Process for the preparation of 1,1,1,3,3,3-hexafluoro-2-propanone

A vapor phase process for the preparation of fluorinated ketones, such as 1,1,1,3,3,3-hexafluoro-2-propanone via oxidation of hydrofluorocarbons, such as 1,1,1,3,3,3-hexafluoropropane, with an oxidizing agent and elemental fluorine at temperatures ranging from 50.degree. C. to 300.degree. C. and residence times ranging from 2 to 60 seconds.

FIELD OF THE INVENTION 
This invention relates to a vapor phase process for the preparation of 
fluorinated ketones, such as, 1,1,1,3,3,3-hexafluoro-2-propanone. In 
particular, the present invention provides a method for preparing 
1,1,1,3,3,3-hexafluoro-2propanone via oxidation of 
1,1,1,3,3,3-hexafluoropropane with an oxidizing agent and elemental 
fluorine (F.sub.2). 
BACKGROUND OF THE INVENTION 
1,1,1,3,3,3-hexafluoro-2-propanone (i.e., hexafluoroacetone or HFA) is used 
as a starting material to prepare hexafluoroisopropylidene (HFIP) bridged 
compounds, which are used as monomers in the synthesis of high performance 
polymers, specialty coatings and pharmaceutical intermediates. 
Incorporation of hexafluoroisopropylidene moiety into the polymer chain is 
known to influence the solubility, processability, oxidative stability and 
electrical properties. 
There are a number of methods to prepare HFA in the literature [Krespan and 
Middleton, J. Fluorine Chem. Rev. 1967, 1, 145] each having certain 
limitations. For example, in the halogen-exchange fluorination of 
hexachloroacetone using anhydrous HF and chromium oxide catalyst, the 
exchange of last chlorine to fluorine is very difficult and the 
intermediate pentafluorochloro-2-propanone is highly toxic. Epoxidation of 
hexafluoropropene to hexafluoropropene oxide followed by isomerization to 
HFA with liquid HF as a solvent requires the use of expensive 
corrosion-resistant reactors. Finally, perfluoroisobutylene, used as a 
starting material to prepare HFA is extraordinarily toxic. 
The preparation of HFA from 1,1,1,3,3,3-hexafluoropropane (i.e., CF.sub.3 
CH.sub.2 CF.sub.3 or HFC-236fa) has several advantages over other methods. 
The starting material, HFC-236fa, can be readily prepared in high yield 
from carbon tetrachloride and vinylidene chloride according to U.S. Pat. 
No. 5,395,997, and the process is amenable to commercial scale-up. The 
by-product produced in the oxidation step of this process is water and the 
process can be operated continuously. The present invention relates to a 
process for the preparation of HFA which can be economically and 
ecologically superior to existing processes. 
1,1,1,3,3,3-Hexafluoropropane (i.e., HFC-236fa) has an atmospheric life 
time of 265 years indicating a slow reaction with hydroxyl radicals. It is 
relatively inert to chlorine and bromine radicals too. For example, Henne 
et al., [J. Amer. Chem. Soc., 67, 1906 (1945)] have reported that 
HFC-236fa resists chlorination completely in bright sunlight and 
bromination of HFC-236fa with elemental bromine at 550.degree.-585.degree. 
C. [L. H. Beck's Thesis, University Microfilms, Inc., The Ohio State 
University, 1959, p 23] yielded only small amount of 
2-bromo-1,1,1,3,3,3hexafluoropropane (i.e., CF.sub.3 CHBrCF.sub.3). 
Reaction of HFC-236fa with elemental fluorine is not known. Poor 
reactivity of HFC-236fa towards chlorine and bromine is attributed to 
heavy shielding of the hydrogens located on the central carbon of 
HFC-236fa by two adjacent trifluoromethyl groups. The same shielding 
effect is expected to prevent any radical attack on those hydrogens 
including fluorine. 
Fluorine is different from other halogens in that fluorine-fluorine bond 
energy is relatively low and carbon-fluorine and hydrogen-fluorine bond 
energies are very high. Reactions with fluorine require very low 
activation energies and fluorine sensitized oxidation and halogenation of 
unsaturated olefins are known in the literature. Miller and co-workers [J. 
Amer. Chem. Soc., 1956, p 2793] have accumulated enough evidence to show 
the role of fluorine as an initiator in the oxidation of trichloroethylene 
and tetrachloroethylene. The use of fluorine to oxidize fluorine 
containing compounds such as hydrofluorocarbons and 
hydrochlorofluorocarbons has not been reported. In an attempt to prepare 
the titled compound, HFC-236fa was reacted with air and fluorine in a 
fluidized bed reactor and the reaction yielded the desired HFA and its 
hydrate in high selectivity and good conversion. The oxidation of 
HFC-236fa in the presence of fluorine provides a route to producing 
1,1,1,3,3,3-hexafluoro-2propanone which is cleaner and cheaper than the 
existing methods. 
