blogfast25
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Fluorination of d-block oxides using Freon
Brauer (p. 261) describes a method for fluorinating UO<sub>3</sub> to UF<sub>4</sub> using Freon 12 - dichloro difluoro
methane - CCl<sub>2</sub>F<sub>2</sub> as a fluorinating agent.
As I'm interested in obtaining various anhydrous fluorides of several d-block elements (mainly for 'thermite analogs' research) but am not
equipped to deal with either HF or F<sub>2</sub>, Freon 12 (although largely banned, it can still be purchased in small quantities for
personal use, mainly A/C replacement) sounds like a very interesting fluorinating agent to me.
Googling unearthed quite a few references to the use of chloro fluoro carbons for the fluorination of metal oxides. Fluorination of thoria to
ThF<sub>4</sub> using Freon 12 for instance is described. Other references refer generically to "many oxides", few are very specific. One
referred to partial fluorination of a mixture of Ta<sub>2</sub>O<sub>5</sub> and TaCl<sub>5</sub> using Freon 12.
Most references date back to pre-1945, from around the time when there was a flurry of activity in developing halocarbons and their applications. It's
reasonable to assume that the use of Freons as fluorinating agents decreased in time with improved and safer handling procedures of HF and
F<sub>2</sub>. Per pound of fluorine, fluorocarbons are of course also more costly than the 'real stuff' and less reactive to boot (one
reference mentioned 3 - 4 hours of reaction time to complete the fluorination of UO<sub>3</sub> to UF<sub>4</sub> using Freon
12 at 400 C).
Still, as far as 'the poor man's fluorinating agent' goes, it would (re)open the possibility of fluorinating stuff like thoria, zirconia, titania and
perhaps some Lanthanides too, without risking life and limb with HF or fluorine.
The free energy ΔG of the reaction:
metal oxide + Freon ---> metal fluoride + by-products
is very roughly equal to the difference in free energy of the fluoride and the oxide, minus the free energy of decomposition of the Freon (for Freon
12 the HoF (298 K) = - 486 kJ/mol). So, other fluorine bearing Freons with lower heats of formation could be more suitable than Freon 12.
Doe anybody here have any experience/thoughts with/on such reactions? A search on this forum yielded nothing...
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unionised
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Just a thought. What are the by-products?
I could see a reasonable yield of phosgene as a potential product and, while it's less nasty than HF or F2 I'd not like to have to deal with it
unexpectedly.
MO + CF2Cl2 -> MCl2 + COCl2
looks plausible to me.
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Fleaker
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Phosgene is the gas product of this reaction and this technique does work in some circumstances but it isn't general. Carbon tet is better suited to
chlorination. Still, there is a reason why fluorine is used over CFS, and it's not because it's cheaper--it's because it works better.
Bear in mind that this reaction must be done at high temperature and there are special precautions you must take in preparation of the precursors.
Check your U2U box.
Neither flask nor beaker.
"Kid, you don't even know just what you don't know. "
--The Dark Lord Sauron
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not_important
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Tetrafluoroethane-1,1,1,2 AKA R134A is often easier to obtain than Freon 12 as being chlorine and bromine free it often escapes regulation, has
been used similarly
| Quote: | | The principle reaction involved is the transformation of BaO into fluorinated form. We have found that 1,1,1,2 tetrafluoroethane (CH2FCF3), a
refrigerant (R-134a) used in modern automotive air conditioning systems, serves perfectly as a source of fluorine. The precursor films were
fluorinated in a tubular furnace in a partial vacuum in an atmosphere of 4 Torr of CH2FCF3 and 200milli-Torr of oxygen, at a temperature of 600 .
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True, they were preparing oxyfluorides but it still might prove useful.
Attachment: PhysCvol415p125.pdf (469kB)
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blogfast25
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| Quote: | Originally posted by unionised
Just a thought. What are the by-products?
I could see a reasonable yield of phosgene as a potential product and, while it's less nasty than HF or F2 I'd not like to have to deal with it
unexpectedly.
MO + CF2Cl2 -> MCl2 + COCl2
looks plausible to me. |
Phosgene is certainly the by-product mentioned in Brauer (for UF4).
I feel it need not always be so:
MO2 + CCl2F2 ---> MF2 + CO2 + Cl2, in the case higher fluorides aren't stable.
is a possibility. But phosgene has to be anticipated, of course
Or:
MO2 + 2 CCl2F2 ---> MF4 + CO2 + CCl4
Which are the exact by-products (including which fluoride - or oxyfluoride) is formed depends mainly on which overall reaction yields the highest
ΔG. As gases are involved, the entropic term - TΔS may have considerable bearing on the outcome.
| Quote: | Originally posted by Fleaker
Phosgene is the gas product of this reaction and this technique does work in some circumstances but it isn't general. Carbon tet is better suited to
chlorination. Still, there is a reason why fluorine is used over CFS, and it's not because it's cheaper--it's because it works better.
Bear in mind that this reaction must be done at high temperature and there are special precautions you must take in preparation of the precursors.
Check your U2U box. |
I didn't say F2 is cheaper than CFs but compared on a pound F/pound F basis it almost certainly is. The cost of producing the CFs is built-in
the use of CFs as a fluorinating agent. And they are much less reactive, of course.
As regards precautions, avoidance of moisture is the main one that springs to mind. Do you see any others?
| Quote: | Originally posted by not_important
Tetrafluoroethane-1,1,1,2 AKA R134A is often easier to obtain than Freon 12 as being chlorine and bromine free it often escapes regulation, has
been used similarly
| Quote: | | The principle reaction involved is the transformation of BaO into fluorinated form. We have found that 1,1,1,2 tetrafluoroethane (CH2FCF3), a
refrigerant (R-134a) used in modern automotive air conditioning systems, serves perfectly as a source of fluorine. The precursor films were
fluorinated in a tubular furnace in a partial vacuum in an atmosphere of 4 Torr of CH2FCF3 and 200milli-Torr of oxygen, at a temperature of 600 .
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True, they were preparing oxyfluorides but it still might prove useful. |
Yes, the easier to obtain R-134a has already been considered because it's easy to obtain and contains a lot of F. It would be interesting to have the
heat of formation of R-134a, as then some thermodynamical calculations would become possible
[Edited on 14-12-2008 by blogfast25]
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