A highly selective process is disclosed for the hydrogenolysis of 2,2-dichlorohexafluoropropane (i.e., CFC-216aa or CF.sub.3 CCl.sub.2 CF.sub.3) to 2,2-dihydrohexafluoropropane (i.e., HFC-236fa or CF.sub.3 CH.sub.2 CF.sub.3) and 2-chloro-2-hydrohexafluoropropane (i.e., 226da or CF.sub.3 CHClCF.sub.3). The process involves reacting the starting material with hydrogen at an elevated temperature of about 300.degree. C. or less in the presence of a catalyst containing a catalytically effective amount of palladium supported on a support of fluorinated alumina and/or aluminum fluoride.

FIELD OF THE INVENTION
 This invention relates to catalytic hydrogenolysis of halofluorocarbons;
 and more particularly to the hydrogenolysis of said materials using
 palladium-containing catalysts.
 BACKGROUND
 Various processes for the catalytic hydrogenolysis of chlorofluorocarbons
 and hydrochlorofluorocarbons are known. For example, British Patent
 Specification 1,578,933 illustrates that mixtures of C.sub.2 Cl.sub.2
 F.sub.4 isomers can be subjected to hydrogenolysis over a particulate
 catalyst of palladium on charcoal (which was intimately mixed with glass
 helices to prevent clogging) or palladium on alumina, to mixtures of
 C.sub.2 H.sub.2 F.sub.4 isomers. U.S. Pat. No. 2,942,036 discloses the
 reaction of 1,2,2-trichloropentafluoropropane with hydrogen in the
 presence of palladium on activated carbon catalyst to produce
 1,2,2-trihydropentafluoropropane. The examples show that under the
 conditions of the experiments one of the products from this reaction is
 CF.sub.3 CH.dbd.CF.sub.2. The carbon support may be treated with aqueous
 HF prior to depositing palladium on the support for the purpose of
 removing silica from the carbon. U.S. Pat. No. 5,171,901 discloses inter
 alia the catalytic hydrogenation of CF.sub.3 CCl.sub.2 CF.sub.3 and/or
 CF.sub.3 CHClCF.sub.3 using suitable catalysts (e.g., palladium).
 Disclosed support materials include activated carbons, aluminas, silicas,
 barium sulfate, spinels, silicates and titanium dioxide. Preferred
 supports are activated carbons and lithium/aluminum spinels. Examples are
 provided wherein 2,2,-dichlorohexafluoropropane and
 2-chloro-2-hydrohexafluoropropane are hydrogenated using palladium
 supported on globular lithium/aluminum spinel. Japanese Patent Application
 Publication Hei 1(1989)-319441 discloses a process where one chlorine atom
 is selectively replaced by hydrogen in 1,1,1-trichlorotrifluoroethane
 using a platinum catalyst. For comparison, a palladium on carbon catalyst
 is disclosed to produce 1,1,1-trifluoroethane as the major product under
 the conditions of the experiment.
 SUMMARY OF THE INVENTION
 The present invention provides a process for the hydrogenolysis of
 2,2-dichlorohexafluoropropane (i.e., CFC-216aa or CF.sub.3 CCl.sub.2
 CF.sub.3) to 2,2-dihydrohexafluoropropane (i.e., HFC-236fa or CF.sub.3
 CH.sub.2 CF.sub.3) and 2-chloro-2-hydrohexafluoropropane (i.e., 226da or
 CF.sub.3 CHClCF.sub.3). The process comprises reacting said starting
 material with hydrogen at an elevated temperature of about 300.degree. C.
 or less in the presence of a catalyst containing a catalytically effective
 amount of palladium supported on a support selected from the group
 consisting of fluorinated alumina, aluminum fluoride and mixtures thereof.
 DETAILED DESCRIPTION
 The catalysts suitable for the process of this invention comprise
 palladium. The palladium is supported on fluorinated alumina, aluminum
 fluoride and/or a mixture thereof. Preferred catalysts consist essentially
 of palladium on an aluminum fluoride or fluorinated alumina support. The
 procedure for preparing such a catalyst is described in U.S. Pat. No.
