Note: Descriptions are shown in the official language in which they were submitted.
CA 02068832 2002-O1-15
25486-12
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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
CA 02068832 2002-O1-15
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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~
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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
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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~~~~
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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~~
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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~~~~~~
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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:
