Field of the Invention
The present invention relates to the preparation of concentrated hexafluorophosphoric
acid (HPF6) complex solutions containing a minimum amount of water.
Background of the Invention
Known processes for the production of hexafluorophosphoric acid include:
1. H3PO4 + 6 HF --→ HPF6 + 4 H2O
2. P2O5 + 12 HF--→ 2HPF6 + 5 H2O
3. H3PO4 + 3 CaF2 + 3H2SO4
--→ HPF6 • 4H2O + 3CaSO4
The preparation of hexafluorophosphoric acid is usually accomplished
by the addition of anhydrous HF to phosphoric acid or phosphorus pentoxide. The
preparation using pure phosphorus pentoxide is difficult since it is a dry air sensitive
powder and therefore some liquid phase is usually added to facilitate blending and
reaction with the HF. This results in a H2O/ HPF6 ratio of
about 3-4, never achieving the theoretical 2.5 ratio of water to HPF6
when using P2O5.
U.S. Pat. No. 3,634,034 to Nickerson et al, which relates to process
3, produces along with phosphorus pentafluoride, HPF6 and calcium sulfate.
Separation problems and disposal of large amounts of calcium sulfate must be addressed
with the process along with sulfate contamination.
HPF6 is not stable at ambient conditions without a stabilizing
coordination complex like water. In fact, very little or no PF5 is absorbed
when bubbled into anhydrous HF at 0°C showing that no complex is formed without
water being present.
The presence of excess water is undesirable in solutions of hexafluorophosphoric
acid because it promotes hydrolysis of the PF6 anion to partially oxygenated
species. In addition, it decreases the overall effectiveness and acidity of the
acid and dilutes the concentration of the acid. It also adds extra shipping weight
to cost. The pure hexafluorophosphoric acid has not been reported under ambient
conditions (atmospheric pressure and room temperature).
Therefore, there exists a need to provide high purity HPF6
in high concentrations that is stable at ambient conditions and can be prepared
in a simple and economical procedure.
Summary Of The Invention
According to the present invention, there is provided a stable hexafluorophosphoric
acid complexed with water having about 1.0 to 3.3 molecules of water to one hexafluorophosphoric
acid molecule at 20°C and the process for its preparation.
According to one embodiment of the invention, there is prepared a
hexafluorophosphoric acid composition compound having at least one molecule of water
per hexafluorophosphoric acid molecule at temperatures below 50°C, preferably 1.6
to 1.7 molecules of water to one hexafluorophosphoric acid molecule and below 20°C.
According to another embodiment of the invention, there is further
prepared a 1.7 mole ratio of H2O/HPF6 complex which is about
83% HPF6 concentration from polyphosphoric acid.
It is, according to a further embodiment of the invention hexafluorophosphoric
acid monohydrate is prepared therefore, an object of the present invention to provide
an improved process for the preparation of a stable high purity hexafluorophosphoric
acid complex in high yields.
Another related object of the present invention is to provide a process
which is highly efficient and economical.
A further object of the present invention is to provide a process
which produces a minimum amount of by-products.
Description Of The Preferred Embodiments
It has found that to stabilize HPF6, adding only enough
water to maintain its vapor pressure at about that of one atmosphere is essential.
Preferably, the amount of water necessary is at least one molecule of water per
hexafluorophosphoric acid molecule below 10°C and more preferably 1.6-1.7 molecules
of water to one hexafluorophosphoric acid molecule at about 20°C. This solution
can be kept in a Teflon bottle, polyethylene bottle or in a nickel alloy low-pressure
Extra HF can also be added, but it does not contribute significantly
to the stabilization of the PF6 anion at these high concentrations. Since
it is desirable to have as little water as possible and maximize the highest concentration
of the hexafluorophosphoric acid, the preferred ratio is about 1.6-1.7. The highest
HPF6 concentrations available from other conventional routes can produce
water/hexafluorophosphoric acid ratios only greater than 3.3 on a practical basis.
According to the present invention, one is able to produce these concentrated
solutions of hexafluorophosphoric acid (ratios from 1 to 3.3) by adding PF5
gas with cooling to various concentrations of HF in water (from 25% to 52% by weight
HF) to produce the desired low ratios or amounts of water in the product. The phosphorous
pentafluoride can contain phosphorus oxyfluoride (POF3).
According to a further embodiment of the invention, one can produce
these same high concentrations of hexafluorophosphoric acid (low water, 11-28% by
weight) by adding PF5 gas to already prepared solutions of hexafluorophosphoric
acid which have water ratios higher than 3 and which also contain the calculated
amount of excess HF to form the desired concentration of hexafluorophosphoric acid
having low water (ratio of 1 to 3).
According to the present invention, a stable colorless crystalline
1:1 HPF6/H2O complex forms below 10°C when PF5
is bubbled into essentially 50% HF/water below 10°C, namely 0-5°C. This complex
appears to melt with decomposition in the 10-12°C range. This crystalline complex
also forms a slurry in solutions with the H2O/HF ratio from 1 to 2 below
10°C. These solid complexes decompose giving off PF5 when the solutions
are warmed to 20°C (room temperature). The equilibrium concentration in a closed
bottle (fluorocarbon or polyolefin) at approximately one atmosphere is a mole ratio
of H2O/HPF6 of 1.6-1.7 (about 83 ± 2% by weight HPF6
and 17 ± 2% water).
According to the present invention, the process is as follows:
1) PF5 + HF (1 to 3.3 moles water for each HF)--→ HPF6
The ratio of H2O/HPF6 is 1.0-3.3.
2) PF5 + HPF6/3.33 H2O + HF --→ HPF6
+ 1-3.3 H2O
The ratio of H2O/HPF6 is 1.0-3.3.
