CA2068832C

CA2068832C - Process for the preparation of pentafluoroethane (r 125)

Info

Publication number: CA2068832C
Application number: CA002068832A
Authority: CA
Inventors: Martin Hoeveler; Martin Schnauber; Martin Schott
Original assignee: Solvay SA
Current assignee: Solvay SA
Priority date: 1991-05-17
Filing date: 1992-05-15
Publication date: 2002-10-29
Anticipated expiration: 2012-05-15
Also published as: DE59203875D1; EP0513823A2; ATE128703T1; JPH07118182A; DK0513823T3; EP0513823B1; EP0513823A3; GR3017745T3; JP3217446B2; ES2078583T3; CA2068832A1

Abstract

Process for the preparation of pentafluoroethane (R 125) The invention relates to a process for the preparation of pentafluoroethane (R 125) from 1,1-dichloro-2,2,2-trifluoroethane (R 123) and hydrogen fluoride in the gas phase, which comprises using a chromium- and magnesium-containing catalyst which is obtainable by precipitating chromium(III) hydroxide by reacting 1 mol of a water-soluble chromium(III) salt with at least 1.5 mol of magnesium hydroxide or magnesium oxide in the presence of water, converting the reaction mixture into a paste containing chromium hydroxide and a magnesium salt, and then drying the paste and treating the residue with hydrogen fluoride at temperatures of from 20 to 500°C.

Description

Process for the preparation of pentafluoroethane (R 125) The present invention relates to a process for the preparation of pentafluoroethane (R 125) by reacting 1,1-dichloro-2,2,2-trifluoroethane (R 123) with hydrogen fluoride in the gas phase. As a chlorine-free compound, R 125 does not damage the ozone layer and is therefore suitable as a substitute, for example, for R 12 (difluorodichloromethane) in refrigeration.
The preparation of R 125 is already known. DE-A 1 443 835 discloses a process in which R 125 is prepared from 1,1,1-trichloro-2,2-difluoroethane (R 122) by chlorine/fluorine exchange by means of anhydrous hydrogen fluoride at 350°C on a chromium oxyfluoride catalyst.
However, yields of R 125 of only 42 $ are achieved, in addition to 48 ~, based on the R 122 employed, of by products which are not described in greater detail. In addition, a molar excess of hydrogen fluoride of more than 9 fold is necessary.
British Patent 901,297 describes the preparation of R 125 by addition and chlorine/fluorine exchange reactions, starting from tetrachloroethene, by means of anhydrous hydrogen fluoride in the gas phase on a chromium oxide catalyst. This process also requires a 7 to 9-fold molar excess of hydrogen fluoride. Yields of R 125 of only about 10 $, based on the tetrachloroethene employed, are achieved. If the starting materials used are 1,1-dichloro-2,2-difluoroethene and chlorotrifluoroethene, which are not readily available and are expensive, yields of R 125 of up to about 29 ~ are achieved, with formation of 16 - 22 $ of R 124 and further secondary components which are not described in greater detail. However, the conversion of the olefins employed is only 62 %.
British Patent 1,307,224 has already disclosed the preparation of R 125 from tetrachloroethene or 1,1-dichloro-2,2-trifluoroethene (R 123) by gas-phase fluorination using hydrogen fluoride in a molar ratio of 1:10 on a chromium oxide catalyst. However, the reactions are not selective. If tetrachloroethene is used, a mixture of 30 % by volume of R 125, 20 $ by volume of R 124, 20 % by volume of toxic R 133a and 20 % by volume of R 123 is formed. If, by contrast, R 123 is used as the starting material, although the yield of R 125 can be increased to 67 % by volume, alongside 21 % by volume of R 124, at the same time 2.5 % by volume of pentafluoro-chloroethane ( R 115 ) , which forms an azeotrope with R 125 and thus cannot be removed therefrom by distillation, are formed .
The invention relates to a process for the preparation of pentafluoroethane (R 125) by reacting 1,1-dichloro-2,2,2-trifluoroethane (R 123 ) with hydrogen fluoride in the gas phase, which comprises using a chromium- and magnesium-containing catalyst which is obtainable by precipitating chromium(III) hydroxide by reacting 1 mol of a water-soluble chromium(III) salt with at least 1.5 mol of magnesium hydroxide or magnesium oxide in the presence of water, converting the reaction mixture into a paste containing chromium hydroxide and a magnesium salt, and then drying the paste and treating the residue with hydrogen fluoride at temperatures of from 20 to 500°C.
The catalyst and its pretreatment (activation) are described in EP-A-0 130 532 (US Patent 4,547,483).
Surprisingly, the process according to the invention allows pentafluoroethane to be prepared in high yield.

