Method for producing phosphorus pentafluoride and method for producing hexafluorophosphate

Abstract

This method for producing phosphorus pentafluoride includes a dehydration step for dehydrating hexafluorophosphate hydrate using sulfuric acid having a sulfur trioxide content of less than 5 mass%.　It is also preferable to include the following steps a) and b).　a) A step for concentrating the sulfuric acid after the dehydration step.　b) A step in which the sulfuric acid is reused in the production of phosphorus pentafluoride by using the sulfuric acid concentrated in step a) in the dehydration of hexafluorophosphate hydrate.　It is also preferable that the dehydration step is performed in the presence of hydrogen fluoride, and that step c) is included before step a).　c) A step for recovering the hydrogen fluoride after the dehydration step.

Description

Method for producing phosphorus pentafluoride and method for producing hexafluorophosphate

[0001] The present invention relates to a method for producing phosphorus pentafluoride and a method for producing hexafluorophosphate.

[0002] Phosphorus pentafluoride is industrially useful and important in the fields of semiconductor materials and battery materials, etc. In particular, it is used as a raw material for lithium hexafluorophosphate, which is used as an electrolyte for lithium-ion batteries and as a catalyst for organic synthesis.

[0003] Currently known methods for producing phosphorus pentafluoride include, for example, the following methods.

[0004] Japanese Patent Laid-Open No. 10-245211 (Patent Document 1) and Japanese Patent Laid-Open No. 60-251109 (Patent Document 2) describe a method of halogen-exchanging phosphorus trichloride with chlorine using HF, and a method of halogen-exchanging phosphorus pentachloride with HF, respectively. ３ + 5HF + Cl ２ → PF ５ + 5HCl (Patent Document 1) PCl ５ + 5HF → PF ５ +5HCl (Patent Document 2)

[0005] In Japanese Patent Laid-Open No. 2001-122605 (Patent Document 3), F ２ The fluorination of phosphorus using P + 5 / 2F has been described. ２ → PF ５ (Patent Document 3)

[0006] U.S. Patent No. 3,634,034 (Patent Document 4) describes a method in which phosphoric acid and calcium fluoride are reacted in the presence of sulfur trioxide (anhydrous sulfuric acid). ３　 + 2H ３ P.O. ４　 + 2CaF ２　 → PF ５　 + HPF ６ ・2H ２ O + HF + 6CSO ４ (Patent Document 4)

[0007] US Publication No. 2001 / 041158 (Patent Document 5) describes a method for generating phosphorus pentafluoride from hexafluorophosphoric acid hydrate using sulfur trioxide, oleum, and fluorosulfonic acid. ６ ・nH ２ O + SO ３ (fuming sulfuric acid) → PF ５ + nH ２ SO ４ (Patent Document 5)

[0008] Japanese Patent Application Laid-Open No. 10-245211 Japanese Patent Application Laid-Open No. 60-251109 Japanese Patent Application Laid-Open No. 2001-122605 U.S. Patent No. 3,634,034 U.S. Publication No. 2001 / 041158

[0009] The methods described in Patent Documents 1 and 2 generate 5 equivalents of by-product HCl. The boiling point of HCl is −84.9° C., which is very close to the boiling point of phosphorus pentafluoride, −84.8° C., making separation by distillation difficult and industrially disadvantageous.

[0010] The method described in Patent Document 3 is difficult to control because of the explosive reaction that accompanies the oxidation of zero-valent phosphorus. ２ is very expensive and therefore economically disadvantageous.

[0011] The method described in Patent Document 4 is unsuitable for industrialization because the reaction itself has a low yield of about 12% and also produces a large amount of calcium sulfate as a by-product.

[0012] The method described in Patent Document 5 provides an excellent yield of phosphorus pentafluoride, but ３ The dehydration of hexafluorophosphate hydrate by the HCl method produces sulfuric acid as a by-product of the hydrate. ３ However, the amount of sulfuric acid continues to increase, so it becomes necessary to dispose of the sulfuric acid. ３ and fluorosulfonic acids constantly produce highly toxic vapors, and SO ３ is an extremely difficult substance to handle due to its explosive properties, and requires special handling techniques.

[0013] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a method for producing phosphorus pentafluoride from hexafluorophosphate hydrate economically while producing less waste than conventional methods.

[0014] As a result of extensive research into achieving the above object, the present inventors have discovered that hexafluorophosphate hydrate (HPF ６ ・nH ２ The present inventors have discovered a method for producing phosphorus pentafluoride by dehydrating phosphorus pentafluoride (O) with sulfuric acid. Furthermore, they have discovered that by using the phosphorus pentafluoride obtained by this method, hexafluorophosphate can be produced inexpensively and economically, with almost no waste products such as sulfuric acid or hydrogen chloride being produced, and have completed the present invention.

[0015] That is, the present invention provides a method for producing phosphorus pentafluoride, which comprises a step of dehydrating hexafluorophosphate hydrate using sulfuric acid containing less than 5% by mass of sulfur trioxide.

[0016] The present invention also provides a method for producing a hexafluorophosphate salt, which comprises a step of reacting phosphorus pentafluoride produced by the above-described method with a fluoride salt in an organic solvent or a hydrogen fluoride solvent according to the following formula (1): PF ５ + MF → MPF ６ ... (1) (In formula (1), M is Li, Na, K, Rb, Cs, Ag, and NH ４ It is one or more selected from the group consisting of

[0017] Hereinafter, embodiments of the present invention will be described. Note that the following embodiments are merely examples for explaining the present invention, and the present invention is not limited to these embodiments. In the following description, when "sulfuric acid" is mentioned, it refers to sulfuric acid (H ２ SO ４ ) and sulfuric acid (H ２ SO ４ ) may refer to a liquid containing

[0018] The method for producing phosphorus pentafluoride in this embodiment includes a step of dehydrating hexafluorophosphate hydrate using sulfuric acid containing less than 5 mass % of sulfur trioxide (hereinafter also simply referred to as the "dehydration step").