SUMMARY OF THE INVENTION 
The present invention provides a process for the preparation of 
1,1,1,3,3,3-hexafluoro-2-propanone by oxidation of 
1,1,1,3,3,3-hexafluoropropane. The vapor phase process includes contacting 
1,1,1,3,3,3-hexafluoropropane with air and fluorine at a temperature of 
from about 50.degree. C. to about 300.degree. C. 
DETAILED DESCRIPTION OF THE INVENTION 
The present invention provides a process to prepare 
1,1,1,3,3,3-hexafluoro-2-propanone by gas phase oxidation of 
1,1,1,3,3,3-hexafluoropropane with air in the presence of fluorine as the 
initiator. The process includes the contacting of HFC-236fa with fluorine 
and an oxidizing agent in a fluidized bed reactor in the presence of a 
particulate phase at temperatures ranging from 50.degree. C.-300.degree. 
C. 
The reactor used in the oxidation process, according to the present 
invention, may be any reactor known in the art, including any simple 
tubular reactor constructed of a metal resistant to attack by reactants. 
Preferably the reactor is constructed with either stainless steel or 
copper. The most preferred metal for the reactor is stainless steel. Since 
reactions involving elemental fluorine are highly exothermic, the reactor 
is usually packed with a material such as alumina, copper metal turnings, 
or the like, generally known in the art to provide favorable mixing of the 
gas streams, good dissipation of heat, and possibly even an active surface 
for heterogeneous reactions. Preferably the reactor is packed with either 
alumina or copper metal turnings with the most preferred packing being 
alumina. The size of the alumina (particulate phase) ranges from 120 to 
320 mesh, with preferred particle size being 180 to 220 mesh. The packing 
materials used for the oxidation process are commercially available. 
The hydrofluorocarbons to be oxidized according to the present invention 
have the general formula: [F(CF.sub.2)n]2CH.sub.2, wherein n=1-5. Examples 
of such hydrofluorocarbons include: (CF.sub.3).sub.2 CH.sub.2, (CF.sub.3 
CF.sub.2).sub.2 CH.sub.2, (CF.sub.3 CF.sub.2 CF.sub.2).sub.2 CH.sub.2, 
(CF.sub.3 CF.sub.2 CF.sub.2 CF.sub.2).sub.2 CH.sub.2, and (CF.sub.3 
CF.sub.2 CF.sub.2 CF.sub.2 CF.sub.2).sub.2 CH.sub.2 with the most 
preferred hydrofluorocarbons for the oxidation being 
1,1,1,3,3,3-hexafluoropropane. 
The products resulting from the oxidation according to the present 
invention have the general formula: [F(CF.sub.2)n]2 C(O), wherein n=1-5. 
Examples of such products include: (CF.sub.3).sub.2 C(O), (CF.sub.3 
CF.sub.2).sub.2 C(O), (CF.sub.3 CF.sub.2 CF.sub.2).sub.2 C(O), (CF.sub.3 
CF.sub.2 CF.sub.2 CF.sub.2).sub.2 C(O) and (CF.sub.3 CF.sub.2 CF.sub.2 
CF.sub.2 CF.sub.2).sub.2 C(O) with the most preferred product from the 
oxidation being 1,1,1,3,3,3-hexafluoro-2-propanone (i.e., CF.sub.3 
C(O)CF.sub.3, HFA). 
The hydrocarbon 1,1,1,3,3,3-hexafluoropropane(CF.sub.3 CH.sub.2 CF.sub.3, 
HFC-236fa) is commercially available and can also be prepared by the 
reaction of 1,1,1,3,3,3-hexachloropropane (CCl.sub.3 CH.sub.2 CCl.sub.3) 
with HF in the presence of antimony catalyst using the procedure disclosed 
in U.S. Pat. No. 5,395,997. 
Elemental fluorine gas used in the process is commercially available and 
the gas is used without any additional purification. Fluorine is usually 
diluted with a carrier gas to reduce the heat generated in the oxidation 
process and to reduce the amount of unwanted by-products as well as to 
adjust the overall residence time of the substrate to be oxidized in the 
reactor by varying the total gas flow rate. Suitable carrier gases include 
nitrogen, helium, argon and air. The preferred carrier gases are air and 
nitrogen with the most preferred carrier gas being air. Dilution of 
fluorine with the carrier gas is accomplished by mixing pure fluorine gas 
with the carrier gas in the concentration required to carry out the 
oxidation. The gases are allowed to mix in a tee connection and the gases 
entering the tee connection are measured with flow meters or rotometers of 
the type well known in the art. 
