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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention concerns a method for defluorinating a perfluoroalkene to
the corresponding more highly unsaturated fluorocarbon.
2. References
U.S. Pat. No. 4,377,717, issued to Anello, et al., on Mar. 22, 1983,
discloses dimerization of hexafluoropropene to perfluoro-2-methylpentene-2
by heating at elevated temperatures in the presence of activated carbon.
Chambers, et al., J.C.S. Perkin I, 1064 to 1067 (1981), and Chem. Comm.,
475 and 476 (1978), describe defluorination of perfluorinated alkenes over
platinum, iron, and cesium fluoride at elevated temperatures to give
perfluorinated dienes and other products.
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. 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.
Ohsaka, et al., Chem. Abstracts 94, 191660 (1981) (Ger. Offen. No.
3,027,229) disclose oligomerization of hexafluoropropene over an alkali
fluoride on carbon or NiO catalyst. Specifically disclosed is the
oligomerization of C.sub.3 F.sub.6 over KF on carbon at 200.degree. C. to
give a mixture of dimer, trimer, and C.sub.9 F.sub.16.
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 two carbon-carbon double bonds
comprising contacting the corresponding perfluoroalkene having at least
one carbon-carbon double bond in which both olefinic carbon atoms are
quaternary with activated carbon to defluorinate the perfluoroalkene,
thereby forming the perfluoroalkene having at least two carbon-carbon
double bonds.
DETAILED DESCRIPTION OF THE INVENTION
The products of the invention, perfluoroalkenes having at least two
carbon-carbon double bonds, are useful as comonomers for preparation of
fluorocarbon-containing polymers and as cross-linking agents in
fluoropolymers. The instant process represents an improvement in prior art
processes for preparing perfluoroalkadienes in that lower temperatures can
be employed and expensive metals such as platinum are not required. In
some cases, isomerization may accompany or precede defluorination. For
example, if perfluoro-3-isopropyl-4-methyl-2-pentene is employed as a
starting material, isomerization to
perfluoro-2,4-dimethyl-3-ethyl-2-pentene precedes defluorination (see
Example 1).
The starting materials for the process of the invention are
perfluoroalkenes having at least six carbon atoms. Perfluoroalkenes
suitable as starting materials have at least one carbon-carbon double bond
in which both olefinic carbon atoms are quaternary. By "quaternary carbon
atom" is meant one to which only carbon atoms are attached. Preferred
reactants are perfluoroalkenes containing 6 to 14 carbon atoms, the
perfluoroalkene having only one carbon-carbon double bond. If the starting
perfluoroalkene has more than one requisite carbon-carbon double bond,
then the product can have more than two double bonds.
Representative perfluoroalkenes suitable as starting materials for the
process of this invention include:
Tetrakis(trifluoromethyl)ethylene,
Perfluoro-2,4-dimethyl-3-ethyl-2-pentene,
Perfluoro-3,4-dimethyl-3-hexene,
Perfluoro-3,4,5-trimethyl-3-heptene, and
Perfluoro-1,2-bis(cyclobutyl)cyclobutene.
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 perfluoroalkene 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 liquid 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 Hexafluoropropene(HFP) Trimers
A 1 mL sample of mixed HFP trimers (C.sub.9 F.sub.18),
perfluoro-3-isopropyl-4-methyl-2-pentene and
perfluoro-2,4-dimethyl-3-ethyl-2-pentene, was heated in a sealed vessel
with 1 g of activated carbon at 400.degree. for 2 hrs. The product was
shown to contain three isomers of C.sub.9 F.sub.16 by GC/MS and fluorine
nmr. Detailed analysis of the GC/MS patterns suggested the structures A,
B, and C for the three defluorinated products. C apparently resulted from
cyclization of B.
##STR1##
EXAMPLE 2
Defluorination of Perfluoro-3,4-dimethyl-3-hexene
A mixture of 1 mL of perfluoro-3,4-dimethyl-3-hexene and 1 g of activated
carbon was heated in a sealed vessel at 400.degree. for 2 hr. The
recovered carbon was extracted with chloroform, and the reaction product
was identified as perfluoro-3,4-dimethyl-2,4-hexadiene by GC/MS and
fluorine nmr spectrum.
EXAMPLE 3
Defluorination of Perfluoro-1,2-bis(cyclobutyl)-cyclobutene
A mixture of 1 mL of perfluoro-1,2bis(cyclobutyl)cyclobutene (C.sub.12
F.sub.18) and 1 g of activated carbon was heated in a sealed vessel at
350.degree. for 2 hr. The recovered carbon was extracted with chloroform,
and the reaction product was shown to contain a mixture of several isomers
of C.sub.12 F.sub.16 and C.sub.12 F.sub.14 by GC/MS.
EXAMPLE 4
Defluorination of Perfluoro-3,4,5-trimethyl-3-heptene
A mixture of 1 mL of perfluoro-3,4,5-trimethyl-3-heptene (C.sub.10
F.sub.20) and 1 g of activated carbon was heated in a sealed vessel at
400.degree. for 2 hr. The recovered carbon was extracted with chloroform,
and the reaction product was identified as a mixture of several isomers of
C.sub.10 F.sub.18 by GC/MS.
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