[0019] For hexafluorophosphate hydrate, for example, H ｘ P.O. ｙ F ｚ (wherein 0≦x≦3, 0≦y≦4, 0≦z≦6, and P is pentavalent) ｘ P.O. ｙ F ｚ It is preferable to use a compound obtained by reacting one or more phosphorus compounds selected from the group consisting of hexafluorophosphate, ...

[0020] The procedure for the dehydration step may, for example, be a method in which the sulfuric acid, a phosphorus compound, and hydrogen fluoride are mixed together, and then the mixture is heated to dehydrate hexafluorophosphoric acid hydrate, which is produced by the reaction of the phosphorus compound and hydrogen fluoride, with the sulfuric acid. Alternatively, a method in which hexafluorophosphoric acid hydrate is mixed with sulfuric acid and heated to dehydrate the hexafluorophosphoric acid hydrate may also be used. During this dehydration, hexafluorophosphoric acid is decomposed into phosphorus pentafluoride and hydrogen fluoride, thereby obtaining phosphorus pentafluoride.

[0021] The phosphorus compound may be in the form of an aqueous solution, but is preferably in a high concentration. A high concentration is preferred because the water content of the resulting hexafluorophosphate hydrate is reduced, allowing for a reduction in the amount of sulfuric acid used in the dehydration step. Specifically, when a liquid containing the phosphorus compound and water is used as the phosphorus compound, the content of the phosphorus compound in the liquid is preferably 50% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, and most preferably 85% by mass or more. For example, when a mixture containing a phosphorus compound having one phosphorus element per molecule and water is used as the phosphorus compound to be reacted with hydrogen fluoride, the proportion of the phosphorus compound in the mixture is preferably 50% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, and particularly preferably 85% by mass or more. Furthermore, when a mixture containing a phosphorus compound having two or more phosphorus elements per molecule and water is used as the phosphorus compound to be reacted with hydrogen fluoride, the proportion of the phosphorus compound having two or more phosphorus elements per molecule in the mixture is preferably 50% by mass or more, more preferably 70% by mass or more, and even more preferably 80% by mass or more. Examples of such a liquid or mixture include commercially available concentrated phosphoric acid, a mixture of phosphoric anhydride and concentrated phosphoric acid, and a mixture of phosphoric anhydride and water.

[0022] As the phosphorus compound, a phosphorus compound containing an oxygen atom is preferable in terms of availability, low cost, and economical. For example, a phosphorus compound containing one phosphorus element in the molecule is preferable. ｘ P.O. ｙ F ｚ As, H ３ P.O. ４ , H.P.O. ３ , H ２ P.O. ３ F, H.P.O. ２ F ２ , POF ３ Pentavalent phosphorus compounds such as H ｘ P.O. ｙ F ｚ As, P ２ O ５ (P.O. ２．５ ) and other H ４ P ２ O ７ (H２ P.O. ３．５ ), H ５ P ３ O １０ (H ５／３ P.O. １０／３ ), H ６ P ４ O １３ (H １．５ P.O. ３．２５ and pentavalent phosphorus compounds in which one or more hydrogen atoms of these compounds have been substituted with fluorine atoms.

[0023] Polyphosphoric acid refers to the general condensed phosphoric acid (HPO ３ ) ｎ (n is a number of 2 or more), and orthophosphoric acid (H ３ P.O. ４ ) by dehydration condensation, or diphosphorus pentoxide (P ２ O ５ ) can be obtained by hydrolysis of the above. One or more hydrogen atoms in the molecule may or may not be substituted with a fluorine atom. Also, a mixture of polyphosphoric acid and orthophosphoric acid may be used as polyphosphoric acid. ２ O ５ A content of 60% by mass or more is preferred in that the amount of sulfuric acid used can be easily reduced, and a content of 80% by mass or more is more preferred.

[0024] The phosphorus compound used as a raw material for producing hexafluorophosphate hydrate is H ３ P.O. ４ , P ２ O ５ and polyphosphoric acid are particularly preferred in that they generate less water, facilitate dehydration of hexafluorophosphate hydrate with sulfuric acid, and are easy to handle, and at least one selected from phosphoric anhydride and polyphosphoric acid is particularly preferred.

[0025] In producing hexafluorophosphate hydrate, hydrogen fluoride is preferably used in an amount of 6 moles or more, more preferably 9 moles or more, and particularly preferably 10 moles or more, per mole of phosphorus element in the phosphorus compound. The amount of hydrogen fluoride used is preferably 50 moles or less, more preferably 35 moles or less, and particularly preferably 30 moles or less, per mole of phosphorus element in the phosphorus compound, in terms of the hydrogen fluoride usage rate and cost benefits. Anhydrous hydrogen fluoride is preferably used as hydrogen fluoride.

[0026] The reaction temperature between hydrogen fluoride and the phosphorus compound is preferably −40 to 40° C., more preferably −20 to 20° C., from the viewpoints of the production efficiency of hexafluorophosphate hydrate and the energy used for cooling.

[0027] The hexafluorophosphoric acid hydrate obtained by the reaction of the phosphorus compound with hydrogen fluoride can be used as is in the dehydration step without any particular treatment of the excess hydrogen fluoride.