The starting solutions of HF/water were prepared by diluting 52% HF(H2O/HF
mol ratio of 1.0) with distilled water to achieve ratios of H2O/HF of
1.1 through 2.2. About 25-35 g. of the specific H2O/HF
mol ratio were placed into a weighed polyethylene bottle containing a magnetic stirrer
under argon and the weight determined. The whole bottle was maintained at 0-5°C
with cooling and stirring. PF5 gas was slowly passed through the solution
until PF5 was no longer absorbed as noted by the fumes coming out of
the vent tube.
The bottle was removed and weighed and the weight gain (amount of
PF5) absorbed noted. The bottle was allowed to warm up to 20°C and vented
carefully over several hours while the gas pressure came to equilibrium. The remaining
amount of absorbed PF5 was noted. The H2O/HPF6
mol ratio in these relatively stable solutions was noted and are given in Table
1. The resulting H2O/HPF6 mol ratio for these stable solutions
Comparative Stabilities of HPF6 Containing Various Ratios of Water
Theorical (based on H2O/HPFPresent) Actual
Mol Ratio of H2O/HPF6
After PF5 Addition
Mol Ratio Found
%Wt. HPF6 Found
0 (no water present-anhydrous HF)
0 HPF6 not stable at 0°C without water present
Stable < 10°C Solid
Warmed to 21°C
Stable at 20°C Solution
Stable at 20°C
Stable at 20°C
Stable at 20°C
Stable at 20°C
3.3 (From PPA as raw material) 1.6 83.1 Stable at 20°C
1.6 mol equiv. HF added
>3.5 Commercial HPF6
Stable at 20°C
No additional PF5 added
Experiment 1 was performed with anhydrous HF without any water added.
Only a trace amount of PF5 dissolved in the anhydrous HF and was stabilized
as HPF6 even at 0°C. As shown by the results listed in Table 1, at least
a H2O/HPF6 mol ratio of 1.0 (below 10°C) and
preferably 1.6 at room temperature is needed to produce a stable HPF6
solution (a vapor pressure less than about 200 mm Hg).
In experiment 6, a lower water HPF6 solution (3.3 H2O/HPF6
mol ratio) prepared by the reaction of polyphosphoric acid (116% H3PO4)
and anhydrous HF was upgraded to an even lower water level by adding 1.3 equivalents
of HF and then about 1.3 mol equivalence of PF5 added at 10°C to produce
a HPF6 acid solution with a H2O/HPF6
mol ratio of 2.0.
The same approach can be done with HPF6 solutions of any
concentration by adding the appropriate amount of HF and then saturating with PF5
to bring the H2O/HPF6 mol ratio below 3.5 and
preferably about 1.5 to 2.0 at 10-20°C. Thereby producing a minimum water content
HPF6 (73-89% HPF6).
Preparation of HPF6 With PPA (116% Phosphoric Acid)
A charge of 1200 g of 116% polyphosphoric acid (equivalent to 14.06
mole phosphoric acid) was added to an 1-gal Inconel reactor stirred with a Teflon
stirrer. (The PPA is warmed to about 60°C to gain flowability of the normally very
viscous material for addition to the reactor.) The stirrer was adjusted to be just
above the surface of the viscous PPA. The reactor is cooled to about 5-15°C with
the stirrer just above the surface of the PPA. 1716 g. of anhydrous HF (about 2%
excess) was slowly added to the stirred reaction mixture with sufficient cooling
to maintain the temperature under 25°C, preferably under 20°C. The rate of reaction
was steady throughout the addition with the mixture becoming quite fluid after about
10% addition. The whole addition using ice water as the coolant took about 1.5 hours.
The reaction mixture was allowed to stir and come up to room temperature. The yield
of hexafluorophosphoric acid is 100% with respect to contained fluoride and phosphorus.
(Note that it is actually an equilibrium mixture and the effective concentration
is about 62% to 65% by PF6 anion precipitation. (Excess HF will bring
the effective concentration up to the theoretical concentration.) The water content
is 29% with a mole ratio of water to calculated hexafluorophosphoric acid of 3.3
The advantages of the present process over previous methods are many.
A solution of 300g of 40.3% by weight hydrofluoric acid was prepared
by adding 54 g water to 246 g of 49% hydrofluoric acid with cooling in a fluorocarbon
polymer container (FEP). The solution was stirred and cooled with an ice bath to
maintain the solution between 10-15°C while PF5 gas was slowly added
to the solution by means of a FEP tube which was below the surface of the stirred
solution. The addition was stopped when 882 g of PF5 gas had been added
and the tubing removed and the container closed. The resulting solution weighed
1180 g and contained 83% by weight hexafluorophosphoric acid with a ratio of H2O/HPF6
of 1.65. At room temperature the bottle when opened had a slight vapor pressure
Preparation of low water hexafluorophosphoric acid using already prepared
HPF6. The 300 g of the hexafluorophosphoric acid from example 2 above
was placed in a FEP bottle and stirred with cooling. Then 59 grams anhydrous HF
were added while maintaining the solution at 10 to 15°C. To this solution was added
369 g PF5 gas over 2 hours through a FEP tube while maintaining the solution
at 10 to 15°C. The resulting solution weighed 125 g and contained 83% HPF6
with a molar ratio of H2O/HPF6 of 1.65.
Comparative Example 1
A commercially available hexafluorophosphoric acid having a high ratio
of water molecules to hexafluorophosphoric acid was placed in a vacuum at 4°C to
lower the amount of water. The product decomposed.
HPF6 · (H2O)x VAC-→ 2HF + POF3
+ (x-1) H2O
While in the foregoing specification we have set out specific procedures
in considerable detail for the purpose of illustrating embodiments of the invention,
it will be understood that such details may be varied widely by those skilled in
the art without departing from the spirit of our invention.