 4,873,381, the entire contents of which are hereby incorporated herein by
 reference.
 The concentration of palladium on the fluorinated alumina and/or aluminum
 fluoride support is typically within the range of from 0.1 to 10% by
 weight of the catalyst. The support can be prepared by fluorination of
 alumina at elevated temperatures. It is preferred that the fluorine
 content of the support be sufficient to provide a fluorine to aluminum
 atomic ratio of at least 2.4. The aluminum fluoride or fluorinated alumina
 support utilized in the instant invention has the advantage of being
 regeneratable by conventional means, which carbon-based supports do not
 have. For example, CF.sub.3 CCl.sub.2 CF.sub.3 can be reacted with
 hydrogen over a catalyst of this invention until the conversion rate of
 CFC-216aa decreased by at least about 20 percent compared to the
 conversion rate of CFC-216aa using fresh catalyst at the same conditions;
 and the catalyst can then be regenerated (e.g., by first treating with air
 or oxygen at elevated temperature, and then reducing with hydrogen).
 The reaction temperature is typically within the range of from about
 100.degree. C. to about 300.degree. C. A preferred range is from about
 100.degree. C. to 250.degree. C. Generally, in order to provide
 substantial hydrogenolysis product yields, the amount of hydrogen used is
 at least about 0.5 mole per mole of the CFC-216aa starting material. To
 provide yields desired in many embodiments, at least stoichiometric
 amounts of hydrogen are used. A considerable excess of hydrogen can also
 be advantageously employed to provide the yields desired in many
 embodiments in addition to serving as a heat sink to reduce the overall
 temperature rise in the reactor. The three-carbon hydrogenolysis product
 from the hydrogenolysis of CFC-216aa contains at least 90% of the fluorine
 atoms contained in the CFC-216aa reacted and less than 5 mole percent of
 said product contains 5 fluorine substituents.
 CFC-216aa utilized in this process can be made by conventional means which
 are well known to the art (see, e.g., U.S. Pat. No. 5,068,472).
 The hydrogenolysis of 2,2-dichlorohexafluoropropane with hydrogen 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 hydrogen halide.
 Pressure is not critical. Atmospheric and superatmospheric pressures are
 the most convenient and are therefore preferred.
 Unreacted 2-chloro-2-hydrohexafluoropropane may be recycled to the reactor
 to produce additional quantities of 2,2-dihydrohexafluoropropane or be
 used as an organic intermediate to produce 2-hydroheptafluoropropane.
 HFC-236fa is useful as a refrigerant, fire extinguishant, heat transfer
 medium, gaseous dielectric, sterilant carrier, polymerization medium,
 particulate removal fluid, carrier fluid, buffing abrasive agent,
 displacement drying agent and power cycle working fluid. In particular,
 HFC-236fa is a highly effective refrigerant.
 The present process has the advantage that the desirable products are
 obtained in extremely high selectivity.

Practice of the invention will become further apparent from the following
 non-limiting examples.
 EXAMPLES
 Preparation of Palladium on Fluorinated Alumina
 Commercial 0.5 weight percent palladium on alumina (21.4 g, 1.6 mm
 extrudates) was placed in a reactor and heated to 175.degree. C. in a flow
 of nitrogen (20 cc/min.) for about 2 hours. At the end of this period the
 nitrogen flow was increased to 50 cc/min. and an HF flow (50 cc/min.) was
 passed through the reactor. After the initial exotherm subsided (about
 three hours), the nitrogen flow was reduced to 20 cc/min. and the HF flow
 increased to 80 cc/min. The reactor temperature was then gradually
 increased to about 400.degree. C. over about a five hour period and
 maintained at 400.degree. C. for an additional 30 minutes. The HF flow was
 then stopped and the reactor purged with nitrogen. The palladium on
 fluorinated alumina prepared by this method was used in the hydrogenolysis
 experiment below.