[0028] The dehydration step and the synthesis of hexafluorophosphoric acid hydrate may be performed in one pot. For example, instead of reacting a phosphorus compound with hydrogen fluoride and then mixing sulfuric acid with the reaction product, the phosphorus compound, hydrogen fluoride, and sulfuric acid may be simultaneously charged into a reaction vessel and heated. Alternatively, one of the phosphorus compound and hydrogen fluoride may be first charged into the reaction vessel, followed by sulfuric acid, and then the other of the phosphorus compound and hydrogen fluoride may be charged into the reaction vessel and heated. Thus, the order in which the phosphorus compound, hydrogen fluoride, and sulfuric acid are charged is not particularly limited. This is because the reaction between a phosphorus compound and hydrogen fluoride is often completed instantaneously, and therefore, simultaneously introducing a phosphorus compound, hydrogen fluoride, and sulfuric acid into one reaction vessel is essentially the same as reacting hexafluorophosphoric acid hydrate with sulfuric acid. For example, a phosphorus compound and sulfuric acid may be mixed, and the water contained in the phosphorus compound may be removed in advance by concentration using sulfuric acid, and then hydrogen fluoride may be added. In this case, before mixing with hydrogen fluoride, the phosphorus compound and sulfuric acid are usually mixed and heated to at least partially remove water from the phosphorus compound. This heating step may be performed under reduced pressure. The heating temperature is preferably 150 to 300°C, more preferably 200 to 250°C. The preferred pressure range for reducing the pressure is the same as the preferred pressure range for step a) described below. High-concentration phosphoric acid with a low water content has high viscosity, which results in poor stirring and heating efficiency, making concentration difficult. However, by concentrating the phosphorus compound as a mixed solution with sulfuric acid, the decrease in stirring and heating efficiency can be suppressed. The sulfuric acid may be added by dropwise addition, lump-sum mixing, or multiple mixing. The hydrogen fluoride may also be added by dropwise addition, lump-sum mixing, or multiple mixing.

[0029] In the dehydration step of the present invention, sulfuric acid is hydrated with water molecules in hexafluorophosphoric acid hydrate, thereby causing dehydration.

[0030] In the present invention, sulfur trioxide (SO ３In this case, sulfuric acid containing less than 5 mass% of sulfur trioxide is used. As a result, in the present invention, by reusing the used sulfuric acid, phosphorus pentafluoride can be produced inexpensively and economically while effectively reducing sulfuric acid waste. Here, sulfuric acid containing less than 5 mass% of sulfur trioxide includes sulfuric acid containing sulfur trioxide as well as sulfuric acid not containing sulfur trioxide. When the total amount of sulfuric acid used is taken as 100 mass%, the proportion of sulfur trioxide is less than 5 mass%, preferably 3 mass% or less, more preferably 1 mass% or less, even more preferably 1000 mass ppm or less, and particularly preferably 100 mass ppm or less. In each example described below, sulfuric acid containing 100 mass ppm or less of sulfur trioxide is used. In contrast, the method described in Patent Document 5 uses sulfuric acid containing less than 100 mass ppm of sulfur trioxide. ３ Alternatively, since fluorosulfonic acid is a water-reactive substance, it forms a covalent bond with water molecules to form sulfuric acid, which causes dehydration from hexafluorophosphate hydrate. Thus, the dehydration mechanism of the present invention is completely different from that of the method described in Patent Document 5. In the decomposition reaction caused by the dehydration of the hydrate, SO ３ Alternatively, the use of sulfuric acid, which has a different dehydration mechanism and a lower dehydration capacity than a strong dehydrating agent such as fluorosulfonic acid, is one of the epoch-making features of the present invention, which is unthinkable from the technical common sense of a person skilled in the art. ３ One method is to heat the solution to above 45°C, which is the boiling point of SO , and check for weight loss. ３ By preparing sulfuric acid samples with known concentrations and measuring the conductivity at each concentration to create a calibration curve, the SO ３ SO in sulfuric acid of unknown concentration ３ The concentration can be determined.

[0031] The dehydration step ２ SO ４ The amount of sulfuric acid (H ２ SO ４The amount of sulfuric acid (H ) used is preferably 1.0 equivalent or more relative to the amount of water, more preferably 1.2 equivalents or more, particularly preferably 1.3 equivalents or more, and most preferably 1.5 equivalents or more. On the other hand, if it is too excessive, the amount of reaction liquid increases, which is industrially undesirable from the viewpoint of reactor efficiency. ２ SO ４ The amount of sulfuric acid (H ) used is preferably 30 equivalents or less, more preferably 20 equivalents or less, and particularly preferably 10 equivalents or less, relative to the amount of water. ２ SO ４ The amount of the sulfuric acid ester used is preferably 1.0 equivalent or more and 30 equivalents or less relative to the amount of water. More preferably, it is 1.2 equivalents or more and 20 equivalents or less, particularly preferably 1.3 equivalents or more and 15 equivalents or less, and most preferably 1.5 equivalents or more and 10 equivalents or less. The water content here refers to the total amount of water in the reaction system, including both the water contained in the sulfuric acid added to the reaction system and the water in the hexafluorophosphate hydrate.

[0032] Furthermore, in the present invention, during the dehydration step, it is preferable that the amount of sulfuric acid is 4 moles or more per mole of elemental phosphorus of the phosphorus component, not only because this improves the production efficiency of phosphorus pentafluoride but also because, when a concentration step (described later) is performed, the amount of heat required for the concentration step can be reduced due to the already high concentration of sulfuric acid used for concentration. On the other hand, during the dehydration step, it is preferable that the amount of sulfuric acid is 20 moles or less per mole of elemental phosphorus of the phosphorus component, and 15 moles or less is more preferable, from the viewpoint of reactor efficiency. Furthermore, during the dehydration step, it is preferable that the amount of sulfuric acid is 4 moles or more per mole of elemental phosphorus of the phosphorus component, and 9 moles or more is more preferable. As for the range, during the dehydration step, it is preferable that the amount of sulfuric acid is 4 moles or more and 20 moles or less, and more preferably 9 moles or more per mole of elemental phosphorus of the phosphorus component, and 15 moles or less is more preferable. The phosphorus component referred to in this specification refers to hexafluorophosphoric acid hydrate that is the target of the dehydration step when commercially available hexafluorophosphoric acid hydrate is mixed with sulfuric acid for dehydration, or when a phosphorus compound and hydrogen fluoride have already been reacted in advance to prepare hexafluorophosphoric acid hydrate, which is then introduced into a separate reaction system and mixed with sulfuric acid for dehydration. Furthermore, when both steps are performed in one pot, such as when a step of reacting a phosphorus compound with hydrogen fluoride to obtain hexafluorophosphoric acid hydrate and dehydration of hexafluorophosphoric acid hydrate are performed simultaneously, the phosphorus compound can be considered as the phosphorus component.

