Fowler process
Encyclopedia
The Fowler Process is an industry and laboratory route to fluorocarbon
s, by fluorinating hydrocarbon
s or their partially fluorinated derivatives in the vapor phase over cobalt(III) fluoride
.
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). 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, at a pilot plant scale, in particular, perfluoro-n-heptane and perfluorodimethylcyclohexane (mixture of 1,3-isomer and 1,4 isomer), and on an industrial scale by Du Pont.
to cobalt(III) fluoride.
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.
The reaction proceeds though a single electron transfer process, involving a carbocation
. This carbocation intermediate can readily undergo rearrangements, which can lead to a complex mixture of products.
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.
Limited (later ICI) was also developing cobalt(III) fluoride technology during the war, prompted by the work in the US. 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. 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. BNFL Fluorochemicals Ltd became F2 Chemicals Ltd in 1998.
Fluorocarbon
Fluorocarbons, sometimes referred to as perfluorocarbons or PFCs, are organofluorine compounds that contain only carbon and fluorine bonded together in strong carbon–fluorine bonds. Fluoroalkanes that contain only single bonds are more chemically and thermally stable than alkanes...
s, by fluorinating hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
s or their partially fluorinated derivatives in the vapor phase over cobalt(III) fluoride
Cobalt(III) fluoride
Cobalt fluoride is the inorganic compound with the formula CoF3. This highly reactive, hygroscopic brown solid is used to synthesize organofluorine compounds...
.
Background
The Manhattan ProjectManhattan Project
The Manhattan Project was a research and development program, led by the United States with participation from the United Kingdom and Canada, that produced the first atomic bomb during World War II. From 1942 to 1946, the project was under the direction of Major General Leslie Groves of the US Army...
required the production and handling of uranium hexafluoride
Uranium hexafluoride
Uranium hexafluoride , referred to as "hex" in the nuclear industry, is a compound used in the uranium enrichment process that produces fuel for nuclear reactors and nuclear weapons. It forms solid grey crystals at standard temperature and pressure , is highly toxic, reacts violently with water...
for uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
enrichment, whether by diffusion or centrifuge. Uranium hexafluoride is very corrosive, oxidising, volatile solid (sublimes at 56 °C). 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
Fluorine
Fluorine is the chemical element with atomic number 9, represented by the symbol F. It is the lightest element of the halogen column of the periodic table and has a single stable isotope, fluorine-19. At standard pressure and temperature, fluorine is a pale yellow gas composed of diatomic...
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, at a pilot plant scale, in particular, perfluoro-n-heptane and perfluorodimethylcyclohexane (mixture of 1,3-isomer and 1,4 isomer), and on an industrial scale by Du Pont.
Chemistry
The Fowler process is typically done in two stages, the first stage being fluorination of cobalt(II) fluorideCobalt(II) fluoride
Cobalt fluoride is a chemical compound with the formula . It is a pink crystalline solid compound. The formula is given for both the red tetragonal crystal, , and the tetrahydrate red orthogonal crystal, . CoF2 is used in oxygen-sensitive fields, namely metal production. In low concentrations, it...
to cobalt(III) fluoride.
- 2 CoF2 + F2 → 2 CoF3
During the second stage, in this instance to make perfluorohexane
Perfluorohexane
Perfluorohexane or tetradecafluorohexane, is a fluorocarbon. It is a derivative of hexane in which all of the hydrogen atoms are replaced by fluorine atoms. It is used in one formulation of the electronic cooling liquid/insulator Fluorinert for low temperature applications due to its low boiling...
, 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 + 28 CoF2
The reaction proceeds though a single electron transfer process, involving a carbocation
Carbocation
A carbocation is an ion with a positively-charged carbon atom. The charged carbon atom in a carbocation is a "sextet", i.e. it has only six electrons in its outer valence shell instead of the eight valence electrons that ensures maximum stability . Therefore carbocations are often reactive,...
. 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, tolueneToluene
Toluene, formerly known as toluol, is a clear, water-insoluble liquid with the typical smell of paint thinners. It is a mono-substituted benzene derivative, i.e., one in which a single hydrogen atom from the benzene molecule has been replaced by a univalent group, in this case CH3.It is an aromatic...
is the feedstock for perfluoromethylcyclohexane, rather than methylcyclohexane
Methylcyclohexane
Methylcyclohexane is a colourless liquid with a faint benzene-like odour. Its molecular formula is C7H14. Methylcyclohexane is used in organic synthesis and as a solvent for cellulose ethers. It is a component of jet fuel and is also a component of correction fluids.-Structure:Monosubstituted...
, 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 IndustriesImperial Chemical Industries
Imperial Chemical Industries was a British chemical company, taken over by AkzoNobel, a Dutch conglomerate, one of the largest chemical producers in the world. In its heyday, ICI was the largest manufacturing company in the British Empire, and commonly regarded as a "bellwether of the British...
Limited (later ICI) was also developing cobalt(III) fluoride technology during the war, prompted by the work in the US. 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. The Flutec business went into a decline, due to a drop in its main application, vapor phase reflow soldering (used in surface-mount technology
Surface-mount technology
Surface mount technology is a method for constructing electronic circuits in which the components are mounted directly onto the surface of printed circuit boards . An electronic device so made is called a surface mount device...
, 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. BNFL Fluorochemicals Ltd became F2 Chemicals Ltd in 1998.