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FIELD OF THE INVENTION
This invention relates to the purification of saturated fluorohalocarbons
and/or fluorohalohydrocarbons containing olefinic impurities, and
especially to a reductive process for removing olefinic impurities.
BACKGROUND OF THE INVENTION
U.S. Pat. No. 2,999,855 discloses a process for the removal of unsaturated
fluorocarbons and saturated fluorohydrocarbons from saturated
perfluorocarbon product streams by treatment with aqueous potassium
permanganate at 20.degree. to 95.degree. C. GB 1,031,409 discloses a
process for the purification of 2,2,2-trifluoro-1-chloro-1-bromoethane
(CF.sub.3 CHBrCl) to remove fluorinated halogen butenes by treatment with
an alcoholic solution of an alkali metal hydroxide or alcoholate.
U.S. Pat. No. 3,004,075 discloses a process for purifying impure saturated
perfluorocarbon of 2-6 carbon atoms in which the impurities consist
essentially of unsaturated highly fluorinated compounds which may also
contain chlorine or hydrogen, which comprises intimately contacting the
impure saturated perfluorocarbon with at least one member of the group
consisting of piperdine, pyrrolidine and pyridine and mixtures thereof, at
a temperature from about 0.degree. to about 80.degree. C, and separating
the purified saturated perfluorocarbon from the reaction mixture.
U.S. Pat. No. 3,218,363 discloses a method of purifying a saturated
perhalocarbon in which the halogen substituents are selected from the
group consisting of fluorine and chlorine, said perhalocarbon containing
impurities selected from the group consisting of (1) hydrogen-containing
compounds of structure R-H wherein R is selected from the group consisting
of perfluorocarbons, perfluorochlorocarbons, polyfluorohydrocarbons and
polyfluorochlorohydro-carbons; comprising subjecting such perhalocarbon to
an energy source consisting essentially of non-radiant heat at a
temperature in the range of from 350.degree. to 700.degree. C in the
presence of O.sub.2 and an initiator selected from the group consisting of
F.sub.2, Cl.sub.2, Br.sub.2 and interhalogen compounds, for a time
sufficient to oxidize the major part of the impurities, to form products
easily removable from said perhalocarbon, but insufficient to cause
degradation of a significant amount of said perhalocarbon.
U.S. Pat. No. 3,381,041 discloses a process for purifying impure 1-9 carbon
fluorinated alkanes consisting of carbon and fluorine, and fluorinated
alkanes consisting of carbon, fluorine and at least one member of the
group consisting of hydrogen, chlorine, bromine and iodine to remove
halogenated alkenes, which comprises contacting an impure fluorinated
compound, at about 0.degree. to 150.degree. C for about 10 seconds to 30
minutes, with a mixture of H.sub.2 SO.sub.4 of at least 50 percent by
weight concentration, and at least one mercury material, in the order of
from 0.1 to 20 percent by weight of the sulfuric acid content of said
mixture.
U.S. Pat. No. 3,696,156 discloses a process for the purification of 2-6
carbon saturated fluoroperhalohydro-carbons, containing only fluorine or
fluorine and chlorine, and contaminated with perfluoro- and
perfluorochloro-olefins, by vapor phase contact at 180-250.degree. C. with
alumina containing .gtoreq.0.1% by weight base selected from alkali(ne
earth) metal (hydr)oxides.
U.S. Pat. No. 4,129,603 discloses a process for reducing the
1,1-difluoro-2-chloroethylene, CF.sub.2 .dbd.CHCl, by-product obtained
from the reaction of 1,1,1-trihalo-2-chloroethane and HF to produce
1,1,1,2-tetrafluoroethane, CF.sub.3 CH.sub.2 F, by contacting the reaction
products with a metal permanganate in a liquid medium.
U.S. Pat. No. 4,158,675 teaches the removal of 1,1difluoro-2-chloroethylene
from the 1,1,1,2-tetrafluoroethane product stream produced as in U.S. Pat.
No. '603 above by vapor phase hydrofluorination of a
1,1,1-trihalo-2-chloroethane, by passing the impure stream together with
HF over a chromium oxide catalyst at lower temperatures (100-275.degree.
C.) than the hydrofluorination temperatures (300-400.degree. C.) of the
tetrafluoroethane production step.
