CA1048545A - Process for preparing hexafluoropropanone-2 - Google Patents

Abstract

Abstract of the Disclosure A process for preparing hexafluoropropanone-2 from hexafluoropropene which comprises contacting hexafluoropropene and oxygen with a fluorinated alumina as a catalyst. The use of fluorinated alumina as the catalyst enables the process to be carried out in a single step, whereas formerly the reaction required two distinct reaction steps.

Description

104~S~S
The present invention relates to a process for preparing hexafluoropropanone-2. More particularly, it relates to a process for preparing hexafluoropropanone-2 from hexafluoropropene with a high efficiency by the use of a specific catalyst.
Hexafluoropropanone-2 is useful by itself as a catalyst for polymerization of perfluorocyclobutene or triazine. Because of its polymerizability, it forms a terpolymer with tetrafluoroethylene and ethylene. It is also useful as the starting material~for the production of bisphenol AF ((C6H4-OH)2C(CF3)2) which is an excellent crosslinking agent for fluorine-containing elastomers.
For production of hexafluoropropanone-2, there is known a method which comprises oxidizing hexafluoropropene and subjecting the resultant 1,2-epoxyhexafluoropropane to rearrangement to obtain hexafluoropropanone-2. For example, hexafluoropropene and oxygen are contacted with activated silica gel at a temperature of 140 to 280C to obtain 1,2-epoxyhexafluoropropane, which is then subjected to rearrange-ment in the presence of a Lewis acid such as aluminum oxide , to give hexafluoropropanone-2 (U.S. patent No. 3,775,439).
Thus, in the conventional method, two steps of reaction are required for the production of hexafluoropropanone-2.
As the result of extensive studies, it has now been found that hexafluoropropanone-2 can be prepared from -hexafluoropropene and oxygen by one step of reaction when a specific catalyst, i.e. fluorinated alumina, is used. The present invention is based on this finding.
According to the present invention, there is provided a process for preparing hexafluoropropanone-2

- 2 --` 1048S4S
from hexafluoropropene by a one step reaction which comprises contacting hexafluoropropene and oxygen in the presence of fluorinated alumina.
The catalysts used in the process of the present invention are fluorinated aluminas, some of which are known as catalysts for the reforming of hydrocarbons and others of which are known as catalysts in the rearrangement of chlorofluorohydrocarbons. While any catalyst known as "fluorinated alumina" can be used in the process of this -invention, but preferably the Eluorinated alumina comprises essentially aluminum, fluorine and oxygen and its fluorine content is preferably from about 0.5 to 50 % by weight.
The fluorinated alumina used as the catalyst is ordinarily prepared by treatment of activated alumina with a fluorinating agent.
Any conventional activated alumina can be used ~n the preparation of the catalyst, e.g. a natural alumina or synthetic alumina, for example a highly porous alumina obtained by calcining -alumina hydrate or ~-alumina hydrate under appropriately controlled conditions. Some of the commercially available activated aluminas contain silica as an agent to facilitate tablet-formation. The presence of silica in an amount up to about 2Q % by weight of the total weight is not disadvantageous for the production of the catalyst to be used in the process of the invention. When the amount is larger than about 50 % by weight, however, the catalytic activity of the resulting catalyst is significantly reduced and industrial use of the catalyst may not be possible.
An inorganic fluorinating agent or an organic fluorinating agent may be used in the preparation of the catalyst.

