Lower perfluoroalkanes are useful as low temperature refrigerants or electrical insulating gases, and as etching agents for semi-conductors.
Methods of manufacturing lower perfluoroalkanes are known, such as:
(A) direct fluorination of carbon;
(B) fluorination or disproportionation of chlorofluoroalkane; and
(C) fluorination or decarbonization of perfluoroalkene.
As an example of the method stated in (A) above, granulated carbon is fluorinated in the presence of fused metal fluoride (in this case, potassium fluoride) in the presence of chlorine (Japanese Toku Ko No. Sho 43-28089) as shown below: EQU C+2CaF.sub.2 +2Cl.sub.2 .fwdarw.CF.sub.4 +2CaCl.sub.2 . . . (1)
However, this method is a reaction in a three phase state of solid, liquid and vapor, and as such, it is a complicated process. Also, it is difficult to control the reaction. Furthermore low conversion rates of raw materials to product are common, and, though not shown in formula (1), chlorofluoromethanes such as chlorotrifluoromethane (CClF.sub.3), are produced as by-products, thus lowering the purity of the final product.
As an example of the method staged in (B) above, chlorofluoromethane is fluorinated in the vapor phase by using chromium fluoride as catalyst and hydrogen fluoride as fluorinating agent (U.S. Pat. No. 2,745,886), as shown below in equation 2, or chlorofluoromethane is disproportionated by using aluminum fluoride as catalyst (U.S. Pat. No. 2,478,201), as shown below in formula 3: EQU CCl.sub.2 F.sub.2 +HF.fwdarw.CClF.sub.3 +CF.sub.4 +HCl . . . (2) EQU CCl.sub.2 F.sub.2 .fwdarw.CF.sub.3 Cl+CF.sub.4 +CFCl.sub.3 +CCl.sub.4 . . . ( 3)
CF.sub.4 to be obtained by these methods is, as shown in formulae (2) and (3) above, in the form of a mixture with other chlorofluoromethane and therefore it is necessary to separate the chlorofluoromethanes. It is difficult to separate CF.sub.4 from CClF.sub.3 and the yield rate of CF.sub.4 is as low as 10-20%.
On the other hand, the method stated in (C) above has an advantage that CF.sub.4 of high purity can be obtained, but it is not satisfactory because of safety problems. The most typical reaction to obtain CF.sub.4 from TFE is the decomposition reaction of TFE through disproportionation as shown below: EQU C.sub.2 F.sub.4 .fwdarw.CF.sub.4 +C . . . (4)
This reaction is of an explosive nature and is accompanied by the release of very large amounts of heat. It produces as a by-product a great deal of carbon, which sticks to the walls of reactor, thus making its industrial application difficult from the viewpoints of both safety and process.
Other reactions are known such as (U.S. Pat. No. 2,351,390); EQU C.sub.2 F.sub.4 +O.sub.2 .fwdarw.CF.sub.4 +CO.sub.2 . . . ( 5) EQU C.sub.2 F.sub.4 +2F.sub.2 .fwdarw.2CF.sub.4 . . . ( 6)
Though these reactions have an advantage of producing no carbon, they have other defects such as production of heat 2.5-3.5 times greater than in equation (4), thus creating the possibility of explosion. Moreover, in formula (6) the elemental state of fluorine which is expensive and very high in reactivity is used. For these reasons, they have not been industrially adopted from safety and economic viewpoints.