This invention relates to a process for the production of hydrofluoroalkanes, particularly 1,1,1,2-tetrafluoroethane and pentafluoroethane.
Several processes have been proposed for the production of 1,1,1,2-tetrafluoroethane, otherwise known as HFC 134a, and pentafluoroethane, otherwise known as HFC 125 which are employed as or as components of replacements for chiorofluorocarbons in the many applications in which chlorofluorocarbons are employed. Amongst such processes is the fluorination of the corresponding chlorine-containing starting material by reacting the starting material with hydrogen fluoride in the liquid or the vapour phase, usually in the presence of a fluorination catalyst.
Thus it has been proposed in United Kingdom Patent Specification No. 1,589,924 to produce HFC 134a by the vapour phase fluorination of 1,1,1-trifluoro-2-chloroethane (HCFC 133a) which is itself obtainable by the fluorination of trichioroethylene as described in United Kingdom Patent Specification No. 1,307,224.
The formation of HFC 134a as a minor product of the fluorination or trichloroethylene is described in United Kingdom Patent Specification No 819,849, the major reaction product being HCFC 133a.
More recently, processes for the production of HFC 134a from trichloroethylene based on a combination of the reaction of trichloroethylene with hydrogen fluoride to produce HCFC 133a and the reaction of HCFC 133a with hydrogen fluoride to produce HFC 134a have been proposed.
In WO 90/08755, the contents of which are incorporated herein by reference, there is described the conversion of trichloroethylene to HFC 134a wherein the two reactions steps are carried out in a single reaction zone with recycle of part of the product stream containing HCFC 133a.
In EP 0 449 614, the contents of which are also incorporated herein by reference, there is described a process for the manufacture of HFC 134a which comprises the steps of:
(A) contacting a mixture of trichloroethylene and hydrogen fluoride with a fluorination catalyst under superatmospheric pressure at a temperature in the range from about 200xc2x0 C. to about 400xc2x0 C. in a first reaction zone to form a product containing 1,1,1-trifluoro-2-chloroethane and hydrogen chloride together with unreacted starting materials,
(B) passing product of step A together with hydrogen fluoride to a second reaction zone containing a fluorination catalyst at a temperature in the range from about 280xc2x0 C. to about 450xc2x0 C. but higher than the temperature in step A to form a product containing 1,1,1-trifluoro-2-chloroethane, 1,1,1,2-tetrafluoroethane and hydrogen chloride,
(C) treating product of step B to separate 1,1,1,2-tetrafluoroethane and hydrogen chloride from 1,1,1-trifluoro-2-chloroethane and unreacted hydrogen fluoride, and
(D) feeding 1,1,1-trifluoro-2-chloroethane obtained from step C together with trichloroethylene and hydrogen fluoride to said first reaction zone (step A).
In EP 0 449 617, the contents of which are also incorporated herein by reference, there is described a process for the production of BFC 134a which comprises the steps of:
(A) contacting a mixture of 1,1,1-trifluoro-2-chloroethane and hydrogen fluoride with a fluorination catalyst at a temperature in the range from about 280xc2x0 C. to about 450xc2x0 C. in a first reaction zone to form a product containing 1,1,1,2-tetrafluoroethane and hydrogen chloride together with unreacted starting materials,
(B) passing product of step A together with trichloroethylene to a second reaction zone containing a fluorination catalyst at a temperature in the range from about 200xc2x0 C. to about 400xc2x0 C. but lower than the temperature in step A to form a product containing 1,1,1-trifluoro-2-chloroethane, 1,1,1,2-tetrafluoroethane, hydrogen chloride and unreacted trichloroethylene and hydrogen fluoride,
(C) treating product of step B to separate 1,1,1,2-tetrafluoroethane and hydrogen chloride from 1,1,1-trifluoro-2-chloroethane, unreacted trichioroethylene and hydrogen fluoride, and
(D) feeding 1,1,1-trifluoro-2-chloroethane obtained from step C together with hydrogen fluoride to said first reaction zone (step A).
However, a problem which is encountered with processes for the production of 1,1,1,2-tetrafluoroethane based on the hydrofluorination of 1-chloro-2,2,2-trifluoroethane and/or trichioroethylene, is that the conversion of 1-chloro-2,2,2-trifluoroethane to 1,1,1,2-tetrafluoroethane is equilibrium limited, there being a maximum conversion of 1-chloro-2,2,2-trifluoroethane to 1,1,1,2-tetrafluoroethane of only about 20% under typical operating conditions.
It has also been proposed to produce pentafluoroethane (HFC 125) by the catalysed fluorination with hydrogen fluoride of chlorotetrafluoroethane (HCFC 124) and/or dichlorotrifluoroethane (HCFC 123) which are themselves obtainable by the fluorination of perchioroethylene with hydrogen fluoride.
The present invention resides in a process for the production of hydrofluoroalkanes, particularly 1,1,1,2-tetrafluoroethane and pentafluoroethane from hitherto unused starting materials, which process in the case of production of 1,1,1,2-tetrafluoroethane is not subject to the aforementioned equilibrium limitation problem.
According to the present invention there is provided a process for the production of a hydrofluoroalkane which comprises contacting a hydrochlorofluoroethane having the formula CClXYCFHZ or a(hydro)chlorofluoroethene having the formula CClAxe2x95x90CFZ in which X and Y are each independently chlorine or fluorine, Z is chlorine or hydrogen and A is chlorine or fluorine provided that where each of X and Y is fluorine then Z is hydrogen in the vapour phase with hydrogen fluoride and a fluorination catalyst and recovering a hydrofluoroalkane from the resulting products.
