. The mixture was filtered by gravity and the solids were dried at 120° C to constant weight to give 110 g of the impregnated catalyst ZnCl2/Cr2O3/Al2O3:. X-ray revealed the amorphous nature of the catalyst. Catalyst C: ZnCI2/Cr2O3/AI2O3:
157.35 g Cr (NO3)3.9H20 and 532.7 g AI(NO3)3.9H20 were dissolved in 25.75 Kg water. A 1.7 % of ammonia solution was added at a uniform rate over a period of 18.25 h to the above solution kept under stirring till the precipitation is complete and the final pH reaches 7.5. The slurry is filtered, washed with water and dried at 120° C till constant weight to obtain 116.7 g of the catalyst.
50 g of the above catalyst was powdered and mixed with a solution of 4.17 g of ZnCI2 in 55 g of water. The mixture is concentrated to dryness on a rotavapor. The solid obtained is shaped into 3 mm tablets and calcined at 400° C in N2 atmosphere for 24 h to obtain 40.5 g of catalyst C. X-ray showed the amorphous nature of the catalyst. Catalyst D: ZnCI2/Cr2O3/AI2O3:
157.35 g Cr((NO3)3.9H2O and 532.7 g AI(NO3)3.9H20 were dissolved in 12.89 kg water. Ammonia solution (5%) was added at a uniform rate over a period of 18.25 h to the above solution kept under stirring till the precipitation is complete and the final pH attains 7.5. The slurry is filtered, washed with water and dried at 120° C till constant weight to obtain 128.7 g of the base catalyst.
50 g of the above catalyst was powdered and mixed with a solution of 4.5 g of ZnCI2 in 45 g of water. The subsequent work-up was done as in the case of catalyst C to obtain 39.5 g of catalyst D. X-ray showed the amorphous nature of the catalyst.
Catalyst E : ZnCl2/Cr2O3/AI2O3:
A mixture of 157.35 g Cr(NO3)3.9H2O and 532.7 g AI(NO3)3.9H2O was dissolved in 6.45 Kg of water. The precipitation was done by adding 1.7% ammonia solution at a constant rate over a period of 19.25 h to the above solution with constant stirring till the pH of the slurry attains 7.5. The slurry was filtered, washed with water and dried at 120° C till constant weight to obtain 116.7 g of the catalyst.
50 g of the above catalyst was powdered and mixed with a solution of 4.5 g of ZnCI2 in 45 g of water. The water was removed as described in the case of catalyst C. The dried catalyst was calcined at 400° C for 24 h and shaped into tablets of 3 mm size to obtain 38.6 g of the catalyst E. X-ray revealed amorphous nature. Catalyst F : ZnCl2/Cr2O3Al2O3
157.35 g of Cr (NO 3) 3.9H2O and 532.7 g of AI(NO3)3.9H2O were dissolved in 6.4 Kg of
water. The precipitation was done by the addition of 1.7% ammonia solution over a period of 12
min. with constant stirring till the precipitation is completed and the final pH of slurry
attained 7.5. The slurry was filtered, washed and dried at 120° C till constant weight to
obtain 136.5 g of the base catalyst.
50 g of the above catalyst was powdered and mixed with a solution of 4.16 g ZnCI 2 in 45 g of water. The water was removed as described in the case of catalyst C. The dried catalyst was calcined at 400° C for 24 h and shaped into tablets of the size 3 mm to obtain 40.5 g of the catalyst F. The X-ray showed amorphous nature. Catalyst G : ZnCl2/Cr3O3/Al2O3
A solution of Cr(NO3)3.9H2O( 157.35 g) and AI(NO3)3.9H2O (532.7 g) in 19.32 Kg. water was prepared and 1.7% ammonia solution was added with constant stirring over a period of 18 h till the pH reaches 7.5. The slurry was filtered, washed and dried at 120° C till constant weight to obtain 149.4 g of the catalyst.
50 g of the above catalyst was powdered and mixed with a solution of 4.5 g of ZnCI2 in 45 g of water. The water was removed on rota-vapor as described in the case of catalyst C and the solid was calcined at 400° C for 24 h and shaped into tablets of 3 mm size to obtain 36 g of the catalyst G. The X-ray showed amorphous nature. Catalyst H : ZnCI2/Cr2O3/AI2O3
58.41 g of Cr(NO3)3.9H2O and 197.9 g of AI(NO3)3.9H2O were dissolved in 1.6 Kg. water. A 3.75 % solution of ammonia was added at a uniform rate over a period of 7h under stirring till the pH reaches 7.5. The slurry was filtered, washed with water and the wet cake obtained was transferred into an autoclave and mixed with 500 g of water.The mixture was stirred in a closed system for 6 h at 90° C. After completion of the thermal treatment the slurry was cooled to 35° C and filtered, washed with water and dried at 120° C till constant weight to obtain 59.5 g Cr2O3/Al2O3 catalyst.