11)48545 Examples of the inorganic fluorinating agent are hydrogen fluoride, silicon tetrafluoride, sulfur fluoride (e.g. sulfur tetrafluoride, sulfur hexafluoride), sulfuryl fluoride, thionyl fluoride, ammonium fluoride (e.g~ acidic ammonium fluoride, neutral ammonium fluoride), etc. Examples of the organic fluorinating agent include fluorohydro-carbons, chlorofluorohydrocarbons, bromofluorohydrocarbons, etc. Fluorine-containing compounds of the formula: CnFaHbX
wherein X is an oxygen atom or a nitrogen atom, n is an integer of 1 to 8 (preferably 1 to 43,~a is an integer of 1 to 2n + m, b is an integer of 0 to 2n + m - 1 and m is an integer of 2 when x is an oxygen atom or an integer of 3 when X is a nitrogen atom, as disclosed in Japanese Patent Publication (unexamined) No. 1578/1972, can be also used as the organic fluorinating agent. The fluorohydrocarbons may be saturated or unsaturated hydrocarbons having not more than 8, preferably not more than 4, carbon atoms in which at least one hydrogen atom is substituted with a fluorine atom.
A higher degree of substitution with fluorine atoms is more preferable. Specific examples are CF4, CHF3, CF3CF3, CHF2CF3' CHF2CHF2' CH3CF3~ CH2FCHF2' CH2=CF2' CF3CF=CF2' --CF2=CF2, etc. The chlorofluorohydrocarbons and the bromo-,: :
fluorohydrocarbons may be saturated or unsaturated hydro-carbons having not more than 8, preferably not more than 4, carbon atoms in which hydrogen atoms are substituted with at least one fluorine atom and at least one chlorine or bromine atom and include specifically CC13F, CC12F2, CHC12F, CHClF2, CClF2CC12F~ CC13CF3r CC12FCC12F~ Ccl3ccl2Fr CClF2CClF2r ~ -CC12FCF3, CF3CCl=CClCF3, CF2BrCFClBr, CF2BrCHClF, CF2BrCF2Br, etc. Examples of the fluorine-containing compounds are :; :

1(~4854S
hexafluoroacetone, hexafluoro-1,2-epoxyethane, decafluoro-diethyl ether, tri(trifluoromethyl)amine, tetrafluoroethyl methyl ether, etc. Among them, perfluoroalkanes such as tetrafluoromethane and perfluoroalkenes such as hexafluoro-propene are particularly preferred.
~ The preparation of the catalyst may be carried out c ~ by various procedures depending on the kind of the fluorinat-ing agent as employed.
When, for instance, hydrogen fluoride or ammonium , 10 fluoride is employed as the fluorinating agent, the acti-vated alumina is contacted with it at a temperature of about 20 to 450C so as to give the fluorinated alumina.
` When sulfur fluoride, sulfuryl fluoride or thionyl ; fluoride is employed, the activated alumina may be contacted with the fluorinating agent at a temperature of about 300 to 500C to give the fluorinated alumina. In some cases, ~! sulfurous compounds may be formed and deposited on the catalyst, but they are not poisonous to the catalytic activity.
When the fluorinating agent is an organic fluorinat-ing agent, the activated alumina may be contacted with it at a temperature of about 100 to 600C, preferably of about 150 to 450C to give the desired fluorinated alumina.
In case of using an organic fluorinating agent, the treatment of the activated alumina with a chlorohydro-; carbon or a bromohydrocarbon may be effected prior to the contact with the organic fluorinating agent. The coexistence ; of a chlorohydrocarbon or a bromohydrocarbon on the contact . of the activated alumina with the organic fluorinating agent '! 30 is sometimes recommendable, since the fluorination of the '!
~ - 5 -.

-1~48S45 activated alumina can be accomplished more smoothly under a lower temperature.
As the chlorohydrocarbon or the bromohydrocarbon, there may be employed a saturated or unsaturated hydrocarbon having not more than 8, preferably not more than 4, carbon atoms in which at least one hydrogen atom is substituted with a chlorine or bromine atom. A higher degree of sub-stitution with chlorine or bromine atoms is more preferable.
Substitution with chlorine or bromine atoms alone or with both of them is admissible. Specific examples are CC14, CHC13, CC13CC13, CHC12CC13, CC12=CC12, CHCl=CC12, CHBr3, CC12Br2, etc. Among them, perchlorohydrocarbons are par- ;
ticularly preferred.
Explaining the preparation of the fluorinated alumina by treatment of the activated alumina with the fluorohydrocarbon and the chlorohydrocarbon or the bromo-hydrocarbon, the activated alumina may be contacted first with the chlorohydrocarbon or the bromohydrocarbon at a temperature of about 100 to 400C (preferably 100 to 200C) and then with the fluorohydrocarbon at a temperature of about 100 to 400C (preferably 100 to 350C), whereby the fluorinated alumina can be obtained.
Alternatively, the activated alumina may be contacted with a mixture of the chlorohydrocarbon or the bromohydrocarbon and the fluorohydrocarbon at a temperature of about 100 to 400C ~preferably 200 to 300C). The mix-ing proportion of the chlorohydrocarbon or the bromohydro-carbon to the fluorohydrocarbon is determined depending on their kinds. In the combination of tetrachloromethane and trichlorotrifluoroethane, for instance, the molar ratio of '' ~