East German Patent 160,718 discloses a process for the separation and
purification of perfluorinated alkanes from mixtures containing
perfluoroolefins and HF which comprises quantitative reaction of olefinic
impurities to form water soluble products by their reaction with reactive
nucleophiles in the presence of acid acceptors, or reaction with alkali
salts of reactive nucleophiles, or reaction with aqueous alkali solutions.
The perfluorinated alkanes form a distinct phase which is separated and
then refined.
The olefinic impurities which are present as impurities in the manufacture
of saturated fluorocarbons and fluorohydrocarbons are particularly
undesirable as contaminants as they may be toxic and for most uses their
concentrations in the saturated products must be lowered to as a low a
level as is practically possible. This is particularly true because the
fluorocarbons and fluorohydrocarbons can be widely used as solvents,
cleaning agents, blowing agents, and refrigerants where toxicity must be
substantially eliminated. Distillation and other conventional physical
methods which may be used to lower the concentrations of olefinic
impurities are generally ineffective when the boiling points are too close
and are generally too costly. Therefore, various chemical treatments have
been proposed, some of which are described above. None of these prior
processes is entirely satisfactory from a commercial viewpoint. The
aqueous alkaline metal permanganate treatments of the U.S. Pat. Nos. '855
and '603 , for example, require that the halocarbon products exiting the
treatment medium be dried (separated from its entrained water) before
further refining, which adds to the expense of the treatment. Moreover,
where saturated halo-hydrocarbon products are being treated, the
possibility exists that some of the valuable saturated material could be
lost to the alkaline oxidative medium along with the unsaturated
impurities. The high temperatures of the U.S. Pat. Nos. '156 and '675 are
objectionable because of the increased cost of unsaturates removal.
Further the U.S. Pat. No. '156 treatment appears limited to perhalocarbons
since hydrogen-bearing halocarbons are possibly susceptible to
dehydrohalogenation to form unsaturated products under the high
temperature alkaline process of the disclosed process.
It is an object of this invention to provide a hydrogenation process for
reducing the concentration of olefinic impurities in fluorocarbons and
fluorohydrocarbons, in particular in such fluorocarbons and
fluorohydrocarbons having 2 to 4 carbon atoms.
Another object is to provide a process as above that operates at relatively
low temperatures.
Still another object is to provide a process as above that reduces the
content of the olefinic impurity in the fluorocarbon and fluorohydrocarbon
process streams without substantial yield loss of the hydrogen-bearing
halocarbon components.
Yet another object is to provide a process as above where the hydrogenated
olefinic impurity can be recovered by distillation and used as a separate
product.
A further object is to provide a process for which no drying is required.
SUMMARY OF THE INVENTION
This invention provides for a process for treating an impure mixture
consisting essentially of at least one olefinic impurity and at least one
saturated halocarbon selected from fluorocarbons and fluorohydrocarbons by
contacting the mixture with a source of hydrogen in the presence of a
hydrogenation catalyst, whereby the olefinic impurity is converted to a
hydrogenated form thereof, to produce a treated mixture consisting
essentially of the hydrogenated form of the olefinic impurity or
impurities and the saturated halocarbon or halocarbons from the impure
mixture. If desired or if necessary, the hydrogenated form of the olefinic
impurity can be separated from the treated mixture by conventional means
to produce a fluorocarbon and/or fluoro-hydrocarbon substantially free of
olefinic impurity or the hydrogenated form of the olefinic impurity. In
addition, if the hydrogenated form of the olefinic impurity is itself
useful, it may be recovered as product.
Such treatment enables the recovery of the saturated fluorocarbons and/or
fluorohydrocarbons directly and substantially free of olefinic impurities
and without significant yield loss. It also enables the recovery of the
fluorocarbons and/or fluorohydrocarbons not only substantially free of
olefinic impurities but also substantially free of water without the need
for an additional drying step.
DETAILS OF THE INVENTION
The invention process is conducted by contacting, either batchwise or
continuously, a substantially dry impure mixture consisting essentially of
olefinic impurity(ies), saturated fluorocarbon(s) and/or
fluorohydrocarbon(s) in gaseous or liquid form with a hydrogenation
catalyst, e.g. Pd/C, Pt/C, Rh/C and Re/C. It is preferably conducted as a
continuous process wherein a gaseous stream of impure mixture is passed
through a particulate solid bed of the hydrogenation catalyst, maintained
at the desired temperature by heating, if necessary. The exit stream, i.e.
treated mixture, from the reactor with the saturated fluorocarbon(s)
and/or fluorohydrocarbon(s) substantially free of olefinic impurities can
be treated, if desired, by conventional means for separating the saturated
fluorocarbons and/or fluorohydrocarbons from the hydrogenated olefinic
impurities.
