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Description  |
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FIELD OF THE INVENTION
This invention relates to a catalytic process for the isomerization of
fluorohydrocarbons, wherein the fluorine and hydrogen exchange places on
the carbon skeleton, using a catalyst composition comprising an aluminium
fluoride.
BACKGROUND OF THE INVENTION
U.S. Pat. No. 2,598,411 discloses the rearrangement of halogen atoms in
perchlorofluoroalkanes by the shifting of like halogens to different
carbon atoms using AlCl.sub.3, AlBr.sub.3, and their mixtures with
antimony halides as catalysts. Carriers such as sintered aluminum fluoride
or activated charcoal, on which the catalysts may be deposited, are
disclosed.
GB 873,212 discloses a process for the manufacture of CF.sub.3 CHClBr by
shifting a fluorine atom and the bromine atom of CF.sub.2 BrCFHCl using
AlCl.sub.3. The patent teaches that if one attempts to isomerize
chlorofluorohydrocarbons with AlCl.sub.3, HCl is lost and the
corresponding ethylene derivative is obtained even under mild conditions.
U.S. Pat. No. 3,087,974 discloses a method of rearranging
perfluorochloroethanes by contacting said perfluorochloroethanes at
temperatures between 150.degree. and 600.degree. C. with a catalyst
prepared by reacting activated alumina with a lower fluorocarbon having
not more than one hydrogen atom. For example, the patent teaches that,
when CH.sub.3 CF.sub.2 Cl is passed over activated alumina at about
300.degree. C., dehydrofluorination occurs and CH.sub.2 =CClF is produced.
JP 53-121710 discloses the isomerization of chlorofluorocarbons containing
one or two hydrogen atoms by contacting them with catalysts represented by
the formula AlCl.sub.x F.sub.y O.sub.z ; where x+y+z=3, 0<x<3, 0<y<3, and
0.ltoreq.z.ltoreq.3/2. In all cases only fluorine and chlorine atoms
exchange positions. The isomerization is more effective, i.e., catalyst
life is prolonged, in the presence of perchloro- or perchlorofluorocarbons
cofeed. The highest temperature used is about 300.degree. C. since, above
this temperature, starting materials are said to be decomposed.
U.S. Pat. No. 3,787,331 discloses catalysts made from AlF3 containing
manganese, chromium and/or nickel which are used for the fluorination of
perchlorofluorocarbon, CF.sub.2 ClCFCl.sub.2 to CF.sub.2 ClCF.sub.2 Cl
with reduced isomerization of the starting material to CF.sub.3 CCl.sub.3.
More isomerization is observed when using pure AlF.sub.3 as catalyst.
DT 2,245,372 discloses the preparation of di- and trifluoroethane compounds
of general formula CF.sub.2 ZCZ.sub.3 or CF.sub.3 CZ.sub.3 in which Z=H,
Cl, Br, or I, by isomerization of CFZ.sub.2 CFZ.sub.2 or CF.sub.2
ZCFZ.sub.2 using as catalyst a mixture of aluminum halides of the type
AlXY.sub.2 and AlX.sub.2 Y in which X and Y are Cl, Br, or I. In one
example Br and F atoms exchange places in CF.sub.2 BrCFHCl to yield
CF.sub.3 CHBrCl.
Processes for the isomerization of hydrogen containing fluorohalocarbons as
discussed above are known. However, the process of the instant invention
provides the isomerization of fluorohydrocarbons, compounds containing
only carbon, fluorine and hydrogen atoms, wherein fluorine and hydrogen
atoms exchange places on the carbon skeleton.
SUMMARY OF THE INVENTION
This invention provides a process for isomerizing saturated C.sub.2 to
C.sub.6 fluorohydrocarbons having lesser thermodynamic stability to
fluorohydrocarbons having greater thermodynamic stability comprising;
contacting in the gaseous phase at a temperature from about 200.degree. C.
to about 475.degree. C. at least one C.sub.2 to C.sub.6 saturated
fluorohydrocarbon with a catalyst composition comprising an aluminum
fluoride.
In the isomerization process of this invention at least one fluorine atom
in the saturated fluorohydrocarbon exchanges places with at least one
adjacent hydrogen atom on the carbon skeleton of the saturated
fluorohydrocarbon to form a more thermodynamically stable isomer of the
saturated fluorohydrocarbon. The process of this invention represents the
first example of an isomerization of a saturated fluorohydrocarbon wherein
at least one fluorine atom and at least one hydrogen atom, located on
adjacent carbon atoms of the carbon skeleton, exchange places.