20~~~3~

The R 124 formed as a by-product in the process can be converted to R 125 by recycling into the reactor. The process has the great advantage that no further by-products are formed in significant amounts.
The process is generally carried out in the manner of a conventional gas reaction on a fixed bed catalyst by passing the gas mixture comprising R 123 and hydrogen fluoride through a heatable reaction tube packed with the catalyst indicated in the claims.
The reaction tube is preferably set up vertically and comprises a material which is sufficiently resistant to hydrogen fluoride, such as nickel or steel.
In order to carry out the process, R 123 and hydrogen fluoride are mixed in the gas state. To this end, the starting materials can be conveyed either in gas form (by heating the storage vessels and feed lines) or in liquid form (by using feed pumps). They are subsequently passed through a preheater or evaporator into the catalyst-filled reactor. Starting substances are advantageously metered in continuously and used in technical-grade purity. R 123 can be prepared, for example, by the process of EP-A-0 349 298.
At atmospheric pressure, the throughput of R 123 is expediently 1 - 90 liters (from about 0.04 to 4 mol), in particular 5 - 50 liters (from about 0.2 to 2 mol), per liter of catalyst and per hour. At higher pressures, the throughput of R 123 can be correspondingly higher. The process is generally carried out at atmospheric pressure or at slightly superatmospheric pressure, from about 10-' to 25 bar, preferably from 1 to 12 bar. In particular, in order to achieve relatively high space-time yields and to reduce the proportion of olefinic by-products in the product gas, the use of superatmospheric pressure (from 2 to 12 bar) is preferred. The molar ratio between HF and - 4 _ R 123 is generally from 2:1 to 10:1, preferably from 2.5:1 to 7:1.
The reaction is generally carried out at temperatures of from 200 to 500°C, preferably at from 320 to 450°C.
The residence time of the gas mixture in the reactor is generally from 1 to 60 seconds, preferably from 5 to 30 seconds.
The conversion of the R 123 employed generally reaches values of about 85 ~. Virtually quantitative conversions can be achieved if the R 123 and R 124 components are removed from the product gas and reintroduced into the reactor together with the fresh gas. This circulating gas procedure offers the advantage of keeping the thermal load on the product low and nevertheless achieving virtually quantitative conversions and high yields.
The reaction products can be worked up by two different methods:
1. The gaseous reaction products are passed through water or aqueous alkali metal hydroxide solution in order to remove the excess hydrogen fluoride and the hydrogen chloride formed in the reaction. After drying, for example by means of calcium chloride, the crude gas mixture is condensed and then separated by fractional distillation into a R 125 fraction and the residual gas mixture comprising R 123 (starting material) and R 124 (intermediate);
this mixture can be fed back into the reactor.
2. In the other work-up method, the gases, after leaving the reactor, are immediately subjected to fractional distillation. In this case, the target product R 125 and the hydrogen chloride are removed as low-boiling components and the residual gas 2~6~~~~

mixture, after mixing with the corresponding amount of fresh gas, is recycled into the reactor. This method has the advantage that the excess hydrogen fluoride is not washed out, but instead remains in the residual gas.
The advantages of the process according to the invention are that high selectivities (> 97 %) are achieved in the sum of R 124 and R 125, the R 124 formed being fed back into the reactor, where it can be converted into further R 125. Side reactions, in particular olefin formation and disproportionation reactions, are suppressed virtually completely. In particular, less than 0.1 $ by volume of R 115 is formed, so that azeotrope formation is avoided.
This also crucially simplifies work-up of the reaction product.
The process represents a considerable technical advance since it enables the preparation of R 125 in high yields from an industrially available starting material (R 123) with optimal utilization of the starting materials, without significant formation of isomerization, dis-proportionation, oligomerization, polymerization or fragmentation products. Associated with this is simple work-up and an easily achievable, high degree of purity of the product.
The process according to the invention is illustrated in greater detail by means of the examples below. The percentages indicated for the GC analyses are area percent.
Experimental Report (Catalyst preparation as described in EP-A 130 532 -US Patent 4,547,483) 200 g of Cr(N03)3 x 9 HZO were dissolved in 1 1 of water.
This solution was added to a mixture of 500 g of ~fl~~?P~~