 Hydrogenolysis of CFC-216aa using Palladium on Fluorinated Alumina Catalyst
 Run No. 1--Liquid CFC-216aa (3 mL/hour) was vaporized and mixed with 20
 cc/minute of hydrogen. This vapor mixture was sent through a 0.5" (12.7
 mm) O.D..times.8" (203 mm) Hastelloy.TM. nickel alloy reactor containing
 15.5 g of the palladium on fluorinated alumina catalyst maintained at
 150.degree. C. using a fluidized sand bath. After nine hours of operation
 under these conditions, organic product analysis using conventional gas
 chromatography indicated that CFC-216aa conversion was essentially
 complete. The hydrogen-containing products included 32.4% HFC-236fa
 (2,2-dihydrohexafluoropropane) and 64.5% HCFC-226da
 (2-chloro-2-hydrohexafluoropropane) in addition to very small quantities
 of other products.
 Run No. 2--Run No. 1 was repeated except that the temperature was
 200.degree. C. After about 15 hours of operation, CFC-216aa conversion was
 still complete. The organic hydrogen-containing products included 45.5%
 HFC-236fa and 52% HCFC-226da.
 Run No. 3--Run No. 2 was repeated except that the hydrogen flowrate was
 increased to 40 cc/minute. The CFC-216aa conversion was complete. The
 organic hydrogen-containing products included 48.9% HFC-236fa and 49.4%
 HCFC-226da.
 Run No. 4--Run No. 3 was repeated except that the temperature was increased
 to 250.degree. C. The organic hydrogen-containing products included 64.5%
 HFC-236fa and 28.0% HCFC-226da.
 Run No. 5--Run No. 4 was repeated except that the hydrogen flow rate was
 reduced to 20 cc/min. CFC-216aa conversion was complete. The organic
 hydrogen-containing products included 63.1% HFC-236fa and 30.1%
 HCFC-226da.
 Comparative Hydrogenolysis of CFC-216aa using Palladium on low-ash
 acid-washed carbon
 Carbon Support
 The carbon support used in the examples was a 4.times.8 mesh (about 4.7
 mm.times.2.4 mm) commercial grade coconut shell carbon which had (before
 washing) an ash content of about 2.6 weight percent. After hydrochloric
 acid washing, the carbon support had an ash content of less than about 0.1
 weight percent.
 Run No. 6--Liquid CFC-216aa (3 mL/hour) was vaporized and mixed with 10
 cc/minute of hydrogen. This vapor-mixture was sent through a 0.5" (12.7
 mm) O.D..times.8" (203 mm) Hastelloy.TM. nickel alloy reactor containing
 7.2 g of 0.5 weight percent palladium supported on low-ash, acid-washed
 carbon maintained at 150.degree. C. using a fluidized sand bath. Organic
 product analysis using conventional gas chromatography indicated that
 about 90% of the starting material had been converted. The
 hydrogen-containing products included 15.7% 2,2-dihydrohexafluoropropane
 (HFC-236fa), 54.3% 2-chloro-2-hydrohexafluoropropane (HCFC-226da), 12.3%
 2-hydropentafluoropropene, and 1.7% 1,2,2-trihydropentafluoropropane
 (HFC-235fa) and small quantities of other compounds.
 Run No. 7--Run No. 6 was repeated except that the hydrogen flowrate was
 increased to 30 cc/minute. Organic product analysis using conventional gas
 chromatography indicated that the starting material conversion was
 essentially complete. The hydrogen-containing products included 24.8%
 2,2-dihydrohexafluoropropane (HFC-236fa), 54.6%
 2-chloro-2-hydrohexafluoropropane (HCFC-226da) and 19.8%
 1,2,2-trihydropentafluoropropane (HFC-235fa) and small quantities of other
 compounds.
 This comparative experiment illustrates that when using palladium supported
 on acid-washed carbon as catalyst for the hydrogenolysis of CFC-216aa
 (where two chlorines of the starting compound are on the middle carbon and
 the two adjacent carbons contain trifluoromethyl groups) an olefin and/or
 a saturated product containing one less fluorine than the starting
 compound can be produced in significant amounts.