[0033] In this embodiment, the concentration (mass percent) of sulfuric acid used in the dehydration step is preferably 80% or more. If the sulfuric acid has this concentration or more, sufficient dehydration from hexafluorophosphate hydrate proceeds in the dehydration step, and a decrease in the yield of phosphorus pentafluoride can be suppressed. It is more preferably 90% or more, and particularly preferably 98% or more. The upper limit of the sulfuric acid concentration is 100%.

[0034] The sulfuric acid may be used in the dehydration step and then concentrated in a sulfuric acid concentration step a) described below (step b below).

[0035] In this embodiment, the step of dehydrating hexafluorophosphate hydrate using sulfuric acid is preferably carried out in the presence of hydrogen fluoride, more preferably anhydrous hydrogen fluoride. The hydrogen fluoride may be introduced as a liquid or may be vaporized and introduced in a gaseous state.

[0036] All steps described herein may be carried out in a batch or continuous manner, and are not particularly limited. Examples of continuous reactors include continuous flow microreactors, line mixers, and continuous tank reactors. Examples of continuous flow microreactors that can be used include the stacked multi-channel reactor SMCR manufactured by Kobe Steel, Ltd.

[0037] The amount of hydrogen fluoride used in the dehydration step is preferably 1 equivalent or more and 100 equivalents or less relative to the phosphorus element of the phosphorus component used in the dehydration step. When the amount is 1 equivalent or more relative to the phosphorus element of the phosphorus component, hydrolysis of hexafluorophosphate hydrate tends to be less likely to proceed during heating. On the other hand, when the amount is 100 equivalents or less relative to the phosphorus element of the phosphorus component, it is possible to prevent the reaction solution temperature from rising too easily, and a decrease in the yield of phosphorus pentafluoride tends to be easily suppressed. From these points of view, the amount of hydrogen fluoride used in the dehydration step is more preferably 3 equivalents or more relative to the phosphorus element of the phosphorus component, and particularly preferably 5 equivalents or more. Furthermore, 50 equivalents or less is more preferred, 30 equivalents or less is even more preferred, 20 equivalents or less is even more preferred, and 15 equivalents or less is most preferred. Therefore, a more preferred range is 3 equivalents or more and 50 equivalents or less, particularly preferably 5 equivalents or more and 30 equivalents or less, and even more preferably 5 equivalents or more and 15 equivalents or less. When sulfuric acid, a phosphorus compound, and hydrogen fluoride are reacted in one pot while using hydrogen fluoride in excess relative to the phosphorus compound, the amount of hydrogen fluoride used in the dehydration step refers to the amount of excess hydrogen fluoride after hexafluorophosphoric acid is produced. The same applies to the explanation of the ratio with sulfuric acid in the next paragraph. In addition, when PF is produced from hexafluorophosphoric acid in the dehydration step, ５ However, this newly generated HF is not included in the amount of hydrogen fluoride used in the dehydration step.

[0038] Furthermore, when hydrogen fluoride is used in the dehydration step, the amount of hydrogen fluoride used in the dehydration step is preferably 0.5 mol or more and 10 mol or less per mol of sulfuric acid used in the dehydration step. An amount of 0.5 mol or more relative to sulfuric acid has the advantage that hydrolysis of hexafluorophosphate hydrate does not proceed easily during heating. On the other hand, an amount of 10 mol or less relative to sulfuric acid has the advantage that the reaction solution temperature does not increase easily, which makes it easy to suppress a decrease in the yield of phosphorus pentafluoride and reduces the reactor efficiency. Furthermore, within this range, sulfuric acid of a predetermined concentration can be easily obtained in the concentration step after dehydration. More preferably, hydrogen fluoride is 0.8 mol or more relative to 1 mol of sulfuric acid used in the dehydration step, with 1.0 mol or more being particularly preferred and 1.2 mol or more being even more preferred. Furthermore, the amount is more preferably 8.0 mol or less, with 5.0 mol or less being particularly preferred and 3.0 mol or less being even more preferred. Therefore, a more preferred range is 0.8 to 8.0 moles of hydrogen fluoride per mole of sulfuric acid used in the dehydration step, particularly preferably 1.0 to 5.0 moles, and even more preferably 1.2 to 3.0 moles.

[0039] The hydrogen fluoride that can be used is that recovered in the dehydration step or the hydrogen fluoride recovery step described below.

[0040] In the dehydration step, a reaction vessel containing sulfuric acid, hexafluorophosphate hydrate, and preferably hydrogen fluoride is typically heated. While phosphorus pentafluoride begins to form at approximately 60°C, it is preferable to gradually increase the temperature. Rapid heating can cause premature distillation of hydrogen fluoride, resulting in hydrolysis of hexafluorophosphate hydrate. Therefore, heating is preferably performed within a temperature range of 60°C to 200°C, more preferably within a temperature range of 60°C to 180°C, and particularly preferably within a temperature range of 60°C to 160°C. In the dehydration step, in the presence of a phosphorus component and sulfuric acid, the time from reaching 25°C to reaching the reaction temperature (maximum temperature during the reaction) is preferably 1 hour or more, more preferably 4 hours or more. Furthermore, in the presence of a phosphorus component and sulfuric acid, the time from reaching 25°C to reaching the reaction temperature (maximum temperature during the reaction) is preferably, for example, 36 hours or less in terms of shortening the reaction time, and more preferably 24 hours or less. These upper and lower limits are particularly preferred for batch-type reactions. In particular, when the reaction is carried out in a continuous system, the residence time of the reaction liquid containing the phosphorus component and sulfuric acid within the above temperature range is preferably 10 seconds or more, more preferably 30 seconds or more, and even more preferably 1 minute or more, from the viewpoint of production efficiency. It is also preferably 2 hours or less, more preferably 1 hour or less, and even more preferably 30 minutes or less. The range is preferably 10 seconds to 2 hours, more preferably 30 seconds to 1 hour, and even more preferably 1 minute to 30 minutes. If the amount of phosphorus pentafluoride generated after heating is small, the liquid may be further circulated by a pump.