magnesium oxide and 240 g of graphite, and the pasty com-position which formed was kneaded intimately.
The pasty reaction product was subsequently pelleted to give cube shapes (0.5 cm edge length) and dried at 100°C
for 16 hours.
1 1 (bulk volume) of the dried catalyst elements (= 600 g) were treated at 200°C with 15 mol of hydrogen fluoride in a tube made from nickel or VA steel having an internal diameter of 5 cm and a length of 100 cm. The hydrogen fluoride treatment lasted about 6 hours. During the treatment, the HF was diluted with N2. The fluori-nation catalyst obtained had a chromium content of 2.3 'k by weight.
Example 1 1,1-Dichloro-2,2,2-trifluoroethane (R 123) and hydrogen fluoride in a molar ratio of 1:5.7 were fed via heated lines to an evaporator, mixed and, in the gaseous state, passed over a bed of 0.5 liter of the catalyst prepared as described in the Experimental Report in a tubular nickel reactor (dimensions of the reactor tube: 54 mm diameter, about 1 m long). The residence time was 10.8 seconds. The electrically heated reactor was kept at an internal temperature of 390°C. The gaseous reaction products leaving the reactor were passed through a water scrubber, which was heated in order to prevent reaction gases condensing out. The gaseous, water-insoluble reaction products were dried by means of a calcium chloride tube and analyzed by gas chromatography.
Analysis gave the following composition:
56.6 ~ of R 125 (pentafluoroethane) 25.6 ~ of R 124 (2-chloro-1,1,1,2-tetrafluoroethane) 16.1 $ of R 123 (2,2-dichloro-1,1,1-trifluoroethane).

2~~~~~~
_ 7 Example 2 The reaction of R 123 with HF was carried out in the apparatus described in Example 1 with the catalyst described in Example 1 at a reactor internal temperature of 410°C, a molar ratio of 1:5 and. a residence time of 10.3 seconds, but otherwise as in Example 1. Analysis of the product by gas chromatography gave the following:
58.3 $ of R 125 25.2 ~ of R 124 14.1 ~ of R 123.
Example 3 A product gas mixture obtained by the procedure of Example 1, washed free from HF and dried (composition according to analysis by gas chromatography: 23.0 ~ of R 123; 31.9 ~ of R 124; 43.2 ~ of R 125) was remixed with HF in a molar ratio of about 1:5 and passed into the tubular reactor described in Example 1. The experiment was carried out at 390°C and a residence time of 9.5 seconds, but otherwise as in Example 1. Analysis of the product by gas chromatography gave:
74.4 $ of R 125 17.7 $ of R 124 5.4 ~ of R 123:

Claims

1. A process for the preparation of pentafluoroethane (R 125) by reacting 1,1-dichloro-2,2,2-trifluoroethane (R 123) with hydrogen fluoride in the gas phase, which comprises using a chromium- and magnesium-containing catalyst which is obtained by precipitating chromium(III) hydroxide by reacting 1 mol of a water-soluble chromium(III) salt with at least 1.5 mol of magnesium hydroxide or magnesium oxide in the presence of water, converting the reaction mixture into a paste containing chromium hydroxide and a magnesium salt, and then drying the paste and treating the residue with hydrogen fluoride at temperatures of from 20 to 500°C.

2. The process as claimed in claim 1, wherein the reaction of R 123 with hydrogen fluoride is carried out in the temperature range of 200-500°C.

3. The process as claimed in claim 1, wherein the reaction of R 123 with hydrogen fluoride is carried out in the temperature range of 320-450°C.

4. The process as claimed in any one of claims 1 to 3, wherein the reaction of R 123 with hydrogen fluoride is carried out at a pressure of 1-12 bar.

5. The process as claimed in any one of claims 1 to 4, wherein the reaction of R 123 with hydrogen fluoride is carried out at a molar ratio of from 2 to 10 mol of HF
per mole of R 123.