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lQ48S45 ~ tetrachloromethane and trichlorotrifluoroethane is desired : ..
; to be about 0.1 - 5 ~
In addition to the procedures as above, the ;,i fluorinated alumina may be produced by any conventional procedure, for instance, as described in Japanese Patent Publications Nos. 11605/1964 and 27748/1968.
When the catalyst is used for a long period of time, carbonaceous materials are deposited on its surface to ~` lower the catalytic activity. In such case, the catalytic activity can be recovered by heating the catalyst in the presence of oxygen or an oxygen-containing material such as air at a temperature of about 350 to 500C.
; The process of the invention can be effected by contacting hexafluoropropene and oxygen with the fluorinated alumina as the catalyst in a per se conventional manner.
Thus, hexafluoropropene and oxygen may be contacted with a fixed bed, moving bed or fluidized bed of the catalyst in an appropriate reaction vessel or tube in a continuous system ; or a closed system.
j 20 The mixing proportion of hexafluoropropene and oxygen is usually about 1 : 10 - 0.1 (molar ratio), pre-.. ..
: ferably about 1 : 2 - 0.3. When the amount of oxygen is ~-smaller than the lower limit of the said range, the con-version rate is low. When the amount of oxygen is larger than the upper limit, the efficiency of apparatus is reduced. In case of necessity, an inactive gas such as ~ carbon dioxide, nitrogen or helium may be employed as the :~ diluent.
The reaction temperature at the contact is usually from about 80 to 300C, preferably from about 100 to 250C.

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When the temperature is lower than the lower limit of the said range, the conversion rate is lowered. When the temper-ature is higher than the upper limit, the yield is reduced.
At a temperature lower than about 80C, the reaction hardly proceeds. At a temperature higher than about 300C, the - yield is extremely low. The reaction pressure may be an atmospheric pressure or a higher pressure. In general, a higher pressure is preferable for increasing the conversion ; rate and the yield. For industrial use, a pressure of about 0 to 20 kg/cm2G is usually adopted. ~ ;
The contact time is determined on the other condi-tions, particularly temperature. At a higher temperature, a ;~
shorter contact time is adopted, and at a lower temperature, a longer contact time is desired, as in case of other usual reactions. In general, a contact time of 30 minutes or less (e.g. 0.5 second) is preferable. A longer contact time results in a higher conversion. From the economical view-point, a proper contact time may be chosen. For instance, a contact time of about l second to 10 minutes is usually adopted in a continuous system in which the temperature is --about 100 to 250C.
As already mentioned, the production of hexa-fluoropropanone-2 from hexaflouropropene has been hitherto effected by two steps of reaction. According to the process of this invention, the production can be effected more efficiently by only one step of reaction. On analyzing the reaction products, formation of 1,2-epoxyhexafluoropropane, which is the intermediate in the conventional process, is not confirmed. In the conventional process, the second step ~ -of reaction is usually carried out in the presence of a , . , , .. . .: . . .

Lewis acid as the catalyst.
While the conventionally known oxidation catalyst (e.g. silica) for hexafluoropropene mainly produces 1,2-epoxyhexafluoropropane, it is revealed that the fluorinated alumina does not produce 1,2-epoxyhexafluoropropane. On the other hand, Lewis acids (e.g. alumina, aluminum trichloride) - known as catalysts for rearrangement of 1,2-epoxyhexafluoro-propane to hexafluoropropanone-2 do not exert any activity for the reaction of hexafluoropropene with oxygen. From these facts, the fluorinated alumina i8 presumed to result in the selective formation of hexafluoropropanone-2 in the oxidation of hexafluoropropene by its unexpected character-istics.
Practical and presently preferred embodiments of the invention are illustratively shown in the following Examples.
Example 1 , (1) Preparation of catalyst:
j In a reaction tube made of Pyrex glass (28 mm in diameter, 1000 mm in length) and vertically set up in an electric furnace, there is charged granular activated alumina having a particle size of 2.3 to 4.7 mm (alumina gel; "Neobead C-4" manufactured by Mizusawa Kagaku Co., Ltd.) (51.25 g). Dehydration is effected under heating at -500C for 1 hour in nitrogen stream, and then the tempera-ture is lowered to 200C. The supply of nitrogen is stopped, and a mixed solution of CC14 and CF2ClCFC12 (1 : 1 in molar ratio) is introduced at a rate of 1 g/min from the top of the reaction tube. The upper part of the alumina layer shows immediately a raise of the temperature up to _ g _ ' `