[0041] When using a continuous process, the heat transfer area per reactor volume can be increased compared to a batch process, improving thermal efficiency. This allows the reactor to be smaller, significantly improving productivity per volume and time.

[0042] Since the generated phosphorus pentafluoride is prone to entrain hydrogen fluoride, it may be cooled using a condenser or the like, and the entrained hydrogen fluoride may be returned to the reactor. This not only makes it easier to prevent hydrolysis of hexafluorophosphate hydrate, but also eliminates the need for operations such as adding hydrogen fluoride to replace the amount of hydrogen fluoride that escapes from the reaction system along with phosphorus pentafluoride. This also has the advantage of reducing the contamination of hydrogen fluoride in the reaction between phosphorus pentafluoride and a fluoride salt in the above formula (1). When cooling using a condenser, the cooling temperature of the cooling water or the like is sufficient as long as it is above the boiling point of phosphorus pentafluoride (−84°C) and below the boiling point of hydrogen fluoride (20°C), but is preferably −80 to 20°C. A temperature of −40 to 10°C is more preferable, and a temperature of −20 to 0°C is even more preferable. When hydrogen fluoride is returned to the reaction vessel during the dehydration step, in order to exert the above-described effects of hydrogen fluoride, it is preferable that hydrogen fluoride remain in the liquid portion of the reaction vessel after the dehydration step so that the amount of hydrogen fluoride is 1% by mass or more, and more preferably 3% by mass or more, relative to sulfuric acid.

[0043] Even if a condenser or the like is not used, the generated phosphorus pentafluoride may be used directly in the next reaction. In this embodiment, hydrogen chloride, which is difficult to separate from phosphorus pentafluoride, is not produced as a by-product, so no special separation operation is required.

[0044] The heating method in the dehydration step is not particularly limited, and a general method can be used. For example, direct heating using a heater or indirect heating by heat exchange with a heat medium may be used.

[0045] The dehydration step is not particularly limited by the pressure at which it is carried out, but is preferably carried out under reduced pressure, since this effectively accelerates the rate of production of phosphorus pentafluoride. However, if the degree of pressure reduction is too high, hydrogen fluoride is likely to be distilled out of the system, so the pressure in the dehydration step is preferably -0.07 to 0.10 MPaG. More preferably, it is -0.05 to 0.05 MPaG, and most preferably, it is -0.03 to 0.01 MPaG. When the dehydration step is carried out under reduced pressure, the pressure is preferably -0.07 to -0.01 MPaG, and more preferably, it is -0.05 to -0.01 MPaG.

[0046] In this embodiment, it is preferable to include the following steps a) and b): a) a step of concentrating the sulfuric acid that has been subjected to the dehydration step, and b) a step of using the sulfuric acid concentrated in the step a) to dehydrate hexafluorophosphate hydrate, thereby reusing the sulfuric acid in the production of phosphorus pentafluoride.

[0047] In this embodiment, the following step c) may be included before step a). c) Step of Recovering Hydrogen Fluoride After the Dehydration Step Here, an example of a method for recovering hydrogen fluoride in step c) is a method in which hydrogen fluoride is distilled from the reaction system by heating in the presence of sulfuric acid during or after the dehydration step, and then recovered using a cooling trap or the like. When cooling using a trap or the like, the cooling temperature of the cooling water or the like is preferably, for example, −60 to 20° C., more preferably −40 to 10° C., and even more preferably −20 to 0° C. According to this step, hydrogen fluoride is recovered from the reaction vessel during or after the dehydration step, making it reusable, and also making it easier to obtain sulfuric acid of higher purity.

[0048] In step c), hydrolysis of unreacted hexafluorophosphoric acid, difluorophosphoric acid which is a hydrolyzate of hexafluorophosphoric acid, and further hydrolyzed monofluorophosphoric acid proceeds from about 100°C. From the viewpoint of promoting these hydrolysis and recovering high-purity hydrogen fluoride and sulfuric acid, it is preferable to heat the inside of the reaction vessel at 100°C or higher in step c). On the other hand, from the viewpoint of suppressing the progress of concentration of sulfuric acid and an increase in water content in the hydrogen fluoride distilled off, it is preferable to heat at 250°C or lower. Furthermore, distillation of hydrogen fluoride from the post-reaction liquid may be promoted by bubbling with an inert gas. Furthermore, to promote the removal of hydrogen fluoride, the reaction may be carried out under reduced pressure conditions. As shown in Example 1 described later, it is possible to make the bottom residue after step c) substantially free of impurities other than orthophosphoric acid, sulfuric acid, and water. If the impurity phosphorus fluoride component (HPF) is present in the reaction vessel after the dehydration step, ６ , H ２ P.O. ３ F, H.P.O. ２ F ２ , POF ３ ) and HSO ３Even if F is generated, these fluorinated phosphorus components and HSO ３ Since F is not highly volatile, heating the reaction solution in step c) causes hydrolysis. When heated, it is usually more likely to undergo hydrolysis before volatilization. As the hydrolysis proceeds, dehydrofluorination of these impurities progresses, liberating anhydrous hydrogen fluoride. For example, H ２ P.O. ３ F, H.P.O. ２ F ２ If so, the chemical formula for the hydrolysis is as follows: HPO ２ F ２ +2H ２ O→H ３ P.O. ４ +2H F H ２ P.O. ３ F+H ２ O → H ３ P.O. ４ +HF In this way, almost all of the fluorine components other than those released to the outside of the system as phosphorus pentafluoride can be recovered as anhydrous hydrogen fluoride. Furthermore, the purity of the sulfuric acid that has been subjected to step c) is high. Although there is no problem in recovering hydrogen fluoride during the dehydration step, it is preferable to recover hydrogen fluoride after the dehydration step, since this can prevent hydrogen fluoride from being discharged to the outside of the system before phosphorus pentafluoride is sufficiently generated in the dehydration step and can reduce the possibility of a hydrolyzate being produced.

