(570x) Epoxidation of Hexafluoropropylene
AIChE Annual Meeting
Wednesday, November 10, 2010 - 6:00pm to 8:00pm
Hexafluoropropylene oxide (HFPO), the epoxide product of the partial oxidation of hexafluoropropylene (HFP), is a highly reactive and versatile intermediate that can be converted to perfluoroalkylvinylethers, used in the production of proton-exchange membranes for fuel cells, used to make heat resistant fluids, high temperature lubricants, high performance elastomers and surfactants. The traditional methods of preparation, as described in the patent literature, include nucleophilic, electrophilic and radical syntheses from hexafluoropropylene using different oxidizers such as hypohalites, hydrogen peroxide and organic peroxides. These processes are, however, economically and environmentally unfavourable. To date, no viable process has been found for the direct gas-phase epoxidation of HFP with molecular oxygen that offers high selectivity towards HFPO. This route, though, offers several advantages including being easier to operate and inherently safer due to the absence of large amounts of organic solvents. It is well known that the gas-phase epoxidation of HFP results from a complex sequence of free-radical reaction steps and requires chemical, photo or thermal initiation. Chemical initiation using, for example, trifluoromethyl hypofluorite, results in a broad product distribution and although this method offers better control over the structure and molecular mass of oligomeric oxidation products, it is not appropriate for epoxide formation. Photo-initiation has been used extensively for the low temperature oxidation of hexafluoropropylene to produce perfluoropolyether polyperoxide. The literature is devoid of a comprehensive and critical study of the thermally initiated partial oxidation process. The difficulty associated with the synthesis of perfluorinated epoxides using the latter method can be attributed to the propensity for the oxirane structure to decompose under the temperatures necessary for initiation or temperatures attained within the reactor due to localized exothermic heat effects, leading to deeper oxidation products. In this work, the thermally initiated gas-phase epoxidation of hexafluoropropylene using molecular oxygen was investigated. A 150m long, copper coil-type tubular reactor, sealed within a high pressure jacket filled with an eutectic heat transfer fluid, was used to study the homolytic partial oxidation reaction at a pressure of 2 atm, within a temperature range of 80-180 °C, a molar ratio of HFP to oxygen in the feed gas of 0.3-6.2 mol/mol and a space time between 10-40 seconds. Feed flow-rates of hexafluoropropylene and oxygen were established by means of Bronkhorst® coriolis and thermal mass flow controllers, respectively. The analysis of reaction products was accomplished using gas-solid chromatography on an alumina PLOT column. The reactor effluent was scrubbed using an aqueous solution of potassium hydroxide prior to venting. A central composite experimental design consisting of twenty experiments was used to probe for optimal operating conditions. Three coded variables, viz. reaction temperature, feed composition and space velocity, were considered. The mathematical relationship between the independent parameters and the response function (yield of HFPO) was approximated by a quadratic response surface model. The results of the experimental study as well as steady-state kinetic simulations suggest that the molar yield of the epoxide product is very sensitive to the concentration of oxygen in the feed gas and changes in temperature above 100 °C. A decrease in selectivity towards hexafluoropropylene oxide at higher oxygen levels is mainly attributed to competitive peroxy radical formation, via an encounter between the copolymer of alkoxyl radicals and HFP and oxygen. However, an interrelation of homolytic and heterolytic processes on the copper reactor surface has also been proposed, where key steps such as initiation, chain termination and propagation may proceed. At sufficiently high oxygen coverages, nucleation and growth of copper oxide commences and selective oxidation activity may be extinguished. Thermal decomposition and isomerization rapidly eliminate the epoxide from the product mixture, thus localized variations in reaction temperature should be avoided through efficient removal of reaction heat.
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