1~48545 , 270C. The zone of high temperature is gradually moved to ~ the lower layer, and after 40 minutes, an equilibrium is attained whereby the whole alumina layer shows a temperaturehigher than the designed temperature by about 10C. Then, the designed temperature ~s raised to 250C. In this case too, there is caused a slight hot spot which moves to the lower layer with lapse of treating time and, after 45 minutes, passes the lowest layer. Further, the designed temperature is raised up to 300C, whereby the raise of temperature is hardly shown. After thé--treatment for 40 minutes, the furnace is allowed to cool, and the catalyst is taken out. The thus obtained catalyst having a fluorine content of 9.9 % by weight is designated as "catalyst I".
The preparation of the catalyst is effected in the same manner as mentioned above under the following condi-tions to obtain the catalysts designated as "catalyst II"
and "catalyst III".

.. _ .... ~ , . Catalyst4/ 2 2 Velocity of Treating time (hr)¦ Fluorine 20 (molar ratio) current of 200C1250C¦300C (% by tion (g/min l weight) ... _ _ , II 0.1 : 1 1.0 0.5 0.7 0.6 9.1 III 0 1 1.0 1 1 0.83 9.3 i (2) Preparation of hexafluoropropanone-2:
The catalyst I obtained in (1) (50 g) (apparent volume, 50 ml) is charged in.to a reaction tube made of Hastelloy C being 18 mm in inner diameter and 1 m in length.
A mixture of hexafluoropropene and oxygen (1 : 0.7 in molar ~ ratio) is introduced therein under the following conditions:
30 temperature, 170C; pressure, 5 kg/cm G (gauge pressure);
amount of supplied gas, 100 ml/min (25C, 1 atmospheric -1((~48S~S
pressure).
The produced gas discharged from the reaction tube at the time of 3 hours after the initiation of introduction of the gaseous mixture is subjected to gas chromatographic, infrared spectrographic and mass spectrographic analyses, whereby the following results are obtained:
Compound Mol %
CF3CF=CF2 65.6 C02 1 . 5 :
COF2 6.6 CF3COF 3 .1 CF3COCF3 15 . 9 CF CFCF2 o ,i 3 .i 0 Examples 2 to 11 and Comparat~ve Examples 1 to 5 Hexafluoropropene and oxygen are contacted with the catalyst in the same manner as in Example 1 (2) under the conditions shown in Table 1. The catalyst used is the same as prepared in Example 1 (1). The composition of the discharged gas at the time of 3 hours after the initiation of introduction of the gaseous mixture is examined by gas chromatographic, infrared spectrographic and mass spectro-graphic analyses. The results are shown in Table 1.

