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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention concerns a method for defluorinating a perfluoroalkane to
the corresponding more highly unsaturated fluorocarbon.
2. References
U.S. Pat. No. 3,192,274, issued to Baranauckas, et al., on June 29, 1965,
discloses a process for removing non-terminal halogens from saturated
perhalocarbons by heating the perhalocarbons over a carbon catalyst at a
temperature between about 275.degree. and 450.degree. C. All of the
disclosed perhalocarbons contain at least one chlorine atom.
Patrick, et al., Chemistry and Industry, 1557 and 1558 (1963), disclose
that passage of certain perfluorodimethylcyclohexanes over unactivated
granular carbon at 600.degree. C. gave a complex mixture of products
including defluorinated products such as decafluoro-p- and -m-xylene. They
also disclose defluorination of such compounds over iron and nickel. The
authors state that perfuoro-1,2-dimethylcyclohexane is normally
defluorinated by passage over iron gauze at 450.degree.-500.degree. to
give decafluoro-o-xylene.
U.S. Pat. No. 2,709,182, issued to Farlow on May 24, 1955, discloses
preparation of tetrafluoroethylene by a process wherein a fluorocarbon of
at least three carbons and of a melting point no higher than 25.degree. C.
is pyrolyzed by heating at a temperature of at least 1500.degree. C.
Pyrolysis by passing the fluorocarbon between carbon electrodes is
specifically disclosed.
U.S. Pat. No. 2,709,192, issued to Farlow on May 24, 1955, discloses a
process for preparing tetrafluoroethylene wherein carbon tetrafluoride or
hexafluoroethane or a mixture of these, is contacted with carbon at a
temperature of at least 1700.degree. C. and the resultant reaction mixture
is rapidly quenched.
U.S. Pat. No. Re. 23,425, issued to Harmon on Oct. 30, 1951, discloses a
process for making completely halogenated polyfluorohydrocarbons
comprising heating at a temperature of at least 125.degree. C. a
completely halogenated ethylene of the formula CX.sub.2 .dbd.CX.sub.2,
wherein X is halogen and at least 2 of the halogens are fluorine. The use
of activated charcoal in the process is disclosed.
Bidinosti, et al., J. Am. Chem. Soc. 83, 3737-3743 (1961), disclose low
pressure pyrolysis of chlorinated methanes and ethanes and of
chlorofluoromethanes and chlorofluoroethanes over graphite to give, in
some cases, dehalogenated products.
Baciocchi, in "The Chemistry of Halides, Pseudo-Halides and Azides", Patai
and Rappoport editors, John Wiley & Sons, New York, 1983, Chapter 5, pages
161 to 201, has reviewed 1,2-dehalogenations and related reactions.
SUMMARY OF THE INVENTION
This invention provides a process for preparing a perfluoroalkene having at
least six carbon atoms and at least one carbon-carbon double bond
comprising contacting the corresponding perfluoroalkane having at least
two adjacent tertiary carbon atoms to defluorinate the perfluoroalkane,
thereby forming the perfluoroalkene having, respectively, at least one
carbon-carbon double bond.
DETAILED DESCRIPTION OF THE INVENTION
The products of the invention, perfluoroalkenes having at least one
carbon-carbon double bond, are useful as comonomers for preparation of
fluorocarbon-containing polymers. The instant process represents an
improvement in prior art processes for preparing perfluoroalkenes in that
lower temperatures can be employed and expensive metals such as platinum
are not required.
The starting materials for the process of the invention are
perfluoroalkanes having at least six carbon atoms. Suitable
perfluoroalkanes have at least two adjacent tertiary carbon atoms. By
"tertiary carbon atom" is meant one to which one fluorine atom is
attached, all other attached atoms being carbon. Preferred reactants are
perfluoroalkanes containing 6 to 14 carbon atoms. When a perfluoroalkane
having only two adjacent tertiary carbon atoms is employed as the
reactant, a perfluoroalk(mono)ene is usually the initial product. Further
defluorination of the perfluoroalk(mono)ene to a perfluoroalkadiene can be
achieved by adjustment of the process conditions, including use of a
longer contact time. If the starting perfluoroalkane has more than one set
of requisite adjacent tertiary carbon atoms, then the product can have
more than two double bonds.
Representative perfluoroalkanes suitable as starting materials for the
process of this invention include:
Perfluoro-2,3-dimethylbutane,
Perfluorodecahydronaphthalene,
Perfluoro-1-methyldecahydronaphthalene,
Perfluorotetradecahydroanthracene,
Perfluorotetradecahydrophenanthrene,
Perfluoro-1,2-dimethylcyclobutane.