DETAILS OF THE INVENTION
The saturated fluorohydrocarbons isomerized in accordance with this
invention are compounds containing only carbon, hydrogen and fluorine
atoms. These saturated fluorohydrocarbons are isomerized to their more
thermodynamically stable isomer or isomers. By more thermodynamically
stable isomers is meant those isomers with the lowest free energy of
formation, as calculated using Benson's Group Contribution Method (S. W.
Benson et. al., Chem. Rev., 69, 279(1969)). For example, in general
thermodynamic stability in the isomer or isomer produced in accordance
with this invention is favored when the fluorine/hydrogen atom exchange is
such that the highest number of fluorine atoms possible exist on terminal
carbon atoms, i.e., -CF.sub.3.
For example, in the case of saturated fluorohydrocarbons having two carbon
atoms, 1,1,2,2-tetrafluoroethane (HFC-134) is isomerized to
1,1,1,2-tetrafluoroethane (HFC-134a); 1,1,2-trifluoroethane (HFC-143) to
1,1,1,-trifluoroethane (HFC-143a); and 1,2-difluoroethane (HFC-152) to
1,1-difluoroethane (HFC-152a).
The catalyst composition utilized in the practice of this invention must
contain an aluminum fluoride, which may or may not be supported. By
aluminum fluoride is meant at least one of AlF.sub.3 and fluorided
alumina. The AlF.sub.3 and/or fluorided alumina can be prepared by any
method known in the art or described hereinbelow.
By fluorided alumina is meant a high fluorine-content composition
comprising aluminum, oxygen, and fluorine in such proportions that the
total fluorine content of the catalyst composition taken as AlF.sub.3
corresponds to, preferably at least 50 weight percent, exclusive of any
supported metal which may be present, and more preferably 80 weight
percent.
The catalyst composition may also contain up to 50% by weight of at least
one metal on a support consisting essentially of aluminum, oxygen, and
fluorine in such proportions that the fluorine content of the catalyst
composition corresponds to an AlF.sub.3 content of, preferably, at least
50% by weight of the catalyst composition exclusive of the metal.
The remainder of the composition may include alumina or aluminum
oxyfluoride. The high-AlF3-content catalyst and supported catalysts can be
prepared in-situ by exhaustive HF fluorination of alumina, optionally
impregnated with at least one metal compound which may be in the form of
the oxide, oxyhalide, halide or pseudohalide or such other form which is
convertible to the fluoride or oxyfluoride under the conditions of the
fluorination pretreatment step described herein. The halides include
fluorides, chlorides, or bromides. The pseudohalides include cyanides,
cyanates, and thiocyanates. The total content of metal, expressed as the
divalent oxide is more than 0.02% but not more than 50% by weight of the
supported catalyst.
Catalysts based on fluorided alumina with or without one or more metal
compounds impregnated therein are preferably prepared prior to use for the
isomerization of saturated fluorohydrocarbon by treatment with a
vaporizable fluorine-containing fluorinating compound, such as HF,
SiF.sub.4, CCl.sub.3 F, CClF.sub.2, CHF.sub.3, or CCl.sub.2 FCClF.sub.2,
at elevated temperatures until the desired degree of fluorination is
obtained, e.g., at about 200.degree. C. to about 450.degree. C. By
vaporizable fluorine-containing fluorinating compound is meant a compound
which will convert the alumina component of the instant invention to the
desired degree of fluorination using the pretreatment conditions described
herein. Such treatments are well known to the art. The treatment with HF
or other vaporizable fluorine-containing compound can conveniently be done
in the reactor which is to be used for isomerizing the saturated
fluorohydrocarbon.
In addition, the invention catalyst composition can also be prepared by
co-precipitation of the metal, if any, and the aluminum as hydroxides
which are thereafter dried and calcined to form the mixed oxides, a
technique well known to the art. The resulting oxide(s), or if desired the
aluminum hydroxide itself, is then pretreated with HF as described in
Example 1 below.
A suitable catalyst may be prepared, for example, as follows:
A quantity of alumina is dried until essentially all moisture is removed,
e.g., for about 18 hours at 100.degree. C. The dried catalyst is then
transferred to the reactor to be used. The temperature is gradually
increased to about 400.degree. C. while maintaining a flow of N.sub.2
through the reactor to remove any remaining traces of moisture from the
catalyst and the reactor. The temperature is then lowered to about
200.degree. C., and HF, diluted with N.sub.2, is passed through the
reactor. The N2 can be gradually reduced until only HF is being passed
through the reactor. At this point the temperature can be increased to
about 450.degree. C. and held at that temperature to convert the
impregnated Al.sub.2 O.sub.3 to a fluoride content corresponding to at
least 50% AlF.sub.3 by weight, e.g., for 15 to 300 minutes, depending on
the HF flow and the catalyst volume.