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': ' ' : ' ~ , . ' -1~48545 Example 12 In a 17 ml-volume autoclave made of stainless steel, the catalyst II obtained in Example 1 (1) (2.0 g) is charged, and the temperature is kept at 130C. A mixture of hexafluoropropene and oxygen (1 1 in molar ratio) is introduced therein under pressure until the pressure becomes 5.0 kg/cm . The composition of gas in the contents is `~ examined by the gas chromatographic analysis 10 minutes and 1 hour after the initiation of the reaction. The results are shown in Table 2.
- Table 2 ~ _ .
Composition of gas (mol ~) CF3CF=CF2 C~4 CO2 COF2 CF3COF ¦CF3COCF~
i After 10 51.2 13.3 3.7 11.1 4.1 ¦ 16.6 ` 15 minutes _ l After 1 38.2 17-0 1~-7 14.2 4-7 ¦ 21-2 ~, ~
Example 13 (1) Preparation of catalyst:
A) In a reaction tube made of Pyrex glass (28 mm in diameter, 1000 mm in length) and vertically set up in an electric furnace, there is charged granular activated alumina as used in Example 1 (1) (50 g). While introducing hexafluoropropene into the reaction tube, the temperature is elevated to 200C, during which the generation of carbon monoxide and carbon dioxide is recognized at 160C. The temperature is maintained at 200C for 1 hourj during which hexafluoropropene is flowed in a rate of 80 ml/min (25C, 1 atm.). Then, the temperature is elevated to 250C, and this temperature is maintained for further 1 hour, during which 1f~48545 hexafluoropropene is flowed in the same proportion as above.
The temperature is further elevated up to 450C in nitrogen '1 stream and then oxygen is passed through in a rate of 100 ml/min (25c, 1 atm.) for 1.5 hours. The thus obtained fluorinated alumina as the~catalyst has a fluorine content of 4.02 % by weight.
B) As in A), granular activated alumina (50 g) is charged in a reaction tube, and the temperature is elevated to 200C in nitrogen stream. While maintaining this temper-ature, hexafluoropropene is flowed in a rate of 80 ml/ min (25C, 1 atm.) for 40 minutes. The temperature is elevated up to 450C in nitrogen stream and then oxygen is passed through in a rate of 100 ml/min (25C, 1 atm.) for 1.5 hours. The thus obtained fluorinated alumina has a fluorine , 15 content of 1.30 % by weight.
C) As in A), granular activated alumina (50 g) is charged in a reaction tube, and the temperature is elevated to 350 - 370C in nitrogen stream. While maintaining this temperature, sulfur hexafluoride is introduced into the reaction tube in a rate of 200 ml/min ~25C, 1 atm.) for a certain period of time as shown in Table 3, whereby the fluorinated alumina having a fluorine content as shown in Table 3 is obtained.
Table 3 ;.
Fluorinated Time for treatment Fluorine content alumina No. (hr) (% by weight) .
Cl 0.5 0.5 C2 1.8 2.5 C3 2.5 3.5 c4 5 5.5 _ _ _ 12.3 1~)4854S
D) In a reaction tube made of Hastelloy C (18 mm in diameter, 1000 mm in length) and vertically set up in an electric furnace, there is charged granular activated alumina having a particle size of 2.3 to 4.7 mm ("Activated Alumina KH-A46" manufactured by Sumitomo Chemical Company, Limited) (50 g). The temperature is elevated to 120C in nitrogen strea~, and then hydrogen fluoride is flowed in a rate of 100 ml/min (25C, 1 atm.) at 120C for 4 hours.
Then, the temperature is elevated up to 420C in nitrogen stream, and this temperature is maintained for 3 hours for elimination of water and hydrogen fluoride, whereby the fluorinated alumina having a fluorine content of 30 ~ by weight is obtained.
E) In a 200 ml volume autoclave made of stainless steel, granular activated alumina having a particle size of 2.3 to 4.7 mm (alumina gel; "Neobead C-4" manufactured by Mizusawa Kagaku) (50 g) is charged, and the atmosphere is reduced to vacuo. Hydorgen fluoride (1 g) is introduced into the autoclave, whereby the generation of heat is observed. After 40 minutes, the temperature is heated at 100C for 1 hour and then elevated to 420C. The contents are maintained at this temperature under a pressure of 0.1 mmHg for 3 hours, whereby water and hydrogen fluoride are-eliminated. The thus obtained fluorinated alumina has a fluorine content of 2.63 ~ by weight.
F) As in D), granular activated alumina is treated with gaseous hydrogen fluoride in a rate of 200 ml/min (25C, 1 atm.) at 140C for 4 hours, whereby the fluorinated alumina having a fluorine content of 49.8 ~ by weight is obtained.