Other useful reactants will suggest themselves to those skilled in the art
upon reading this disclosure.
By "activated carbon" is meant an amorphous carbon having high adsorptivity
for gases, vapors, and colloidal solids. Such activated carbons are
typically formed from the carbon-source by heating to about 800.degree. to
900.degree. C. with steam or carbon dioxide to confer upon the carbon a
porous internal structure. Any of the well-known activated carbons can be
used in the practice of this invention as well as any carbons activated
according to the disclosure provided herein or any of the techniques known
in the art to improve carbon adsorptivity. Commercially available
activated carbons useful in the process of this invention include those
sold under the following trademarks: Darco.TM., Nuchar.TM., Columbia
SBV.TM., Columbia MBV.TM., Columbia MBQ.TM., Columbia JXC.TM., Columbia
CXC.TM., Calgon PCB.TM., and Barnaby Cheny NB.TM.. The source, grade, or
form of the activated carbon is not critical. However, it is preferred to
use granules to facilitate use in tubular reactors. The size of the
granules is not critical but it is preferred to employ granules having an
average mesh size of about 1/25 to 1/4 of the reactor diameter.
In the process of the invention the perfluoroalkane is contacted with
activated carbon at a temperature of from about 300.degree. to about
500.degree. C., preferably from about 350.degree. to 450.degree. C.
The process of this invention can be carried out readily in liqud or gas
phase using well-known chemical engineering practice, which includes
continuous, semi-continuous, or batch operations. The process is
conveniently carried out at atmospheric pressure, although either higher
or lower pressures can be employed. The type of reactor vessel used is not
critical so long as it is able to withstand the temperatures and pressures
employed. Reactor vessels of stainless steel are typically used although
other materials such as nickel-based corrosion resistant alloys, such as
Hastelloy.TM. alloy and tantalum can be used. The activated carbon can be
used in a fixed bed or a fluidized bed configuration.
Contact times can vary from fractions of a second to 2 hours or more.
Contact time is not critical since appreciable defluorination occurs even
with relatively short contact times. For example, in a continuous flow
process, a contact time as short as about 0.1 sec can be employed. In a
batch process, a contact time of about 2 hr or longer can be used. When a
continuous flow process is employed, contact time is calculated using the
following equation.
##EQU1##
The invention is further illustrated by the following examples in which all
parts and percentages are by weight and all degrees are Celsius unless
otherwise noted. unless otherwise specified, the activated carbon employed
in the examples comprised 12 to 30 mesh (2.00 mm-600 .mu.m) granules
having a surface area of over 1000 m.sup.2 /g (Calgon PCB.TM.) as
determined by standard nitrogen adsorption methods.
EXAMPLE 1
Defluorination of Perfluoro-2,3-dimethylbutane
A liquid flow of 2 mL/hr of perfluoro-2,3-dimethylbutane and 5 mL/min of
nitrogen was passed over a bed of 2.5 g of activated carbon at various
temperatures. The carbon was contained in a 1 cm diameter.times.10 cm long
stainless steel reactor which was heated in a fluidized sand bath.
Effluent from the hot reactor was transported directly to a gas
chromatograph and analyzed in a 6.10 m.times.0.32 cm (20 ft.times.1/8 in)
column of C.sub.9 fluorocarbon acrylate on a diatomaceous earth support
isothermally at 32.degree.. In addition to a peak for starting material at
14.6 minutes, two product peaks were observed. A liquid sample was
collected and analyzed by both fluorine nmr and GC/MS on a fluorosilicone
capillary column and the products were assigned as
perfluoro-2,3-dimethylbutene and perfluoro-2,3-dimethylbutadiene, the
result of successive defluorinations of the starting perfluoroalkane and
the intermediate perfluoroalkene. Table 1 summarizes the proportions of
the two products as the temperature was raised from 300.degree. to
400.degree. and the flow rates varied. The samples were taken at regular
intervals over a 2-hr period.
TABLE 1
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Defluorination of Perfluoro-2,3-dimethylbutane.sup.(1)
Sample Alkane Olefin Diene
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1 41 12 40
2 55 20 20
3 62 21 9
4 71 16 2
5 79 13 1
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.sup.(1) All numbers are area percent. The sum is not 100% because minor
amounts of unidentified products were also formed.
EXAMPLE 3
Defluorination of Perfluorotetradecahydrophenanthrene
A mixture of 2 g of activated carbon and 1 mL of
perfluorotetradecahydrophenanthrene, admixed with the corresponding
anthracene derivative, was heated in a sealed vessel at 400.degree. for 4
hr. The recovered carbon was extracted with chloroform, and a fluorine nmr
spectrum was obtained on the extract. The spectrum was consistent with a
product mixture of perfluorooctahydrophenanthrene and
perfluorooctahydroanthracene in about a 1:1 molar ratio.
