United States Patent Office Patented June 15, 1971
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3,584,999 United States Patent Office Patented June 15, 1971 Alternatively, the POF may be pumped into a reactor 3,584,999 containing the hydrogen fluoride and the reaction carried MANUFACTURE OF PHOSPHORUS out at 10-100° C. or higher. PENTAFLUORIDE Robert A. Wiesboeck, Atlanta, Ga., assignor to United The mole ratio of the reactants is not critical; however, States Steel Corporation, Pittsburgh, Pa. best results were obtained when 2-4 (preferably 3) moles No Drawing. Filed Jan. 16, 1968, Ser. No. 698,128 of hydrogen fluoride are employed per mole of phos Int, C. C01b. 25/10 phoryl fluoride. U.S. C. 23-205 10 Claims Approximately one-half of the POF is converted to PFs of high purity. If the product is removed from the O reactor at temperatures above 20° C., some hydrogen. ABSTRACT OF THE DISCLOSURE fluoride and a trace of phosphoryl fluoride are also ob Phosphoryl fluoride is reacted with hydrogen fluoride tained. These impurities can be separated by fractional to form phosphorus pentafluoride and hexafluorophos condensation at -40° C. phoric acid, and the hexafluorophosphoric acid may be The remaining liquid phase of the reaction mixture is reacted with sulfur trioxide, pyrosulfuric acid or fluoro essentially 60-65 percent hexafluorophosphoric acid. The sulfonic acid to form additional phosphorus pentafluoride. yield of phosphorus pentafluoride can be increased by The hexafluorophosphoric acid from any source may be reaction of the remaining liquid with sulfur trioxide, as reacted with the sulfur trioxide and/or pyrosulfuric acid described hereinafter. Total conversion of POF to PFs, to liberate phosphorus pentafluoride. employing both methods, ranged from 70-80 percent. 20 Starting with the hexafluorophosphoric acid produced as above described or obtained from any source, the hexa fluorophosphoric acid solutions contain the acid as an BACKGROUND AND SUMMARY equilibrium mixture involving hydrogen fluoride and Conventional processes for the manufacture of phos mono- and difluorophosphoric acid, as follows: phorus pentafluoride are based on the halogen exchange 25 of phosphorus pentachloride with a fluoride; i.e., arsenic EF 4HF trifluoride, HPOF) - H(POF) - H(PFo) 3PCls--5AsF->3PF--5AsCls H2O 2H2O or on the chlorofluorination of phosphorus trifluoride, The composition of commercial 65 percent hexafluoro 30 phosphoric acid has been shown to consist of 25 mole 5PF--3Cl-)3PF-2PCl percent HPFs, 1 mole percent HPOF2, 1 mole percent Both methods require extensive fractionation to separate HPOF, 7 mole percent HF and 66 mole percent H.O. mixed halides from phosphorus pentafluoride. Attempts to concentrate 65 percent HPF6 lead to de I have discovered that phosphorus pentafluoride can composition, shifting the equilibrium to mono- and di be produced in high purity without resorting to complex 35 fluorophosphoric acid and hydrogen fluoride: operations. Any impurities in the product are of such nature that they can easily be removed by inexpensive procedures. In contrast to the conventional methods, the phosphorus pentafluoride produced is free of chlorine. 40 However, I have found removal of water without shift Phosphoryl fluoride reacts with hydrogen fluoride to ing the equilibrium can be achieved by chemical means. form phosphorus pentafluoride, If a solution of hexafluorophosphoric acid is treated with sulfur trioxide, preferably in the cold, the water of the POF--2HF->PF-HO system is chemically bonded through the formation of The resulting water shown in the above equation is sulfuric acid. immediately consumed by the formation of hexafluoro 45 On warming, the hexafluorophosphoric acid cannot phosphoric acid hydrate, hydrolyze and is forced to decompose into phosphorus POF-3HF-1 mole HO)->HPF2HO pentafluoride and hydrogen fluoride, The hexafluorophosphoric acid produced as above de 50 scribed or from any source may be reacted with a water Depending on the quantities of sulfur trioxide em removing agent such as sulfur trioxide or pyrosulfuric acid ployed, part or all of the hydrogen fluoride is converted to obtain phosphorus pentafluoride. to fluorosulfonic acid, Hexafluorophosphoric acid is stable at room tempera ture and above only as a solution in highly polar media 55 such as water. I have found that removal of water by It is advantageous to use a minimum of 1.0-1.3 moles chemical means liberates phosphorus pentafluoride in of sulfur trioxide per mole of water present in the hexa substantial amounts. Typical reagents for the removal of fluorophosphoric acid. The sulfur trioxide can be con water are sulfur trioxide, pyrosulfuric acid or fluorosul densed into a cooled reactor containing the hexafluoro fonic acid, 60 phosphoric acid or it can be added from a pressurized container at room temperature or above. Solutions of sul fur trioxide in inert solvents, i.e., tetrachloroethylene, are The liberated phosphorus pentafluoride is obtained in also suitable. high purity. The liberated PFs is of high purity and contains only