[0049] The fluorine ion concentration in the reaction solution after step c) is preferably 1,000 ppm by mass or less, more preferably 100 ppm by mass or less, because recovery efficiency is high. The fluorine ion concentration can be measured by ion chromatography.

[0050] Regarding step a), the method for concentrating sulfuric acid is not particularly limited, and known methods can be used. For example, general concentration by vacuum reduced pressure concentration, electrolysis, or electrodialysis may be performed. For example, in the case of vacuum reduced pressure concentration, in order to successfully obtain sulfuric acid of the desired concentration, the absolute pressure in the reduced pressure reaction vessel is preferably in the range of 1 to 100 hPa, more preferably in the range of 1 to 20 hPa. Furthermore, the heating temperature in the step of concentrating sulfuric acid is preferably 150°C or higher, more preferably 200°C or higher. Furthermore, it is preferably 300°C or lower, more preferably 250°C or lower. The range is preferably 150 to 300°C, more preferably 200 to 250°C.

[0051] In step a), the concentration of sulfuric acid after concentration is preferably 80% by mass or more. If the concentration is this or higher, sufficient dehydration from hexafluorophosphate hydrate proceeds in the step of reusing the concentrated sulfuric acid in the production of phosphorus pentafluoride, and a decrease in the yield of phosphorus pentafluoride can be suppressed. It is more preferably 90% by mass or more, and particularly preferably 97% by mass or more. The upper limit of the sulfuric acid concentration after concentration is 100% by mass, and 99% by mass or less is preferred from the viewpoint of reducing the energy required for concentration.

[0052] The concentration of sulfuric acid before concentration is preferably 75% by mass or more, more preferably 90% by mass or more, from the viewpoint of successfully obtaining high-concentration sulfuric acid by the concentration in step a), and is preferably, for example, 98% by mass or less, from the viewpoint of ease of production.

[0053] In step a), when concentrating sulfuric acid, phosphoric acid, which is a raw material for hexafluorophosphoric acid hydrate, may be added and concentrated simultaneously. For example, by mixing 85% by mass phosphoric acid with the sulfuric acid obtained after step c) and concentrating them simultaneously, it is possible to simultaneously remove the 15% by mass water contained in the phosphoric acid and the water hydrated to sulfuric acid in the dehydration step, thereby obtaining a highly concentrated mixed solution of phosphoric acid and sulfuric acid with a low water content. Furthermore, as mentioned above, the lower the water content of phosphoric acid, the higher the viscosity, which reduces the stirring and heating efficiency and makes concentration more difficult. Therefore, mixing phosphoric acid and sulfuric acid and concentrating them is also preferable in this respect.

[0054] As described above, the obtained sulfuric acid can be subjected to the dehydration step again to be used for dehydrating new hexafluorophosphoric acid hydrate, thereby avoiding the generation of sulfuric acid waste.

[0055] Next, a method for producing a hexafluorophosphate salt according to this embodiment will be described. The method for producing a hexafluorophosphate salt according to this embodiment includes a step of reacting phosphorus pentafluoride produced by the above-described method with a fluoride salt in an organic solvent or a hydrogen fluoride solvent according to the following formula (1): PF ５ + MF → MPF ６ + ... (1) (In formula (1), M is Li, Na, K, Rb, Cs, Ag, and NH ４ It is one or more selected from the group consisting of

[0056] The organic solvent used in the step of producing the hexafluorophosphate salt is preferably a non-aqueous solvent such as an ether or a carbonate, specifically, dimethoxyethane, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, polyethylene glycol dimethyl ether, tetrahydrofuran, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, ethyl propyl carbonate, dipropyl carbonate, methyl butyl carbonate, ethyl butyl carbonate, dibutyl carbonate, methyl pentyl carbonate, ethyl ... Examples of suitable ethers and carbonates include ethyl carbonate, dipentyl carbonate, methylheptyl carbonate, ethylheptyl carbonate, diheptyl carbonate, methylhexyl carbonate, ethylhexyl carbonate, dihexyl carbonate, methyloctyl carbonate, ethyloctyl carbonate, dioctyl carbonate, methyltrifluoroethyl carbonate, ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, and 2,3-pentylene carbonate. Because the resulting reaction solution can be used as an electrolyte solution as is, dimethyl carbonate (DMC) and ethylene carbonate are more preferred. These ethers and carbonates may be used in combination of two or more.

[0057] The reaction temperature in the step of producing the hexafluorophosphate salt is preferably −20° C. to 100° C. If the temperature is −20° C. or higher, adverse effects on stirring due to an increase in the viscosity of the solvent can be prevented. If the temperature is 100° C. or lower, decomposition of the hexafluorophosphate salt and the solvent and progress of solvent evaporation can be suppressed. The reaction temperature is more preferably 0° C. to 50° C., and even more preferably 10° C. to 30° C.

[0058] The reaction time in the step of producing the hexafluorophosphate salt is not particularly limited because the reaction between phosphorus pentafluoride and the fluoride salt is very fast, and depends on the introduction rate of phosphorus pentafluoride.