The oxidizing agent useful in the present invention is selected from air, 
molecular oxygen, and mixtures of nitrogen and oxygen. Preferably the 
oxidizing agent is either air or mixtures of nitrogen and oxygen, with the 
most preferred oxidizing agent being air. The oxidizing agents useful for 
the present invention are commercially available. 
The hydrofluorocarbons used in the present invention are either gases or 
liquids. For liquid hydrofluorocarbons, an inert carrier gas such as 
nitrogen is used to carry it into the vapor phase by purging the 
hydrofluorocarbon container with the carrier gas. 
The oxidation process according to the present invention may be carried out 
either as a batch or a continuous process. However, for large scale 
production, the process is conducted preferably in a continuous flow 
system by passing vapors of carrier gas, fluorine and hydrofluorocarbon 
through a tubular reactor containing the particulate phase. 
The oxidation process for the present invention is preferably conducted in 
the temperature range of about 50.degree. C. to about 300.degree. C. At 
the lower end of the temperature range, conversion of the starting 
material is very low, while at higher temperatures the amount of 
by-products increase and the selectivity is diminished. More preferably 
the temperature ranges from 100.degree. C.-250.degree. C., with the most 
preferable range being 150.degree. C.-225.degree. C. Pressure is not 
critical for the present invention and it is most convenient to operate 
the oxidation process at approximately atmospheric pressure with the only 
pressure above atmospheric being due to back pressure of the system. 
Useful residence time ranges from about 2 seconds to 100 seconds, 
preferably from 30-60 seconds. Residence times may be adjusted by changing 
the volume of the particulate phase, the reaction temperature or the total 
gas flow rates. 
Since the role of fluorine gas is to initiate the reaction of 236fa with 
oxygen, the molar ratio of fluorine to hydrofluorocarbon should be kept at 
minimum in order to limit the amount of unwanted by-products. A useful 
molar ratio of F.sub.2 /hydrocarbon ranges from 0.01 to 3.0 with the 
preferable ratio being from 0.01 to 0.5. 
Based on reaction stoichiometry, the required ratio of oxygen present in 
the oxidizing agent to hydrofluorocarbon is 1 and preferably the 
concentration of the oxygen be kept high to increase the conversion of the 
hydrofluorocarbon. Most preferred ratio of oxygen to hydrofluorocarbon 
ranges from 2 to 10. 
The oxidation process for the present invention is preferably operated in 
such a way that either the fluorine conversion is high or unreacted 
fluorine is returned to the reactor. Similarly the process is operated in 
such a way that unused oxidizing agent is returned to the reactor. 
The resulting oxidized compounds may be separated from the product stream 
via any known separation or purification method known in the art such as 
neutralization and distillation.

EXAMPLES 
The following examples serve to illustrate the invention, but are not 
intended to limit the scope of the invention. 
The fluidized bed reactor consisted of a vertical stainless steel pipe 
(2'.times.2" ID) which was threaded at both the ends to accept caps. A 20 
micron porous disc was used at the bottom to retain the particulate phase 
and to distribute the entering gaseous materials. A 1/4" stainless steel 
tube fitted at the end with a porous frit of 15 micron pores was used to 
introduce the organic materials. Fluorine and air were introduced through 
a common 1/4" pipe from the bottom. The reactor was initially filled to a 
depth of 8" (400 ml) with 180 mesh alumina and heated with the air flow. 
After thermal equilibrium was reached, fluorine was introduced and finally 
organic was fed from the top. Reactor temperatures were monitored using 
thermocouples near the reaction and sample exiting zones. Fluorine flow 
rate to the reactor was measured with a Teledyne Hastings-Raydist, Model 
ST-M mass flow meter and controlled with a needle valve. Matheson 
rotometer and flow meters were used to measure the air and organic flow 
rates. The cylinder containing HFC-236fa was pressurized with nitrogen (60 
psi) to facilitate the upward flow of the organic material through a check 
valve. Gases exiting the reactor from the top were directed through a 
stainless steel tube(1'.times.1" ID) packed with anhydrous sodium fluoride 
to trap HF. The HF-free vapors were then passed into a product trap 
consisting of a one-liter stainless steel cylinder cooled to -78.degree. 
C. using dry ice/isopropyl alcohol mixture. Uncondensed gases exiting the 
product trap were passed through an aqueous potassium hydroxide solution. 
At the conclusion of the run, the fluorine and organic flows were shut off 
and the stainless steel cylinder containing the products was disconnected 
and its contents were analyzed by gas chromatography, GC-MS and NMR. 
The GC analysis was performed on a Hewlett-Packard Series II 5890 Gas 
Chromatograph coupled with a 3396 integrator. 