DETAILED DESCRIPTION 65 small amounts of POF and HF which can be removed by fractional condensation. Phosphoryl fluoride and hydrogen fluoride are prefer On a plant scale, I would prefer to maintain the hexa ably combined in the cold, warmed to room temperature, fluorophosphoric acid in a pool at a temperature of about and subsequently heated under autogenous pressure to 0-100° C. (preferably about 30° C.) and to introduce about 60-80° C. The volatile material may then be vented 70 the SO3 below the surface of the pool. Instead of SOs, and the phosphorus pentafluoride recovered therefrom pyrosulfuric acid, fluorosulfonic acid and the like may be by condensing or by any other suitable method. used. 3,584,999 3 4 Specific examples illustrative of the invention may be I claim: set out as follows: 1. In a process for the preparation of phosphorus pen EXAMPLE tafluoride, the steps of reacting phosphoryl fluoride with hydrogen fluoride to form phosphorus pentafluoride and Commercial 65 percent hexafluorophosphoric acid (22.5 hexafluorophosphoric acid, and reacting the resulting hex g.) was placed in an aluminum cylinder, cooled to -196 afluorophosphoric acid with an agent selected from the C. and evcuated. Sulfur trioxide (45.0 g.) was condensed group consisting of sulfur trioxide, pyrosulfuric acid and into the cylinder and the mixture was allowed to warm to fluorosulfonic acid to produce additional phosphorus pen room temperature. A pressure of 48 p.s. i. developed, tafluoride. which increased to 81 p.s. i. on heating to 60° C. The O 2. The process of claim 1 in which said phosphoryl formed phosphorus pentafluoride (6.8 g.) was collected fluoride and said hydrogen fluoride are combined in the by expansion into an evacuated cold trap maintained cold and heated to about 60-80 C. at -196° C. 3. The process of claim 1 in which the phosphoryl flu EXAMPLE II oride is pumped into a reactor containing the hydrogen fluoride and reacted therein at a temperature of about Sulfur trioxide (11.0 g.) was condensed into 75 per 10-100° C. cent hexafluorophosphoric acid (10.5 g.) at -196° C. 4. The process of claim 1 in which 2-4 moles of hy as described in Example I. A pressure of 58 p.s. i. devel drogen fluoride are employed per mole of phosphoryl oped on warming the mixture to 60° C. After one hour, fluoride. the volatile material was expanded into an attached cold 5. In a process for the preparation of phosphorus pen trap maintained at -196° C. The obtained phosphorus tafluoride, the steps of reacting hexafluorophosphoric acid pentafluoride (4.2 g.) corresponded to a 62 percent con containing water with a water-removal agent selected version of the hexafluorophosphoric acid. from the group consisting of sulfur trioxide, pyrosulfuric acid and fluorosulfonic acid, and combinations thereof, EXAMPLE III heating the reactants under pressure, and recovering the volatilized material. Hydrogen fluoride (41.0 g.) was condensed into an 6. The process of claim 5 in which sulfur trioxide is evacuated aluminum reactor at -80 C. and pressurized introduced in an inert solvent. with phosphoryl fluoride (65.0 g.). The reaction mixture 7. The process of claim 5 in which the hexafluorophos was allowed to warm to room temperature. A pressure phoric acid is cooled to about -196° C., the sulfur tri of 130 p.s.i. developed. After heating to 50 C. for one 30 oxide condensed and combined therewith, and the reac hour, volatile material was vented into a cold (-196 tants warmed to produce phosphorus pentafluoride, and C.) stainless steel cylinder. A total of 42 g. of phos the volatile material is recovered as product. phorus pentafluoride was obtained. 8. The process of claim 5 in which at least 1.0-1.3 moles of sulfur trioxide are employed per mole of water EXAMPLE IV present in the hexafluorophosphoric acid. 9. The process of claim 5 in which a pool of hexaflu An evacuated aluminum cylinder was charged with hy orophosphoric acid is maintained at about 0-100° C. drogen fluoride (7.2 g.) and subsequently pressurized and sulfur trioxide is introduced into the pool. with phosphoryl fluoride (11.3 g.) at 10° C. Within 30 10. In a process for the preparation of phosphorus minutes, a permanent pressure of 62 p.s. i. developed. After 40 pentafluoride, the steps of reacting, in the cold, phos heating to 60° C. for one hour, the cylinder was cooled phoryl fluoride with two to four equivalents of hydrogen to 25 C. and volatile material vented into a trap main fluoride to form phosphorus pentafluoride and hexafluo tained at -196° C. A total yield of 7.5 g. of phosphorus rophosphoric acid, removing, at a temperature below pentafluoride was obtained. --20 C., the gaseous reaction products, heating under EXAMPLE V pressure the remaining hexafluorophosphoric acid with greater than a stoichiometric amount of an agent selected Hexafluorophosphoric acid (45.5 g.) containing 75 per from the group consisting of sulfur trioxide, pyrosulfuric cent HPF was added dropwise to fluorosulfonic acid acid and fluorosulfonic acid and removing the volatized (50.0 g.) at 20-30 C, while stirring.