A metal containing catalyst may be prepared by impregnating alumina with a
solution, usually aqueous, of one or more of the metal compounds described
above. The amount of metal expressed as the divalent oxide, will be
between about 0.02 to 50 weight percent of the alumina support, preferably
not more than 20 weight percent, and more preferably 0.1 to 10 weight
percent. The impregnated alumina can be dried until essentially all
moisture is removed and treated in the same manner as described above for
alumina.
Another suitable procedure for the catalyst preparation is to add ammonium
hydroxide to a solution of Al(NO.sub.3).sub.3 and, if present, a metal in
the form of a water soluble compound, such as Ni(NO.sub.3).sub.2. The
ammonium hydroxide is added to the nitrate solution to a pH of about 8.8.
At the end of the addition, the solution is filtered, the solid obtained
is washed with water, dried and slowly heated to 500.degree. C., where it
is calcined. The calcined product is then treated with a suitable
fluorine-containing compound as described above.
The isomerization of the saturated fluorohydrocarbon in the presence of the
catalyst of the instant invention is conducted at 200.degree. C. to
475.degree. C., preferably about 300.degree. C. to 450.degree. C. and most
preferably about 350.degree. C. to 450.degree. C.
The contact time can vary widely depending on the degree of conversion
desired and generally will be about 30 to 180 seconds, preferably about 60
to 90 seconds.
The saturated fluorohydrocarbons may be fed as is or diluted with oxygen or
an inert gas such as nitrogen, helium or argon.
In accordance with this invention any mixture of saturated
fluorohydrocarbons can be utilized. In practice, for ease of separation,
isomerization of one saturated fluorohydrocarbon at a time is recommended.
In practice, as the isomerization progresses, both any unreacted saturated
fluorohydrocarbon and the resulting isomer can be recycled to the reactor
until the desired degree of isomer purity is obtained.
The isomerization of saturated fluorohydrocarbons may be conducted in any
suitable reactor, including fixed and fluidized bed reactors. The reaction
vessel should be constructed from materials which are resistant to the
corrosive effects of the hydrogen fluoride which may be formed in small
amounts, such as Hastelloy.RTM. alloy and Inconel.RTM. alloy.
Pressure is not critical. Atmospheric and superatmospheric pressures are
the most convenient and are therefore preferred.
The fluorocarbons of this invention are useful as refrigerants, blowing
agents, propellants, cleaning agents, solvents, and intermediates for the
preparation of other fluorocarbons.
EXAMPLES
In the following illustrative Examples, all parts and percentages are by
weight and all temperatures are Celsius. All product compositions are area
percent.
General Procedure for Fluorination
The reactor (0.5 inch ID.times.5" Inconel.RTM. alloy pipe) was charged with
alumina, metal compound supported on alumina, or aluminum fluoride, as
described in the following examples, and placed in a sand bath. The bath
was gradually heated to 400.degree. C. while nitrogen at 50 cc/min was
passed through the reactor to remove traces of water.
When the reactor was charged with an alumina support, the temperature was
lowered to 200.degree. C., and HF and N.sub.2 gas (1/4 molar ratio) were
passed through the reactor. The N.sub.2 flow was decreased with time until
neat HF was being passed through the reactor. At this point the
temperature was gradually raised to 450.degree. C. and maintained there
for 15 to 300 minutes. The fluorine content of the catalyst composition
corresponded to an AlF.sub.3 content, exclusive of any added metal, of at
least 50%.
GeneraI Procedure for Isomerization
The temperature was adjusted to the indicated value, followed by the
initiation of flow of the fluorohydrocarbon and, optionally, with air or
an inert gas. All flows were adjusted to give the indicated molar ratios
and contact times in the Examples. The reactor effluent was sampled
on-line by a Hewlett Packard 5890 gas chromatograph using a 20
foot.times.1/8" ID stainless steel column containing Krytox.RTM.
perfluorinated polyether on an inert support and with a helium flow of 35
cc/minute. Gas chromatographic conditions were 70.degree. for 3 min
followed by temperature programming to 180.degree. C. at a rate of
6.degree. C./min.
