Kinetics and mechanism of thermal gas-phase oxidation of hexafluoropropene in the presence of trifluoromethylhypofluorite, CF3OF

Abstract

The oxidation of hexafluoropropene with molecular oxygen in the presence of CF₃OF has been studied at 303.0, 313.0 and 323.4 K, using a conventional static system. The initial pressure of CF₃OF was varied between 1.7 and 16.6 Torr, that of C₃F₆ between 10.0 and 120.2 Torr and that of O₂ between 82.0 and 599.8 Torr. Several runs were made in the presence of 325.3–499.2 Torr of N₂ . Major products were COF₂, CF₃C(O)F, CF₃OC(O)F and a new compound, CF₃OCF₂OCF₂C(O)F. The latter was characterised by the determination of its relative molecular mass and its IR spectrum, which is consistent with the calculated one using theoretical approximations. Small amounts of CF₃OCF₂C(O)F were also formed. The oxidation is an homogeneous chain reaction, attaining, at the pressure of O₂ used, the pseudo-zero-order condition with respect to O₂ as a reactant. It is independent of the total pressure. The basic steps are: the thermal generation of CF₃O^• radicals by abstraction of the fluorine atom from CF₃OF by C₃F₆, chain initiation by the addition of CF₃O^• to olefin leading, in the presence of O₂, to the formation of haloalkoxy radicals, which decompose via the C–C scission to give products containing C(O)F group and CF₃^• radicals, reforming CF₃O^•. The full mechanism is postulated.