1. Field of the Invention
This invention relates to a process for producing a carbonyl fluoride compound, and more particularly to a process for producing a carbonyl fluoride compound useful as an intermediate for producing perfluoro (lower alkyl vinyl ethers).
2. Description of the Prior Art
Perfluoro (lower alkyl vinyl ethers), for example, perfluoro (propyl vinyl ether), perfluoro (ethyl vinyl ether), perfluoro (methyl vinyl ether), etc. are important compounds as raw material monomers for producing fluorine-containing resin, fluorine-containing rubber, etc.
These perfluoro (lower alkyl vinyl ethers) are prepared by addition reaction between a carbonyl fluoride compound such as perfluoroacetyl fluoride (CF.sub.3 COF), carbonyl fluoride (COF.sub.2), etc. as a raw material and perfluoroalkene oxide and successively COF.sub.2 -releasing reaction (U.S. Pat. No. 3,291,843).
For example, ##STR1##
CF.sub.3 COF used as a raw material for the reaction has been so far synthesized by reaction of trichloroacetyl chloride (CCl.sub.3 COCl) with hydrogen fluoride and successibe halogen exchange (Chemical Abstract, Vol. 78, 42856z). However, the product CF.sub.3 COF contains chlorine as an impurity and when the product CF.sub.3 COF is used in the above-mentioned reaction as the raw material, there is a high possibility that the desired product perfluoro (alkyl vinyl ether) contains chlorine-based impurities.
COF.sub.2 likewise used in the above-mentioned reaction as the raw material is generally prepared by halogen exchange reaction between phosgene (COCl.sub.2) and a metal fluoride (CHEMISTRY OF ORGANIC FLUORINE COMPOUNDS, 2nd Ed. page 678, 1976). Phosgene is highly toxic and contains unwanted COClF as an impurity. Thus, COF.sub.2 of high purity cannot be obtained from such a raw material phosgene.
Furthermore, perfluoro (propyl vinyl ether) has been so far prepared by oligomerization of hexafluoropropylene oxide as a raw material (U.S. Pat. Nos. 3,250,807 and 3,250,808). In the oligomerization reaction, only the desired 2-(heptafluoropropoxy) propionyl fluoride is not selectively formed, but polyether compounds having a high degree of polymerization [n is 1 or higher in the general formula (XII) which will be given later] are by-produced at the same time. This is a problem.
Hexafluoropropylene oxide used in the oligomerization reaction as the raw material is an expensive compound, and the by-production of the polyether compounds having a high degree of polymerization will lower the yield of the desired product, increasing its cost. Though the trimer and tetramer (n=1-2) among the by-products can be used as surfactants, the pentamer or higher obigomers (n=3 or higher) has no special application. This is another problem.
The present inventors have made extensive studies to find a process for producing a carbonyl fluoride compound containing no chlorine-based impurities in the process for producing perfluoroacetyl fluoride from the trichloroacetyl chloride and the process for producing carbonyl fluoride from the phosgene, and have found that the problem can be effectively solved by thermal decomposition of a polyether compound obtained by reaction of tetrafluoroethylene with oxygen under irradiation of ultraviolet rays.
Furthermore, the present inventors have made extensive studies to produce perfluoropropionyl fluoride as a useful compound from oligomers such as trimer and higher oligomers of hexafluoropropylene oxide by-produced when perfluoro (propyl vinyl ether) is produced from hexafluoropropylene oxide, and have found that the problem can be effectively solved by thermal decomposition of the oligomers in the presence of an activated carbon catalyst. Furthermore, the present inventors have found that the thermal decomposition process can be effectively applied not only to the hexafluoropropylene oxide oligomers, but also to tetrafluoroethylene oxide oligomers.