EXAMPLE 3
Defluorination of Perfluorodecahydronaphthalene
A 1 mL sample of a mixture of cis and trans isomers of
perfluorodecahydronaphthalene was heated in a sealed vessel with 1 g of
activated carbon at 450.degree. for 2 hr. The fluorine nmr spectrum of the
resulting defluorinated product indicated the presence of
perfluoro-9-octalin.
EXAMPLE 4
Defluorination of Perfluoro-1-methyldecahydronaphthalene
In a flow reactor similar to the one employed in Example 1, a liquid flow
of 1 mL/hr of perfluoro-1-methyldecahydronaphthalene and 5 mL/min of
nitrogen was passed over 3 g of activated carbon at 400.degree.. Liquid
was collected for one hour and analyzed by both F-19 nmr and GC/MS. Still
present were three isomers of the starting materials, C.sub.11 F.sub.20.
One peak with an empirical formula of C.sub.11 F.sub.18 was seen, which by
fluorine nmr had the double bond in the 9-10 position. Two isomers of
C.sub.11 F.sub.14 were seen, which by nmr were identified as perfluoro-1-
and perfluoro-2-methyltetralin, present in 7 and 1 area percent,
respectively.
EXAMPLE 5
Defluorination of Perfluorodecahydronaphthalene
This Example demonstrates the use of several different types of activated
carbon for the defluorination reaction. In a flow reactor similar to that
employed in Example 1, a liquid flow of 1 mL/hr of a mixture of cis- and
trans-perfluorodecahydronaphthalenes and 5 mL/min of nitrogen was passed
over a bed of a designated activated carbon at a temperature of
450.degree.. Liquid was collected for about 0.5 hr, and the product was
analyzed by GC/MS on a fluorosilicone capillary column. The product
obtained was perfluoro-9-octalin. The results ae summarized in Table 2.
TABLE 2
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Perfluorodecahydronaphthalene Defluorination
over Various Activated Carbons.sup.(1)
Recovered Starting Material
Product
Carbon Trans-Isomer Cis-Isomer 9-Octalin
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None 53 43 0
Control 55 41 <1
(Graphite)
Control 50 23 1
(Colloidal)
Graphite.sup.(2)
Nuchar .TM.
53 17 24
Darco .TM.
55 38 3.6
2:1 C/SiO.sub.2
48 30 10
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.sup.(1) All numbers are area percent.
.sup.(2) The high surface area colloidal graphite was a commercial sample
(Acheson Aquadag .TM.) which was subsequently isolated by freezing,
filtering, washing and drying.
CONTROLS A-G
A gaseous flow of 5 mL/min of perfluorobutane was passed over a bed of 5 g
of 8-30 mesh (2.36 mm-600 .mu.m) activated carbon pellets at various
temperatures. The carbon was contained in a 1 cm in diameter by 10 cm in
length Vycor.TM. glass reactor which was heated in a split-tube furnace.
Effluent from the hot reactor was transported directly for analysis at
room temperature to a gas chromatograph equipped with a 20 m capillary
column of --OCH.sub.2 CF.sub.3 derivatized silicone oil and a flame
ionization detector. Retention times for perfluorobutane (1.42 min),
perfluoro-cis-2-butene (1.45 min) and perfluoro-trans-2-butene (1.62 min)
were determined using known standards. The results are summarized in the
table below.
TABLE 3
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Relative Area (%)
Control Temp. (.degree.C.)
Total Area Butane
t-2-Butene
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A 275 120 100 0
B 325 95 100 0
C 375 208 100 0
D 425 267 100 0
E 475 159 100 0
F 525 28 62 36
G 575 2 57 43
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These controls show that the defluorination process of the prior art, e.g.
Baranauckas, et al. patent, cannot be used to predict the reactions of
perfluorocarbons. This patent requires the presence of a chlorine atom on
one of the adjacent carbon atoms of the fluorocarbon. These experiments
show that when a chlorine is not present, i.e. use of perfluorobutane
instead of 2-chlorononafluorobutane, defluorination does not occur at all
within the temperature range of 275.degree. C. to 450.degree. C. specified
in the patent. And when temperatures above the range specified in the
patent are used, degradation occurs to the extent that insignificant
amounts of the butane and butene survive.
It is also clear from these controls that the process of the present
invention requires a critical relationship of the elements in the
perfluoroalkane, namely, the perfluoroalkane must contain at least two
adjacent tertiary carbon atoms.
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