Fowler process

The Fowler Process is an industry and laboratory route to perfluorocarbons, by fluorinating hydrocarbons or their partially fluorinated derivatives in the vapor phase over cobalt(III) fluoride.

Background

The Manhattan Project required the production and handling of uranium hexafluoride for uranium enrichment, whether by diffusion or centrifuge. Uranium hexafluoride is very corrosive, oxidising, volatile solid (sublimes at 56°C [http://www.itcilo.it/actrav/actrav-english/telearn/osh/ic/7783815.htm] ). To handle this material, several new materials were required, including a coolant liquid that could survive contact with uranium hexafluoride. Perfluorocarbons were identified as ideal materials, but at that point no method was available to produce them in any significant quantity.

The problem is that fluorine gas is extremely reactive. Simply exposing a hydrocarbon to fluorine will cause the hydrocarbon to ignite. A way to moderate the reaction was required, and the method developed was to react the hydrocarbon with cobalt(III) fluoride, rather than fluorine itself.

After World War II, much of the technology that had been kept secret was released into the public domain. The March 1947 issue of Industrial and Engineering Chemistry presented a collection of articles about fluorine chemistry, starting with the generation and handling of fluorine, and going on to discuss the synthesis of organofluorides and related topics. In one of these articles Fowler et al. describe the laboratory preparation of numerous perfluorocarbons by the vapour phase reaction of a hydrocarbon with cobalt(III) fluoride, [Fowler, R. D.; Burford, W. B.; Hamilton, J. M.; Sweet, R. G.; Weber, C. E.; Kasper, J. S.; Litant, I. Ind. Eng. Chem., "39", 1947, p292-298] at a pilot plant scale, in particular, perfluoro-n-heptane and perfluorodimethylcyclohexane (mixture of 1,3-isomer and 1,4 isomer), [Burford, W. B.; Fowler, R. D.; Hamilton, J. M.; Anderson, H. C.; Weber, C. E.; Sweet, R. G.; Ind. Eng. Chem., "39", 1947, p319-329] and on an industial scale by Du Pont. [Benner, R.G; Benning, A.F.; Downing, F. B.; Irwin, C. F.; Johnson, K. C.; Linch, A. L.; Parmalee, H. M.; Wirth, W. V. Ind. Eng. Chem., "39", 1947, p329-333]

Chemistry

The Fowler process is typically done in two stages, the first stage being fluorination of cobalt(II) fluoride to cobalt(III) fluoride.

:2 CoF₂ + F₂ → 2 CoF₃

During the second stage, in this instance to make perfluorohexane, the hydrocarbon feed is introduced and is fluorinated by the cobalt(III) fluoride, which is converted back to cobalt(II) fluoride for reuse. Both stages are performed at high temperature.

:C₆H₁₄ + 28 CoF₃ → C₆F₁₄ + 14 HF

The reaction proceeds though a single electron transfer process, involving a carbocation. [Sandford, G. Tetrahedron, "59", 2003, p437-454] This carbocation intermediate can readily undergo rearrangements, which can lead to a complex mixture of products.

Feedstocks

Typically hydrocarbon compounds are used as the feedstocks. For cyclic perfluorocarbon, the aromatic hydrocarbon is the preferred choice, so for example, toluene is the feedstock for perfluoromethylcyclohexane, rather than methylcyclohexane, as less fluorine is required. Often partially fluorinated feedstocks are used, for example, bis-1,3-(trifluoromethyl)benzene to make perfluoro-1,3-dimethylcyclohexane. Although these are considerably more expensive, they require less fluorine and more importantly, they generally give higher yields, as the cabocation rearrangements are much less likely.

Flutec perfluorocarbons

In the UK, Imperial Chemical Industries Limited (later ICI) was also developing cobalt(III) fluoride technology during the war, prompted by the work in the US. [Dawson, A. M. Imperial Chemical Industries Limited, General Chemical Division, Research Department Report R/GC/1685, 1943] The process was later commercialized by the Imperial Smelting Company (later ISC Chemicals) under the tradename Flutec at Avonmouth near Bristol. They were originally produced on the pilot plant, and so were designated PP1, PP2, PP3, etc. The designation has remained to this day.

ISC Chemicals become part of RTZ in 1973, and that part of the business was transferred to Rhone-Poulenc in 1988 [http://www.rhodia.co.uk/media/cws/rhodia_in_the_country/global_locations/uk/docs/plant_data_sheet/en/Avonmouth_v1_m56577569830657403.pdf] . The Flutec business went into a decline, due to a drop in its main application, vapor phase reflow soldering (used in surface-mount technology, and six years later the Flutec business was purchased by BNFL Fluorochemicals Ltd and transferred to Preston, Lancashire, where it has been developed in to several new applications, in particular in medicine [http://www.fluoros.co.uk/data/medical_applications.php] . BNFL Fluorochemicals Ltd became F2 Chemicals Ltd in 1998.

References