Production of a hexafluorophosphate salt and of phosphorous pentafluoride

Abstract

A process for producing a hexafluorophosphate salt comprises neutralizing hexafluorophosphoric acid with an organic Lewis base, to obtain an organic hexafluorophosphate salt. The organic hexafluorophosphate salt is reacted with an alkali hydroxide selected from an alkali metal hydroxide (other than LiOH) and an alkaline earth metal hydroxide, in a non-aqueous suspension medium, to obtain an alkali hexafluorophosphate salt as a precipitate. A liquid phase comprising the non-aqueous suspension medium, any unreacted organic Lewis base and any water that has formed during the reaction to form the precipitate, is removed. Thereby, the alkali hexafluorophosphate salt is recovered.

Claims

1. A process for producing a hexafluorophosphate salt, the process comprising neutralizing hexafluorophosphoric acid with an organic Lewis base, to obtain an organic hexafluorophosphate salt; reacting the organic hexafluorophosphate salt with an alkali hydroxide selected from an alkali metal hydroxide (other than LiOH) and an alkaline earth metal hydroxide, in a non-aqueous suspension medium, to obtain an alkali hexafluorophosphate salt as a precipitate; and removing a liquid phase comprising the non-aqueous suspension medium, any unreacted organic Lewis base and any water that has formed during the reaction to form the precipitate, thereby to recover the alkali hexafluorophosphate salt.

2. The process according to claim 1, which includes reacting phosphoric acid with anhydrous hydrogen fluoride or aqueous hydrofluoric acid, to obtain the hexafluorophosphoric acid.

3. The process according to claim 1, wherein the organic Lewis base is an organic amine.

4. The process according to claim 3, wherein the organic amine is selected from pyridine, imidazole, and pyrole.

5. The process according to claim 4, wherein the organic amine is pyridine.

6. The process according to claim 1, wherein the alkali hydroxide is selected from sodium hydroxide and potassium hydroxide.

7. The process according to claim 6, wherein the alkali hydroxide is sodium hydroxide.

8. The process according to claim 1, wherein the non-aqueous suspension medium is an organic solvent.

9. The process according to claim 8, wherein the organic solvent comprises methanol or ethanol.

10. The process according to claim 9, wherein the organic solvent comprises ethanol.

11. The process according to claim 1, wherein the non-aqueous suspension medium is an aprotic medium.

12. The process according to claim 11, wherein the aprotic medium comprises an alkyl carbonate, a tetrahydrofuran ether, or acetonitrile.

13. The process according to claim 1, wherein the removal of the liquid phase is effected by decanting excess liquid phase from the precipitate, and heating the precipitate to a temperature up to 200 C. to evaporate residual liquid phase present on the precipitate.

Description

(1) The invention will now be described in more detail with reference to the accompanying drawings and the following non-limiting Examples.

(2) In the drawings,

(3) FIG. 1 shows, in simplified flow diagram form, a process for producing pure gaseous phosphorus pentafluoride (PF.sub.5), in accordance with the invention;

(4) FIG. 2 shows, for Example 1, a plot of conductivity vs volume during titration of HPF.sub.6 solution with NaOH;

(5) FIG. 3 shows, for Example 1, an EDX elemental scan of C.sub.5H.sub.5NHPF.sub.6;

(6) FIG. 4 shows, for Example 1, a SEM image of the synthesized C.sub.5H.sub.5NHPF.sub.6;

(7) FIG. 5 shows, for Example 1, the .sup.13C NMR spectrum of the synthesized solid C.sub.5H.sub.5NHPF.sub.6;

(8) FIG. 6 shows, for Example 2, FTIR spectra of the synthesized MPF.sub.6-salts or pyridine complexes;

(9) FIG. 7 shows, for Example 2, Raman spectra of the synthesized MPF.sub.6-productsLiPF.sub.6-pyridine complex (top line or spectrum), NaPF.sub.6 salt (middle line or spectrum) and KPF.sub.6 salt (bottom line or spectrum);

(10) FIG. 8 shows, for Example 3, a process flow diagram of the experimental set-up used for the thermal decomposition of KPF.sub.6 and NaPF.sub.6;