The invention may be applied to the purification of saturated fluorocarbons
and fluorohydrocarbons and mixtures thereof, whatever their source, that
contain one or more fluorine atoms in the molecule and are contaminated
with olefinic impurities. Included are perfluoro- and fluorohydrocarbons
composed of: carbon and fluorine and carbon, hydrogen and fluorine. The
saturated fluorocarbons and/or fluorohydrocarbons preferably contain 2 to
4 carbon atoms, more preferably 2 to 3, most preferably 2 because of their
greater commercial importance.
The saturated fluorocarbons and/or fluorohydro-carbons include cyclic as
well as acyclic compounds represented by the empirical formula C.sub.n
H.sub.m F.sub.p, where n is an integer from 2 to 4, m is an integer from 0
to 9, and p is an integer from 1 to 10, provided that m +p equals 2n +2
when the compound is acyclic and equals 2n when the compound is cyclic.
In one embodiment the fluorocarbons are fluoroalkanes, i.e. acyclic,
represented by the above empirical formula where n is 2 or 3, m is 0 to 7
and p is 1 to 8.
In another embodiment the fluorohydrocarbons are acyclic hydrogen-bearing
alkanes, where n is 2, m is 1 to 5, p is 1 to 5, and m +p is 6.
In still another embodiment the compounds to be treated are fluorinated
hydrogen-bearing alkanes where n is 3, m is 1 to 7, p is 1 to 7, and m +p
is 8.
Representative saturated halocarbons that can be treated in accordance with
the method of the invention when contaminated with olefinic impurities
include fluorocarbons such as CF.sub.3 CF.sub.3, CF.sub.3 CF.sub.2
CF.sub.3, CF.sub.3 CF.sub.2 CF.sub.2 CF.sub.3, and cyclo-C.sub.4 F.sub.8;
and fluorohydrocarbons such as CHF.sub.2 CF.sub.3, CHF.sub.2 CHF.sub.2,
CF.sub.3 CH.sub.2 F, CF.sub.3 CH.sub.3, CHF.sub.2 CH.sub.3, CH.sub.2
FCH.sub.3, CF.sub.3 CF.sub.2 CHF.sub.2, CF.sub.3 CF.sub.2 CH.sub.3,
CH.sub.3 CHFCH.sub.3, CH.sub.3 CH.sub.2 CH.sub.2 F, and (CF.sub.3).sub.2
CHCF.sub.3.
The invention process is capable of removing a wide variety of
carbon-carbon unsaturated compounds when present in the impure mixture,
notably C.sub.2 to C.sub.4 olefins composed of carbon and hydrogen and
optionally bearing halogen substituents, generally fluorine, chlorine
and/or bromine and boiling over a wide range. The invention process is
particularly effective for the removal of the C.sub.2 olefinics, the most
commonly occurring of the olefinic impurities in the fluorocarbons and
fluorohydrocarbons described above.
Representative olefinic impurities that can be removed from the saturated
fluorocarbons and fluorohydro-carbons include hydrocarbon and halocarbon
olefins including cis and trans isomers thereof, such as: CH.sub.2
.dbd.CH.sub.2, CH.sub.2 .dbd.CHCl, CHCl.dbd.CHCl, CH.sub.2 .dbd.CC.sub.12,
CHCl.dbd.CCl.sub.2, CF.sub.2 .dbd.CH.sub.2, CF.sub.2 .dbd.CF.sub.2,
CF.sub.2 .dbd.CHF, CF.sub.2 .dbd.CClF, CH.sub.2 .dbd.CClF, CH.sub.3
CF.dbd.CH.sub.2, CF.sub.3 CF.dbd.CF.sub.2, CF.sub.3 CH.dbd.CHCF.sub.3,
(CF.sub.3).sub.2 C.dbd.CF.sub.2, and CClF.sub.2 CF.sub.2 CF.dbd.CF.sub.2.