1~48545 G) Granular activated alumina as in D) (50 g) is immersed in a 20 % aqueous solution of ammonium fluoride for 30 minutes. The activated alumina is taken out from the ammonium fluoride solution, dried at room temperature under reduced pressure and then charged in a reaction tube made of v Hastelloy C. The reaction tube is heated at 550C for 5 -hours, during which nitrogen is passed through. The ob-tained fluorinated alumina has a fluorine content of 4.5 by weight.
` 10 H) As in D), granular activated alumina (40 g) is treated with sulfuryl fluoride (300 ml/hour, at 25C, 1 atm.) at 427C for 3 hours, whereby the fluorinated alumina having a fluorine content of 1.5 % by weight is obtained.
(2) Preparation of hexafluoropropanone-2:
The catalyst obtained in (1) (40 g) is charged into a reaction tube made of Hastelloy C being 18 mm in inner diameter and 1 m in length. A mixture of hexafluoro-propene and oxygen (1 : 1 in molar ratio) is introduced therein under the following conditions: temperature, 175C;
pressure, 1 atm.; amount of supplied gas, 80 ml/min (25C, 1 atm.). The composition of the discharged gas is examined by gas chromatographic analysis. The results are shown in Table 4. -..
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xample 14 ` (1) Preparation of catalyst:
In a reaction tube made of Hastelloy C (18 mm in -inner diameter, 1000 mm in length) and vertically set up in an electric furnace, ther~ is charged granular activated ~ I .
alumina as used in Example 1 (1) (50 g). Dehydration is effected at 450C for 2 hours in nitrogen stream, and then the temperature is lowered to 170C. The supply of nitrogen is stopped, and a mixture of C2F6 and 2 tl: 1 in molar ratio) is introduced at a rate of 80 ml/min (25C, 1 atm.) from the top of the reaction tube. The temperature of the alumina layer shows a temporary elevation and, after 30 minutes, indicates the recovery to 170C. At this tempera-ture, the introduction of the said mixture is continued for 1.5 hours, during which the production of CO2 in trace is detected, Then, the temperature is raised to 200C, and the introduction of the mixture is continued for 4 hours, during which the formation of a small amount of CO2 is confirmed.
Thereafter, the inner temperature rises gradually, and the production of CF3COCF3 and CF4 is detected with the increase ' of the produced amount of CO2, at which the inner tempera-ture reaches to 220C in 2 hours. The reaction tube is allowed to cool by passing nitrogen through the same, and the catalyst is taken out. The fluorine content of the thus obtained catalyst is 1.8 % by weight. Deposition of carbonaceous materials is not recognized.
(2) Preparation of hexafluoropropanone-2:
The catalyst obtained in (1) t40 g) is charged into a reaction tube made of Hastelloy C being 18 mm in inner diameter and 1 m in length, and the temperature is .. . . . .. . .. : , . . .

1~48545 elevated to 160~C. ~exafluoropropene and oxygen are - introduced into the reaction tubé at amounts of 30 ml/min and of 20 ml/min (25C, 1 atm.) respectively, whereby the inner temperature is rapi~ly elevated to 190C and an S equilibrium is attained at this temperature. The com-position of the gaseous mixture taken out from the reaction tube at this time is as follows: CF4, 1.2 mol~; C02, 10.5 mol%; CF3COCF3, 2.1 mol%; C3F6, 86.2 mol~. Then, the inner temperature is gradually lowered and reaches to a constant temperature of 177C about 15 hours after the initiation of introduction of hexafluoropropene and oxygen. At this time, the gaseous mixture from the reaction tube shows the following composition: CF4, 4.8 mol%; CO2, 1.1 mol%; COF2,

3.8 mol%; CF3COF, 1.2 mol%; CF3COCF3, 13.3 mol%; C3F6, 75.8 mol%. The supply of hexafluoropropene and oxygen is further continued until the elapse of about 30 hours from the initiation, during which the proportion of CF4, COF2 and CF3COCF3 is a little increased with an tendency of the decrease of CO2and CF3COF. The fluorine content o the catalyst taken out from the reaction tube after the finish of the reaction is 8.30 % by weight.
Example 15 In a reaction tube made of Hastelloy C, the catalyst C4(40 g) is charged, and the temperature is elevated to 150C in nitrogen stream. Then, C3F6 and 2 are introduced into the reaction tube respectively in amounts of 40 ml/min and of 20 ml/min (25C, 1 atm.) under a gauge pressure of 3 kg/cm2G. After 20 hours, the gaseous mixture taken out from the reaction tube has the following com-position: CO, 0.5 mol%; CF4, 1.5 mol%; Co2, 1.0 mol%; COF2, : - .: : : . . , : : ...................... . . .

~ ~ `
; 1048S45 12.0 mol%: CF3COF, 12.3 mol%; CF3COCF3, 26.2 mol~; C3F6, 46.5 mol~.

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Claims (4)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for preparing hexafluoropropanone-2 from hexafluoropropene by a one-step reaction which comprises contacting hexafluoropropene and oxygen in the presence of a fluorinated alumina.

2. A process according to claim 1, wherein the fluorinated alumina contains fluorine in a concentration of 0.5 to 50 % by weight.

3. A process according to claim 2, wherein the fluorinated alumina is the one prepared by reacting activated alumina with a fluorinating agent.

4. A process according to cliam 1, wherein the contact is carried out at a temperature of 80 to 300°C.