(11) FIG. 9 shows, for Example 3, a FTIR spectrum of the gaseous products formed after thermal decomposition of KPF.sub.6 in helium at 600 C.;

(12) FIG. 10 shows, for Example 3, a FTIR spectrum of the gaseous products from thermal decomposition of NaPF.sub.6 salt;

(13) FIG. 11 shows, for Example 3, a FTIR spectrum of a commercial PF.sub.5 gas; and

(14) FIG. 12 shows, for Example 3, a thermogravimetric (TG) graph of the decomposition of NaPF.sub.6

(15) One embodiment of the invention described hereunder, encompasses the synthesis of C.sub.5H.sub.5NHPF.sub.6 (pyridinium hexafluorophosphate) as a precursor for obtaining for example pure KPF.sub.6 salt, the conversion of C.sub.5H.sub.5NHPF.sub.6 to the salt and the isolation of the pure salt, which is a good PF.sub.5 gas generator.

(16) Referring to FIG. 1, reference numeral 10 generally indicates a process for producing pure phosphorus pentafluoride (PF.sub.5) gas, in accordance with the invention.

(17) The process 10 includes a first reaction stage 12, with a H.sub.3PO.sub.4 feed line 14 as well as an HF feed line 16 leading into the stage 12. In the stage 12, H.sub.3PO.sub.4 and HF react to give hexafluorophosphoric acid and water, in accordance with reaction (1):
6HF+H.sub.3PO.sub.4HPF.sub.6+4H.sub.2O(1)

(18) The reaction products from the stage 12 pass, along a flow line 18, to a second reaction stage 20. A pyridine (C.sub.5H.sub.5N) addition line 22 leads into the stage 20. In the reaction stage 20, the hexafluorophosphoric acid is neutralized by means of pyridine, which thus constitutes an organic Lewis base, in accordance with reaction (2):
HPF.sub.6(aq)+C.sub.5H.sub.5N.fwdarw.C.sub.5H.sub.5NHPF.sub.6(s)(2)

(19) The solid reaction product from the stage 20 passes, along a flow line 24, to a stage 26, with a solid KOH addition line 27 as well as an ethanol (solvent) (EtOH) addition line 28 also leading into the stage 26. In the stage 26, the organic hexafluorophosphate salt that is formed in the stage 20, is reacted with the KOH in accordance with reaction (3):

(20) ##STR00001##

(21) The reaction products from the stage 26 pass along a line 29 to a separation stage 30 where the precipitate, i.e. KPF.sub.6, is separated from a liquid phase comprising regenerated pyridine, water and ethanol.

(22) The liquid phase passes from the stage 30 along a flow line 32 to a stage 34 where the pyridine is separated from the ethanol. The pyridine and water are recycled from the stage 34, along a flow line 36, to the stage 20, while the ethanol is recycled, along a flow line 38, to the stage 26.

(23) The solid, wet KPF.sub.6 passes from the stage 30, along a transfer line 39, to a drying stage 40 where it is dried at a temperature of 100 C. to 200 C. The dried KPF.sub.6 passes from the stage 40 along a transfer line 42 to a thermal decomposition stage 44 in which the KPF.sub.6 is thermally decomposed at a temperature up to 600 C., in accordance with reaction (4):
KPF.sub.6.fwdarw.PF.sub.5+2KF.sub.(s)(4)

(24) The resultant pure gaseous PF.sub.5 is withdrawn from the stage 44 along the line 46. The KF that is produced in accordance with the reaction 4 is withdrawn from the stage 44 along a line 48, to a stage 50. A Ca(OH).sub.2 addition line 52 leads into the stage 50. In the stage 50, the Ca(OH).sub.2 reacts with the KF in accordance with reaction (5) to yield KOH and CaF.sub.2:
2KF+Ca(OH).sub.2.fwdarw.KOH+CaF.sub.2(5)

(25) These reaction products pass from the stage 50 along a line 54 to a separation stage 56 where the KOH is separated from the CaF.sub.2. The KOH is recycled from the stage 56 to the stage 26, along a line 58.