Since most of the olefinic impurity content of the saturated fluorocarbons
and fluorohydrocarbons is usually removable at reasonable cost by standard
physical methods such as fractional distillation, the quantity of the
olefinic impurities remaining to be treated by the method of this
invention will generally be less than about 1% by weight, and more usually
will lie in the range of about 0.5% down to about 0.001% by weight of the
saturated halocarbon, or about 5000 ppm down to about 10 ppm. The
invention process has been found to be effective to lower the unsaturated
content to below 10 ppm and in most cases to below the gas chromatographic
detection limit.
The invention process is especially applicable to the purification of
saturated fluorocarbons and fluorohydrocarbon products obtained by
reaction of HF with a chlorine- or bromine-containing precursor of the
products. Included, for example, is CH.sub.3 CF.sub.3 contaminated with
vinylidene chloride, CH.sub.2 .dbd.CCl.sub.2, obtained by
hydrofluorination of methylchloroform in the presence or absence of
catalyst; CF.sub.3 CH.sub.2 F obtained by catalytic reaction of HF,
CX.sub.3 CH.sub.2 Cl or CX.sub.2 =CHCl, where X =Cl or F, and containing
1,1-difluoro-2-chloroethylene, CF.sub.2 .dbd.CHCl, as impurity. All these
reactions, the conditions employed and the compositions of the product
streams produced thereby are well known in the prior art.
The impure mixture consisting essentially of saturated fluorocarbons and/or
fluorohydrocarbons and olefinic impurities is contacted with hydrogen in
the presence of a hydrogenation catalyst containing a Group VIII metal or
rhenium, supported or unsupported, with Pd/C being preferred, at about
50.degree. to about 300.degree. C, and more preferably 50.degree. to about
200.degree. C in any suitable reactor, including fixed and fluidized bed
reactors.
The contact time under the conditions of this invention should be
sufficient to allow hydrogenation of the olefinic impurity without
substantially affecting the saturated halocarbon. Contact time can vary
widely, but, generally, will be about 5 to 100 seconds, preferably about
10 to 30 seconds.
The molar ratio of hydrogen to the saturated fluorocarbons and/or
fluorohydrocarbons can vary widely, but generally ranges from about 0.1 to
2, preferably about .5 to 1.5, and more preferably about 0.5 to 1.0.
Hydrogen can be fed either in the pure state or diluted with an inert gas,
e.g., nitrogen, helium or argon.
While vapor phase reactions are preferred, the hydrogenation reactions may
also be done in the liquid phase.
The hydrogenated form of the olefinic impurities can be separated and
recovered by conventional means such as distillation.
Pressure is not critical. Atmospheric and superatmospheric pressures are
the most convenient and are therefore preferred.
EXAMPLES
In the following illustrative examples of the invention, parts and
percentages are by weight and temperatures are in degrees Celsius unless
otherwise specified. All pertinent compositions are given in area percent.
The reactor effluent was sampled on-line with a Hewlett Packard HP 5890
gas chromatograph using a 20 foot long, one-eighth inch diameter, column
containing Krytox.degree. perfluorinated polyether on an inert support and
a helium flow of 35 cc/min. Gas chromatographic conditions were 70.degree.
C. for three minutes followed by temperature programming to 180.degree. C.
at a rate of 6.degree. C./minute.
EXAMPLES 1-4
A 3/8" dia. stainless steel U-tube of 15 mL capacity was charged with 0.5%
Pd/C (6.65 g, 4/8 mesh) and impure 1,1,1,2-tetrafluoroethane (HFC-134a)
containing ppm CHCl.dbd.CF.sub.2 (FC-1122) was passed over the catalyst,
together with hydrogen, at 150.degree. C. The flow rates of H.sub.2
(cc/min) and the impure HFC-134a (mL/hr) together with the hydrogenation
results are shown in Table 1. The products exiting the reactor were
analyzed by gas chromatography and found to contain less than 10 ppm
FC-1122, which is the gas chromatograph detection limit, in three of the
four examples, and 10 ppm FC-1122 in the fourth. Additionally, HFC-134a
was recovered substantially unchanged by this treatment.
TABLE 1
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FC- HFC-
Run H.sub.2 HFC-134a
% HFC- 1122 142.sup.1
EX. Time Flow Flow 134a (ppm) (ppm)
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1 8.2 h 10 5 99.6 <10 ND.sup.2
2 16.7 10 5 99.6 <10 350
3 20.7 10 10 99.6 10 330
4 23.2 10 20 99.6 <10 240
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.sup.1 HFC142 = CH.sub.2 ClCHF.sub.2
.sup.2 nondetectable, <10 ppm
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