In a particular embodiment of the process for producing 1,1,1,2-tetrafluoroethane, the hydrochlorofluoroethane has the formula CClXYCFH2 and the (hydro)chlorofluoroethene has the formula CClAxe2x95x90CFH.
We have found that the product gases from the process for producing 1,1,1,2-tetrafluoroethane comprise a greater molar proportion of 1,1,1,2-tetrafluoroethane than is obtained when 1-chloro-2,2,2-trifluoroethane is used as the starting material.
The starting materials for the process are CCl3CFH2, CCl2FCFH2, CClF2CFH2, CCl2FCClFH, CCl3CHFCl, CCl2xe2x95x90CFH, CClFxe2x95x90CFH, CCl2xe2x95x90CFCl and CClFxe2x95x90CFCl. We prefer to employ CCl2xe2x95x90CFH or CCl2FCFH2, especially CCl2xe2x95x90CFH for the production of 1,1,1,2-tetrafluoroethane and CCl2FCClFH or CClFxe2x95x90CFCl for the production of pentafluoroethane since these materials are more readily available.
Processes for the production of the starting materials of the present invention are known. Thus for example CCl2xe2x95x90CFH may be produced from trichloroethylene, as described in the Journal of Organic Chemistry 28, 112 (1963), or from tetrachloroethane as described in EP 537560.
Suitable fluorination catalysts are those which yield the desired hydrofluoroalkane as a product of the reaction with a yield of greater than 20%, preferably greater than 25%, based on the starting material processed and include catalysts based on chromia or chromium oxyfluoride, and the fluorides or oxyfluorides of other metals, for example magnesium and aluminium. Activity promoting amounts of other metals, for example zinc and nickel may also be present; we particularly prefer to employ a catalyst comprising zinc on chromia as described fully in published European Patent Application No. 502605, the contents of which are incorporated herein by reference.
The relative proportion of hydrogen fluoride to starting material which is employed may vary within wide limits although it is generally preferred to employ a stoichiometric excess of hydrogen fluoride. The stoichiometrically required molar ratio depends upon the particular starting material. Where the starting material is the preferred 1,1-dichloro-2-fluoroethene, the stoichiometrically required molar ratio of hydrogen fluoride to 1,1-dichloro-2-fluoroethene is 3:1. The molar ratio of hydrogen fluoride to the starting material, for example 1,1-dichloro-2-fluoroethene, will usually be at least 2:1, and preferably is at least 4:1 and especially at least 6:1 and substantially greater excesses of hydrogen fluoride, for example up to 50:1, may be employed if desired.
The temperature at which the process is carried out is preferably at least 180xc2x0 C. and more preferably at least 200xc2x0 C. or 220xc2x0 C. but may be significantly lower than the temperatures typically employed for the conversion of 1-chloro-2,2,2-trifluoroethane to 1,1,1,2-tetrafluoroethane. Preferably the temperature is not greater than 350xc2x0 C., especially not greater than 330xc2x0 C.
The process may be carried out at atmospheric pressure although superatmospheric pressure, say up to about 30 bar is preferred.
The contact time is preferably in the range from about 0.1 seconds to about 10 seconds, preferably in the range from about 0.5 seconds to about 5 seconds at atmospheric pressure.
As described previously, hydrofluorination of trichloroethylene via the intermediate 1-chloro-2,2,2-trifluoroethane is used for the production of 1,1,1,2-tetrafluoroethane. If desired the process of the present invention may be combined with processes for the production of 1,1,1,2-tetrafluoroethane based on trichloroethylene/1-chloro-2,2,2-trifluoroethane,
Thus and according to a preferred embodiment of the invention, a hydrochlorofluoroalkane and/or a(hydro)chlorofluoroalkene as hereinbefore defined, for example 1,1-dichloro-2-fluoroethene is fed as a second starting material to processes for the production of 1,1,1,2-tetrafluoroethane employing trichloroethylene as the starting material.
The co-feeding of trichloroethylene and the second starting material such as 1,1-dichloro-2-fluoroethene may be effected in the processes described in our published European Patent Applications Nos 0 449 617, and 0 449 614, the contents of which are incorporated herein by reference.
The invention is illustrated but not limited by the following Examples.
The 1,1-dichloro-2-fluoroethene (HCFC 1121a) used in the Examples was synthesised via the ethanolic potassium hydroxide dehydrochlorination of trichlorofluoroethane (HCFC 131) as described in J. Am. Chem. Soc., 1936, 58, 402. The resulting crude product was washed with water, dried with magnesium sulphate and then fractionally distilled with the fraction boiling between 32xc2x0 C. and 34xc2x0 C. being collected; this fraction was analysed and found to be pure HCFC 1121a.
The 1,1-dichloro-1,2-difluoroethane (HCFC 132c) used in the Examples was synthesised by oxyfluorination of 1,1-dichloroethane (vinylidene chloride) using lead (IV) oxide in anhydrous hydrogen fluoride as described in J. Am. Chem. Soc., 1945, 67, 1639. The reaction was carried out in a Hastalloy C autoclave and yielded a considerable amount of the co-product 1,1-dichloro-1-fluoroethane (HCFC 141b). The resulting reaction mixture was fractionally distilled and a fraction comprising 60% HCFC 141b and 40% HCFC 132c by weight was collected; it was not feasible to separate the HCFC 132c from the HCFC 141b by distillation.