(26) The CaF.sub.2 passes from the stage 56 to a reaction stage 60 along a line 59. A H.sub.2SO.sub.4 addition line 62 leads into the stage 60. In the stage 60, the CaF.sub.2 reacts with the H.sub.2SO.sub.4 in accordance with reaction (6) to produce solid CaSO.sub.4 as well as HF:
CaF.sub.2+H.sub.2SO.sub.4.fwdarw.2HF+CaSO.sub.4(s)(6)

(27) The reaction products from the stage 60 pass along a flow line 64, to a stage 66 where the HF is separated from the CaSO.sub.4. The CaSO.sub.4 is withdrawn from the stage 66 along a line 68, while the HF is recycled to the stage 12 along a line 70.

(28) Hexafluorophosphoric acid (HPF.sub.6) is a complex ionic mixture of weak and strong acids which constantly decompose at room temperature. In order to determine a good estimate for the stoichiometric quantity of pyridine required to neutralize only the stronger HPF.sub.6 component, the reaction end point was predetermined by conductivity titration using HPF.sub.6 and NaOH solutions. The molar concentration obtained from this titration end point value was used to determine the stoichiometric quantities of C.sub.5H.sub.5N and HPF.sub.6 for the formation of a pure C.sub.5H.sub.5NHPF.sub.6 compound.

EXAMPLE 1

(29) In order to determine the molar concentration of HPF.sub.6 in the acid solution for the stoichiometric addition of pyridine in stage 20 of FIG. 1, a sodium hydroxide standard solution of 0.1 M concentration was used to titrate an HPF.sub.6 solution because NaOH does not form a precipitate during the reaction. A 600 l aliquot of HPF.sub.6 solution was diluted to 100 ml with distilled water and titrated with the 0.1 M solution of NaOH. An Orion 4 Star conductivity meter fitted with a platinum electrode was used to measure the conductivity of the reaction mixture during titration. The solution was constantly stirred with a magnetic stirrer to ensure OH/H+ equilibrium. Conductivity changes were measured after every addition of 5 ml of titrant. The corresponding conductivity value and volume were recorded. The end point of the titration is marked by the vertex point or bend in the conductivity graph where the steep decline in conductivity values due to depleted strong acid ions (FIG. 2) changes to a more moderate slope, which is determined by the intersection of the tangents to the straight sections of the graph as shown. The thus determined end point corresponds to 40.8 ml of NaOH in 100 ml of HPF.sub.6 solution, which equalises to 0.00408 mol NaOH and translates to a molar concentration of the HPF6 of 6.80 mol per liter.

(30) This molar concentration was then applied during the reaction of HPF.sub.6 with pyridine to produce pyridinium hexafluorophosphate (C.sub.5H.sub.5NHPF.sub.6). The reaction of pyridine with HPF.sub.6 is very exothermic and therefore water is used as a cooling medium to minimize volatilization of the reactants and improve the yield. Pyridine (18 ml) was slowly added (drop wise) to a commercial HPF.sub.6 solution (10 ml) purchased from Alfa Aeser diluted to 200 ml with distilled water. A product precipitated out. The precipitate was filtered using a Whatman No. 42 filter paper and dried overnight in an oven at 110 C. In the repeat experiment, the water previously recovered during filtration was topped up to 200 ml with distilled water and re-used. This helped to minimize the loss of the product through solubility and thus improved the yield.

(31) The precipitated product comprised a white powder of pyridinium hexafluorophosphate, and was obtained with an average yield of 95% based on the recovery from repeated experiments. This powder was characterized using inductively coupled plasma (ICP), nitrogen, oxygen, sulphur and carbon combustion process and other techniques such as EDX (FIG. 3) and ISE (ion selective electrode, particularly fluoride ion). Table 1 lists the elemental composition of the pyridinium hexafluorophosphate powder obtained by different techniques.

(32) TABLE-US-00001 TABLE 1 Percentage elemental composition of C.sub.5H.sub.5NHPF.sub.6 Percent Composition (m/m) Element Theoretical ICP EDX Combustion ISE F 50.6 39 52.39 H 2.7 N 6.2 5.8 5.55 C 26.7 24.2 P 13.8 13.40 11.21

(33) Scanning electron microscope (SEM) photos show that the compound has small particles of approximately 40 m in diameter (FIG. 4). NMR results (FIG. 5) confirm that there is a strong electron withdrawing group in the pyridinium hexafluorophosphate, which supports the conclusion that pyridinium hexafluorophosphate as a compound was formed.