EXAMPLE I
Isomerization of CHF.sub.2 CHF.sub.2
A. Al(OH).sub.3 (38.98 g) was dissolved in 48% aqueous HF (100 mL) in a
polyethylene tray. The solution was evaporated at room temperature over 72
h in a fume hood. The resulting white residue was dried at 110.degree. C.
for 48 h. The solid was then heated in air at 5.degree. C./min to
500.degree. C. and held at this temperature for 3 h. After cooling, the
solid was crushed and sieved to yield 17.47 g of 12 .times.20 mesh granule
B. The isomerization procedure was followed using the AlF.sub.3 catalyst
(15.7 g, 25 mL) prepared in . A 1/1 molar ratio mixture of CHF.sub.2
CHF.sub.2/ air was passed over this catalyst at 425.degree. C. with a
contact time of 60 seconds. After about 30 minutes of operation a 99.0%
conversion of HFC-134 to HFC-134a with a selectivity of 94.6% was
observed.
EXAMPLES 2-7
Isomerization of CHF.sub.2 CHF.sub.2
A. A sample of 1/12" alumina extrudate was dried at 110.degree. C. for 18 h
in air. A portion (100 g) of this dried alumina was added to a solution of
LaCl.sub.3.6Hhd 2O (12.4 g) and distilled water (175 mL) in a large
evaporating dish. The slurry was dried on a hot plate, with occasional
stirring for 3 h. The solid was then dried at 110.degree. C. for 18 h in
air and treated with HF as described above in the general procedure for
fluorination.
B. The isomerization procedure was followed using the above described
catalyst (19.1 g, 30 mL) containing 4.9% lanthanum. A 1/2 molar ratio
mixture of CHF.sub.2 CHF.sub.2/ N.sub.2 was passed over this catalyst with
a contact time of 60 s. The results are shown Table 1.
TABLE 1
______________________________________
Ex. Temp. % CHF.sub.2 CHF.sub.2
% CF.sub.3 CH.sub.2 F
% CF.sub.2 .dbd.CHF
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2 300.degree. C.
96.0 1.9 --
3 350.degree.
93.2 4.5 --
4 375.degree.
84.2 13.3 --
5 400.degree.
61.9 33.4 1.7
6 425.degree.
33.6 59.5 4.8
7 450.degree.
21.3 68.1 9.0
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EXAMPLES 8-11
Isomerization of CHF.sub.2 CHF.sub.2
A. A sample of 1/12" alumina extrudate was dried at 110.degree. C. for 18 h
in air. A portion (100 g) of this dried alumina was added to a solution of
MnCl.sub.2.4H.sub.2 O (6.6 g) in distilled water (175 mL) in a large
evaporating dish. The slurry was dried on a hot plate, with occasional
stirring for 3 h. The solid was then dried at 110.degree. C. for 18 h in
air and treated with HF as described above in the general procedure for
fluorination.
B. The isomerization procedure was followed using the above prepared
catalyst (19.5 g, 30 mL) containing 1.9% manganese. A 1/2 molar ratio
mixture of with a contact CHF.sub.2 CHF.sub.2/ N.sub.2 was passed over
this catalyst time of 60 seconds. The results are shown in Table 2.
TABLE 2
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Ex. Temp. % CHF.sub.2 CHF.sub.2
% CF.sub.3 CH.sub.2 F
% CF.sub.2 .dbd.CHF
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8 350.degree. C.
96.6 0.6 --
9 400.degree.
87.4 7.5 0.3
10 430.degree.
58.6 33.8 2.0
11 450.degree.
54.1 37.4 4.5
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EXAMPLES 12-13
Isomerization of CHF.sub.2 CHF.sub.2
A. A sample of 1/12" alumina extrudate was dried at 110.degree. C. for 18 h
in air and treated with HF as described above in the general procedure for
fluorination.
B. The isomerization procedure was followed using the above prepared
catalyst (30 mL). A 1/2 molar ratio of CHF.sub.2 CHF.sub.2 /N.sub.2 was
passed over this catalyst with a contact time of 30 seconds. The results
are shown in Table 3.
TABLE 3
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Ex. Temp. % CHF.sub.2 CHF.sub.2
% CF.sub.3 CH.sub.2 F
% CF.sub.2 .dbd.CHF
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12 350.degree. C.
62.5 33.5 1.5
13 400.degree.
59.3 35.0 3.2
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EXAMPLE 14
Isomerization of CHF.sub.2 CHF.sub.2
A. A sample of 1/12" alumina extrudate was dried at 110.degree. C. for 18 h
in air and treated with HF as described above in the general procedure for
fluorination.
B. The isomerization procedure was followed using the above prepared
catalyst (30 mL). A 1/2/.2 molar ratio mixture of CHF.sub.2 CHF.sub.2
/N.sub.2 /O.sub.2 was passed over this catalyst at 400.degree. C. with a
contact time of 60 seconds. After 7.3 hours on-line, the product stream
contained 86.5% HFC-134, 10.3% HFC-134a, and 1.8% CF.sub.2 =CHF.
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