Example 1 
The above described reactor was purged with air at the rate of 400 
cc/minute and heated to 160.degree. C. When the temperature of the reactor 
stabilized, fluorine was introduced at a flow rate of 40 cc/minute. After 
the initial exotherm subsided (.about.5 minutes), HFC-236fa was introduced 
at the flow rate of 15 g/hour. While feeding 19.6 g (129 mmoles) of 
HFC-236fa in one hour 20 minutes, three exiting gas samples were collected 
at the beginning, middle and end of the feeding of HFC-236fa and analyzed 
by GC, GC-MS, GC-IR and .sup.19 F NMR. 
Reaction conditions, product distribution and conversion of 236fa for the 
three samples (entry no. 1-3) are presented in Table I. GC-MS: 
166(M.sup.+), 69 (base peak); GC-IR: 1795(w) , 1350(m), 1220(s, doublet), 
975(s) and 700(m). .sup.19 F NMR [CDCl.sub.3, CFCl.sub.3 int. ]: -63.7 
ppm(t, J=9.5 Hz, due to CF.sub.3 CH.sub.2 CF.sub.3), -76.1ppm(s, due to 
CF.sub.3 COCF.sub.3) and -83.1ppm(s, broad, due to CF.sub.3 
COCF.sub.3.xH.sub.2 O). 
Example 2 
The fluidized bed reactor and procedure of Example 1 were utilized except 
the fluorine flow rate was reduced to 20 cc/minute. Air and 236fa flow 
rates were maintained at 400 cc/min. and 50 cc/min., respectively. While 
feeding 20.5 g(135 mmoles) 236fa in one hour 25 minutes, two gas samples 
were collected at the beginning (entry no. 4) and end (entry no. 5) of the 
organic feeding and analyzed by GC. Conversion of 236fa, reaction 
conditions and product distribution are summarized in Table I. This 
example demonstrates that oxidation of HFC-236fa to HFA can be carried out 
with less than stoichiometric amount of fluorine. 
Example 3 
The fluidized bed reactor and procedure of example 1 were utilized except 
the reactor was initially heated to 55.degree. C. with an air flow of 400 
cc/minute. Fluorine and 236fa flow rates were maintained at 40 cc/min. and 
50 cc/min., respectively. While feeding 14.5 g(95.4 mmoles) 236fa in 45 
minutes, two gas samples were collected at the beginning (entry no. 6) and 
end (entry no. 7) of the organic feeding and analyzed by GC. Conversion of 
236fa, reaction conditions and product distribution are summarized in 
Table I. This example demonstrates that oxidation of HFC-236fa to HFA can 
be carried out at the lower end of the temperature range, viz, 50.degree. 
C.-60.degree. C., albeit in low conversion. Example 4 
The fluidized bed reactor and procedure of Example 1 were utilized except 
the air flow rate was increased to 600 cc/minute. F2 and 236fa flow rates 
were maintained at 40 cc/min. and 50 cc/min., respectively. While feeding 
12.5 g(82.2 mmoles) 236fa in 30 minutes, two gas samples were collected at 
the beginning (entry no. 8) and end (entry no. 9) of the feeding and 
analyzed by GC. Conversion of 236fa, reaction conditions and product 
distribution are summarized in Table I. The effect of concentration of the 
oxidizing agent and fluorine on conversion and selectivity in the 
oxidation reaction is demonstrated in this example. 
TABLE 1 
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Temperature (C.) 
Molar 
Flow Rates 
Conv. 
GC Yield (area %) 
Reaction 
Exit 
ratio 
(CC/min) 236fa HFA Other 
No. 
zone zone 
F2/236fa 
Air 
F2 236fa 
(%) 236fa 
HFA 
hydrate 
pdts. 
__________________________________________________________________________ 
1 165 153 0.94 389 
38 50 84.2 
15.8 
27.3 
51.3 5.6 
2 169 154 0.94 388 
38 50 69.7 
30.3 
56.1 
13.1 0.5 
3 177 169 0.94 389 
38 50 11.0 
89.1 
-- 10.7 0.2 
4 168 160 0.48 383 
19.3 
50 47.1 
52.9 
16.5 
27.6 3.0 
5 191 174 0.48 376 
19.4 
50 19.0 
81.0 
8.5 
8.6 1.9 
6 53 52 0.94 390 
39 50 4.5 95.5 
3.8 
0.4 0.4 
7 60 58 0.94 390 
39 50 0.1 99.9 
0.01 
-- 0.08 
8 159 156 1.19 600 
40 45 46.3 
53.7 
21.4 
-- 24.9 
9 155 157 1.79 600 
60 50 65.3 
34.8 
58.6 
-- 6.7 
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