The above catalyst (25 g) was powdered and mixed with a solution of 1.0 g of zinc chloride in 17 g of water. The water was removed on rotavapor and dried to obtain 27 g of the catalyst. The catalyst was shaped into tablets of 3 mm size and calcined at 400 °C for 24 h to get 18.67 g of catalyst H. Catalyst l:Cr 2O3/AI2O3
Following the procedure described for catalyst A the co-precipitated catalyst is prepared starting with 95 g of Cr(NO3)3.9H20 and 603 g AI(NO3)3.9H20 dissolved in 3.4 Kg. water and 10% ammonia solution to obtain 103 g of calcined catalyst-l. Catalyst J : ZnCI 2/Cr 20 3/AI20 3
92 g of catalyst - A was suspended in a solution of 16.35 g zinc chloride in 100 g of water and the mixture was slowly vaporised to dryness on a rotavapor under vacuum. The product obtained was dried to constant weight at 120° C to obtain 110 g of catalyst - J.
oatalyst K: ZnCI 2/Cr 20 3/AI 20 3
92 g of catalyst A was suspended in a solution of 24.65 g of zinc chloride in 100 g of water and the mixture was slowly vaporised to dryness on rotavapor under vaccum. The product obtained was dried to constant weight at 120 ° C to obtain 119 g of catalyst - K. Bulk chromia catalyst Cr 203
The procedure described in Inorganic synthesis (1946) Vol. Il.pp 190-191, was followed to reduce Cr03 with ethanol to obtain CrOOH, which was filtered, washed with water, dried at 120° C till constant weight. The product was powdered, shaped into 3 mm tablets and calcinated at 400 °C for 24 h in nitrogen atmosphere. General method of fluorination:
The experimental set up consists of separate feed lines for HF and TCE, vaporiser and a 90 cm long 1" i.d. inconel tubular reactor, pressure relief trap, alkali scrubber, drier, condenser and a receiver cooled in dry ice-acetone mixture. A sample of the product stream is drawn periodically from a sampling valve between the drier and condenser. The temperatures in different zones are maintained by electrically heated block furnaces and PID controllers.
The catalyst is loaded into the tubular reactor and pretreated with nitrogen at 400 ° C for 24 h. The temperature is then lowered to 150° C and a slow stream of HF is introduced along with nitrogen. After the initial exothermicity nitrogen is slowly withdrawn while raising the temperature of the catalyst bed to 375° C. The fluorination is continued until the moisture content in the exit HF is below 1%. The bed temperature of the catalyst is then brought and maintained at the reaction temperature and TCE is introduced into the system along with HF. The feed quantity of HF and TCE were adjusted to give the desired mole ratio and W/F. The product stream is scrubbed with aq.KOH solution and then condensed in a trap cooled in dry ice-acetone. The composition of the product stream is determined by GC after reaching steady state and is based on the peak areas. The fluorination experiments were carried out both at atmospheric pressure and under pressure as indicated in the examples given below:
Fluorination of TCE at atmospheric pressure:
Example – 2
Fluorination of TCE under pressure
Example – 3
Fluorination of TCE at atmospheric pressure using catalysts prepared under different
The advantage of the co-precipitated chromia-alumina catalyst impregnated with zinc lies in the economisation of relatively costly chromium salt as compared to bulk chromia. The co-precipitated catalyst exhibits longer life and higher crushing strength as compared to an
impregnated chromia-alumina catalyst used in fluorination reactions. The conversions and selectivity for HCFC133a in the fluorination of TCE are consistently high and are in the range above 96%. The impregnation of the co-precipitated chromia-alumina catalyst with zinc reduces the relative percentage of strong acid sites which are responsible for the formation of side products like HFC-125, HFC-143a and HCFC-1122 thus increasing the selectivity.
We Claim :
1. A process for the preparation of 2-chloro-1,1,1-trifluoroethane which comprises contacting in the gas phase a feed comprising trichloroethylene and HF with a co-precipitated chromia alumina catalyst impregnated with zinc compound such as herein described at a temperature of 275° to 400°C optionally under pressure and recovering of 2-chloro-1,1,1-trifluoroethane in a conventional manner from the product stream.
2. A process as claimed in claim 1 wherein the co-precipitated chromia alumina catalyst contains chromium:aluminium in the atomic ratios 1:1 to 1:14.
3. A process as claimed in claims 1-2 wherein the amount of zinc in the co-precipitated chromia alumina catalyst is in the range of 2-12% by weight.
4. A process as claimed in claims 1-3 wherein the mole ratio of anhydrous hydrogen fluoride and trichloroethylene are in the range of 6:1 to 12:1.
5. A process as claimed in claims 1-4 wherein the ratio of catalyst to feed (w/f) ranges from 65-150 g.h./mole.
6. A process as claimed in claims 1-5 wherein the contacting is carried out in the pressure range 15-210 psig.
7. A process for the preparation of 2-chloro-1,1,1-trifluoroethane substantially as herein described with reference to the examples.
|Indian Patent Application Number||61/DEL/1999|
|PG Journal Number||13/2009|
|Date of Filing||12-Jan-1999|
|Name of Patentee||COUNCIL OF SCIENTIFIC & INDUSTRIAL RESEARCH|
|Applicant Address||RAFI MARG NEW DELHI-110 001, INDIA|
|PCT International Classification Number||C07C 19/08|
|PCT International Application Number||N/A|
|PCT International Filing date|