EXAMPLE 2

(34) In a laboratory simulation of the stage 26 of FIG. 1, the chemical reaction between pyridinium hexafluorophosphate and an alkali metal hydroxide such as sodium or potassium hydroxide in the presence of ethyl alcohol forms the XPF.sub.6 salt (where X is sodium or potassium), while liberating pyridine gas, leaving a solid alkali metal hexafluorophosphate as a product (in accordance with equations or reactions 7 and 3 respectively).

(35) ##STR00002##

(36) For example, the sodium hexafluorophosphate was synthesised by adding 0.8 g of NaOH pellets to a 50-ml ethanol solution and then reacting with 4.5 g of suspended C.sub.5H.sub.5NHPF.sub.6, previously synthesised as described above. The mixture was continuously stirred for 10 minutes, during which time a precipitate formed. The liquid phase containing water, the pyridine and ethanol was decanted. The precipitate was filtered and dried overnight at 90 C. in an oven to remove impurities and excess pyridine. The resulting white powder was stored in a glove box filled with nitrogen.

(37) For the synthesis of the potassium hexafluorophosphate, 1.1 g of KOH powder was reacted in the place of NaOH, and the procedure outlined for the synthesis of the sodium salt was followed.

(38) When applying the reaction of pyridinium hexafluorophosphate to LiOH, the inventors found that lithium hexafluorophosphate could not be obtained in this direct synthesis method (Equation 8) as is the case for sodium and potassium salts, but instead forms a stable LiPF.sub.6-pyridine complex.

(39) ##STR00003##

(40) Comparative analyses with FTIR and Raman spectra (FIGS. 6 and 7) confirm that the Li-species contains significant amounts of pyridine.

(41) In contrast, the reactions between pyridinium hexafluorophosphate and the sodium or potassium hydroxide apparently do not form complexes, but instead rapidly form precipitates of the pure salts of NaPF.sub.6 and KPF.sub.6, with little or no traces of pyridine, particularly after mild treatment at elevated temperatures (FIG. 6).

(42) Thus, the inventors have found that alkali metal cations other than Li.sup.+ such as sodium and potassium cations do not form stable intermediate complexes as opposed to the lithium cation. Apparently, the reaction progresses without delay to form precipitates of pure salts and liberate pyridine and water into the suspension medium. Furthermore, the favourable reaction conditions for the formation of pure sodium and potassium hexafluorophosphate salts present an opportunity for their use as good sources of pure PF.sub.5 gas, a precursor in the synthesis of LiPF.sub.6.

EXAMPLE 3

(43) In a laboratory simulation of stage 44 of the process 10, the synthesized KPF.sub.6 and NaPF.sub.6 salts can be subjected to thermal decomposition, e.g. in a tube reactor system (FIG. 8) with the aim of decomposing the salts into a phosphorus pentafluoride gas and a metal fluoride residue according to Equation 9 (also exemplified more specifically by equation or reaction (4) above).

(44) ##STR00004##
where M=K or Na.

(45) The laboratory simulation of stage 44, as depicted in FIG. 8, comprises a single 2.54 cm diameter tube reactor 102 manufactured from stainless steel. The tube reactor 102 consists of thick stainless steel walls. The tube reactor 102 is fitted with an electric heating device 104. Uniform heating of the tube reactor 102 is made possible by using the heater 104, which is a conventional heater designed for tube reactors. The tube reactor 102 is fitted with a temperature controller 108.

(46) An inlet tube 110 is connected to an upstream end of the tube reactor 102, and is fitted with a pair of spaced valves 112, 114. Between the valves 112, 114 leads a vacuum line 116 fitted with a valve 118.

(47) The line 110 leads from a PF.sub.5 gas cylinder 120 and is fitted with a valve 122, a forward pressure regulator 124 and another valve 126. A bypass line 128 leads from upstream of the flow regulator 124 to downstream of the valve 126 and is fitted with a valve 130. The PF.sub.5 gas source was included in the design because PF.sub.5 was used as a passivation gas and a reference standard prior to commencing of the thermal decomposition experiments by purging the system and measuring the reference point in the FTIR cell.