[0059] When a hydrogen fluoride solvent such as anhydrous hydrogen fluoride solvent is used in the step of producing the hexafluorophosphate, the hexafluorophosphate is separated by concentration or crystallization. The concentration method is not particularly limited, and vacuum concentration or the like may be used. The crystallization temperature is not limited as long as it is not lower than −84°C, which is the melting point of hydrogen fluoride, and not higher than the boiling point of hydrogen fluoride (20°C), and is preferably −60 to −20°C.

[0060] When an organic solvent commonly used as an electrolyte solvent, such as dimethyl carbonate, is used in the process for producing the hexafluorophosphate salt, the organic solvent may be used as is as the electrolyte solution, or may be concentrated or diluted to adjust the concentration.

[0061] M in formula (1) includes Li, Na, K, Rb, Cs, Ag, and NH ４ It is preferable that the element be one selected from the group consisting of Li, Na, and K, because there is a high demand for the element as a battery material, a catalyst for organic synthesis, and the like, and the element has a high availability. In particular, it is preferable that the element be one selected from Li, Na, and K, and it is most preferable that the element be Li.

[0062] The present invention provides the following: [1] A method for producing phosphorus pentafluoride, comprising a dehydration step of dehydrating hexafluorophosphoric acid hydrate using sulfuric acid containing less than 5% by mass of sulfur trioxide.

[0063] [2] The method for producing phosphorus pentafluoride according to [1], wherein the dehydration step is carried out in the presence of hydrogen fluoride.

[0064] [3] The method for producing phosphorus pentafluoride according to [1] or [2], wherein the dehydration step is carried out under atmospheric pressure or under reduced pressure.

[0065] [4] The method for producing phosphorus pentafluoride according to any one of [1] to [3], wherein the dehydration step is carried out under a pressure of −0.07 to 0.10 MPaG.

[0066] [5] A method for producing phosphorus pentafluoride according to any one of [1] to [4], comprising the following steps a) and b): a) concentrating the sulfuric acid that has been subjected to the dehydration step, and b) using the sulfuric acid concentrated in step a) to dehydrate hexafluorophosphate hydrate, thereby reusing the sulfuric acid in the production of phosphorus pentafluoride.

[0067] [6] The method for producing phosphorus pentafluoride according to [5], wherein the dehydration step is carried out in the presence of hydrogen fluoride, and the method further comprises, before the step a), the following step c): c) a step of recovering the hydrogen fluoride that has been subjected to the dehydration step.

[0068] [7] The method for producing phosphorus pentafluoride according to [5] or [6], wherein the concentration of the sulfuric acid concentrated in the step a) is 80% by mass to 100% by mass.

[0069] [8] The hexafluorophosphate hydrate may be H ｘ P.O. ｙ F ｚ (wherein 0≦x≦3, 0≦y≦4, 0≦z≦6, and P is pentavalent), and hexafluorophosphoric acid hydrate obtained by reacting hydrogen fluoride with one or more phosphorus compounds selected from the group consisting of compounds represented by the formula (I) and (II),

[0070] A method for producing a hexafluorophosphate salt, comprising a step of reacting phosphorus pentafluoride produced by the method according to any one of [1] to [8] with a fluoride salt in an organic solvent or a hydrogen fluoride solvent according to the following formula (1): PF ５ + MF → MPF ６ ... (1) (In formula (1), M is Li, Na, K, Rb, Cs, Ag, and NH ４ It is one or more selected from the group consisting of

[0071] The present invention will be described in detail below with reference to examples and comparative examples, but the present invention is not limited thereto. Furthermore, unless otherwise specified, all of the examples and comparative examples were carried out under atmospheric pressure.

[0072] Example 1 First Step: Synthesis of Hexafluorophosphoric Acid Hydrate 70.1 g (3.50 mol) of anhydrous HF was introduced into a 500 mL perfluoroalkoxyalkane (PFA) vessel and cooled to −10° C. in an ice bath, and 20.0 g (0.17 mol, water: 0.17 mol) of 85% by mass phosphoric acid was added dropwise over 30 minutes. The mixture was heated to room temperature (25° C.) and stirred. １９ F-NMR and ３１HPF ６ The production of was confirmed (yield: 100%). １９ HF and HPF in F-NMR ６ Peak of, ３１ HPF for P-NMR ６ The hexafluorophosphate hydrate prepared in this step was 40.8 g (0.17 mol, water: 0.86 mol) of HPF ６ ・5H ２ Equivalent to O.

[0073] Second step: Synthesis of phosphorus pentafluoride and synthesis of lithium hexafluorophosphate HPF prepared in the same manner as in the first step ６ ・5H ２ 90.1 g of a HF solution of O (0.17 mol, water: 0.86 mol, HF: 2.48 mol) was cooled to -10 ° C. in an ice bath, and 221.1 g of 96% by mass sulfuric acid (2.18 mol, water: 0.44 mol) was added dropwise over 40 minutes. After completion of the addition, the temperature inside the reaction vessel was gradually raised to approximately 150 ° C. over 32 hours using an aluminum block heater. The HF distilled from the reaction system was returned to the reaction vessel as much as possible by cooling the condenser to -20 ° C. The generated phosphorus pentafluoride was collected as lithium hexafluorophosphate in a trap containing approximately 200 mL of a LiF / DMC solution containing 4.7 g (0.18 mol) of LiF. As a result, the amount of lithium hexafluorophosphate produced in the first 4 hours was 0.043 mol (yield 25%), and the production rate of lithium hexafluorophosphate at this time was 1.6 g / hour. Finally, 0.145 mol (yield 85%) of lithium hexafluorophosphate was obtained. The reaction temperature for producing lithium hexafluorophosphate was 25°C. The liquid composition in the reaction vessel was １９ F-NMR and ３１ Analysis by P-NMR and ICP-OES (Agilent Technologies, 5900 ICP-OES) showed that HPF ６ 0.1% by mass, H ２ P.O. ３ F0.2% by mass, HPO ２ F ２ 0.8% by mass, POF ３0.006% by mass and HSO ３ F 2.6 mass%, hydrogen fluoride 3.3 mass%, H ３ P.O. ４ The total weight of the aqueous solution was 0.01% by weight, sulfuric acid was 83.6% by weight, and the remainder was water.

