Gas-phase epoxidation of hexafluoropropylene.
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A process was developed for the production of hexafluoropropylene oxide via the gas-phase oxidation of hexafluoropropylene with molecular oxygen. The non-catalytic oxidation reaction was investigated in an isothermal, laminar flow reactor at a total pressure of 4.5 bar and over the temperature range of 453 to 503 K. Feed mixtures comprising 20 to 67% HFP in oxygen were used at total flow-rates between 150 and 550 cm3 min-1. The reactor was fabricated from 1/8 inch nominal sized, copper refrigeration tubing and was 114.3 m long. The reactor was used in the form of a helical coil. Gas-chromatographic analysis was used for reactant and stable product quantification. The reaction gave hexafluoropropylene oxide, trifluoroacetyl fluoride and carbonyl fluoride as major products. Minor products included tetrafluoroethylene and hexafluorocyclopropane. The oxidation reaction also produced high molecular weight oligomers that were retained on the inner surface of the reactor tube. The operating conditions for the non-catalytic oxidation were optimized independently for HFPO selectivity and yield using quadratic response surface methodology. A maximum HFPO selectivity of 55.81% was identified at 478.2 K, a HFP/O2 molar feed ratio of 1.34 mol mol-1 and a space time of 113 seconds. An optimum HFPO yield of 40.10% was identified at 483.2 K, a HFP/O2 molar feed ratio of 1.16 mol mol-1 and a space time of 121 seconds. Using the weighted-sum-of-squared-objective-functions (WSSOF) multi-response optimization technique, a combined optimum HFPO selectivity and yield of 56% and 40%, respectively, was obtained at 480 K, with a HFP/O2 molar feed ratio of 1.21 and a space time of 118 seconds. This represented the best trade-off between these two performance criteria. A kinetic reaction scheme involving 8 species and 7 reactions was developed, based on the results of the experimental study, and was used to model the non-catalytic oxidation of HFP. The initial steps in this scheme encompassed the addition of oxygen to the double bond of the fluoro-olefin and transformation of the resultant dioxetane intermediate to form HFPO and the haloacetyl fluorides. Subsequent steps included the thermal decomposition of HFPO to yield CF3COF, C2F4 and c-C3F6, as well as elimination of C2F4, and to a lesser extent CF3COF, through oxidation. Rate parameters for the oxidation reactions were determined through a least-squares minimization procedure. The investigation was extended by considering the catalyzed synthesis of HFPO. Four different catalysts were studied viz., 1wt% Au/A12O3, 1wt% Au/ZnO, 10wt% CuO/SiO2 as well as 10wt% CuO/SiO2 doped with caesium. The gold-based catalysts were found to be completely inactive for the oxidation reaction. The caesium promoted, copper-based catalyst appeared to be the most stable and active, with no observable decomposition to copper fluoride. At 453 K, a HFP/O2 molar feed ratio of 0.86 and a weight-hourly-space-velocity of 0.337 h-1, a HFPO selectivity of 85.88% was obtained. This was significantly greater than what was achieved in the non-catalytic system.