(48) The laboratory installation also includes a helium gas cylinder 132 from which leads a line 134 into the line 110 downstream of the valve 126. The line 134 is fitted with a forward pressure regulator 136, a flow indicator 138 and a valve 140. The FTIR gas cell 150 is a flow through 10 cm gas cell fitted with ZnSe windows, and with a pressure transducer 152.

(49) A line 142 leads from the downstream end of the tube reactor 102 and is fitted with a pressure transducer 144, a valve 146 and a further valve 148. The line 142 leads into the gas cell 150.

(50) A line 154 leads from the gas cell 150 to a vacuum generator (not shown) and is fitted with a valve 154, a pressure indicator 156 and a valve 158.

(51) In use, a head space of the tube reactor 102 is evacuated before each run, whereafter helium is allowed to flow continuously through the reactor 102 and the gas cell 150 at atmospheric pressure. The pressure inside the reactor 102 is monitored by the pressure transducer 144. The pressure inside the gas cell is monitored using the pressure transducer 152, while the system pressure is monitored using the pressure transducer 156. The pressures in the gas cylinders 120, 132 are regulated using the forward pressure regulators 124 and 136 respectively. A thermocouple 106 monitors the reaction temperature, while a thermocouple 108 monitors the reactor and heater temperature. The gas cell 150 is evacuated by opening valves 154 and 158 and closing valve 148. Gaseous substances within the system can be analyzed as and when needed by charging the gas cell 150 to a maximum pressure of 1.5 bar, closing the valves 148, 154 and then collecting data using the infrared spectrometer.

(52) Thermal decomposition of KPF.sub.6 and NaPF.sub.6 is performed under a constant helium flow rate of 100 ml/min, heating the potassium hexafluorophosphate to 600 C. and the sodium hexafluorophosphate to 400 C. respectively. This is followed by constantly monitoring the thermal decomposition pressures (pressure transducer 144) and temperatures (thermocouple 106) on their respective indicators. The PF.sub.5 gas generated through thermal decomposition of each of these salts is analysed by allowing the IR spectrometer to measure a spectrum in time and to collect data as and when desired. The results are shown in FIGS. 9 and 10, and can be compared with the FTIR spectrum of commercially available PF.sub.5 gas (FIG. 11).

(53) A pre-heating step at temperatures of up to 300 C. under drying conditions and before PF.sub.5 gas is liberated essentially eliminates HF. Impurities other than HF are eliminated by starting with pure feed materials. In FIG. 12, the bend in the TG graph of NaPF.sub.6 around 300 C. represents the endpoint of the volatilisation of impurities prior to commencement of the evolution of the pure PF.sub.5 gas.

(54) The possibility of synthesizing pure PF.sub.5 gas from thermal decomposition of MPF.sub.6 salts according to the present invention allows this gas to be produced in a high purity form for further use as a precursor towards synthesis of pure LiPF.sub.6. Using the PF.sub.5 gas as a precursor is normally realised through a process of applying a known LiPF.sub.6 synthesis process (prior art), e.g. by passing the synthesized PF.sub.5 gas through lithium fluoride suspended in anhydrous hydrogen fluoride as described before.

(55) The proposed PF.sub.5 gas production process of the invention is unique and differs from current industrial processes for producing PF.sub.5 gas because it makes it possible to avoid the use of expensive fluorine gas as fluoride source; instead it uses inexpensive hydrogen fluoride but avoids the tedious and environmentally unfriendly chloride route. Advantages of this process are that no gaseous mixtures are formed which require expensive equipment for separation and purification and the process is also designed to recover or recycle most reagents, e.g. the sodium (or potassium) ions used in an intermediate steps can be recovered and re-used in the process, while pyridine and ethanol are recycled. The fluoride that is not bound to the product (PF.sub.5) is recoverable as CaF.sub.2 which can be recycled to produce HF to feed back into the process as shown in FIG. 1. This makes the process of producing PF.sub.5 gas from thermal decomposition of M.sup.+PF.sub.6.sup.(M.sup.+=K.sup.+ or Na.sup.+) and then synthesizing pure UP F.sub.6 via known processes economically viable.