[0074] Third step: Distilling off HF from the bottom of the reactor. The bottom of the reactor in the second step was distilled off with N ２ The mixture was heated for 12 hours at an internal temperature of 100°C using an aluminum block heater while bubbling (up to 2.0 L / min). The distilled HF was collected using a trap cooled to -20°C and a water trap. The F ion concentration of the bottoms was measured by ion chromatography (manufactured by Thermo Fisher Scientific Inc., IC-5000) and found to have been reduced to 40 mass ppm or less. The liquid composition in the reaction vessel after this step was １９ F-NMR and ３１ Analysis using P-NMR, ICP-OES, and Karl Fischer moisture analyzer (Nitto Seiko Analytech Co., Ltd., CA-310) revealed that HPF ６ , H ２ P.O. ３ F, H.P.O. ２ F ２ , POF ３ and HSO ３ The total of F is 0 mass%, hydrogen fluoride is 0 mass%, H ３ P.O. ４ The content was 1.2% by mass, sulfuric acid was 91% by mass, and the remainder was water.

[0075] Fourth step: sulfuric acid concentration of HF distillation bottom residue The 91% by mass sulfuric acid from which HF was distilled in the third step was transferred to a glass three-neck flask, the pressure was reduced to 10 hPa using a diaphragm pump, and the mixture was heated at an internal temperature of 250 ° C. for 5.5 hours using an aluminum block heater. The water concentration of the bottom residue was measured using a Karl Fischer moisture meter (manufactured by Nitto Seiko Analytech Co., Ltd., CA310), and was found to have been reduced to 2.0% by mass or less. The concentration of the concentrated sulfuric acid was 97% by mass.

[0076] Fifth step: Reuse of sulfuric acid after concentration 38.4 (1.92 mol) g of anhydrous HF was introduced into a 500 mL PFA vessel, cooled to -10 ° C. in an ice bath, and 11.1 g (0.11 mol, water: 0.09 mol) of 85% by mass phosphoric acid was added dropwise over 30 minutes. After heating to room temperature (25 ° C.) and stirring, 105.1 g (1.04 mol, water: 0.18 mol) of 97% by mass sulfuric acid concentrated in the fourth step was added dropwise over 40 minutes. After completion of the dropwise addition, the temperature inside the reaction vessel was gradually raised to about 150 ° C. over 31 hours using an aluminum block heater. The HF distilled from the reaction system was returned to the reaction vessel as much as possible by cooling the condenser to -20 ° C. The generated phosphorus pentafluoride was collected as lithium hexafluorophosphate in a trap containing approximately 200 mL of LiF / DMC solution containing 3.1 g (0.12 mol) of LiF. As a result, 0.025 mol (23% yield) of lithium hexafluorophosphate was generated in the first 4 hours, and the lithium hexafluorophosphate generation rate at this time was 0.94 g / hour. Lithium hexafluorophosphate was finally obtained in a 74% yield (reaction temperature: 25°C). Since sulfuric acid is reused, no sulfuric acid waste is generated.

[0077] [Example 2] 15.4 g of polyphosphoric acid (0.17 mol as phosphorus element, 80 mass % P) was placed in a 500 mL PFA container. ２ O ５ , manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was introduced, and while cooling to -10°C in an ice bath, 168.0 g (1.65 mol, water: 0.37 mol) of 96% by mass sulfuric acid was added dropwise over 10 minutes. Then, 79.0 g (3.95 mol) of anhydrous HF was added dropwise over 60 minutes. After heating to room temperature (25°C) and stirring, １９ F-NMR and ３１ HPF by P-NMR ６The formation of lithium hexafluorophosphate was confirmed (yield: 100%). The temperature inside the reaction vessel was gradually raised to approximately 133°C over 22 hours using an aluminum block heater. HF distilled off from the reaction system was returned to the reaction vessel as much as possible by cooling the condenser to -20°C. The generated phosphorus pentafluoride was collected as lithium hexafluorophosphate in a trap containing approximately 200 mL of a LiF / DMC solution containing 4.9 g (0.19 mol) of LiF (reaction temperature: 25°C). As a result, 0.071 mol (yield: 41%) of lithium hexafluorophosphate was formed in the first 4 hours, and the formation rate of lithium hexafluorophosphate at this time was 2.7 g / hour. Ultimately, 0.161 mol (yield: 93%) of lithium hexafluorophosphate was obtained.

[0078] Example 3: 128.3 g (1.11 mol, water: 1.07 mol) of 85% by weight phosphoric acid was introduced into a 1000 mL PFA container, and 1153.7 g (11.54 mol, water: 1.28 mol) of 98% by weight sulfuric acid was added dropwise over 10 minutes at room temperature. Next, while cooling to -10°C in an ice bath, 400.3 g (20.01 mol) of anhydrous HF was added dropwise over 120 minutes. The prepared solution was introduced at 45 mL / min (approximately 75 g / min) using a plunger pump into a 10 m ¼-inch PFA tube that had been preheated to 100°C in an oil bath. The preheated reaction solution was introduced into a 120 mL SiC (silicon carbide) multi-channel reactor (SMCR (registered trademark) manufactured by Kobelco Environmental Solutions Co., Ltd.) heated with oil at 200°C (residence time: 2.7 minutes). After heating, the mixture was separated into gas and liquid, and the liquid component was collected in a 1000 mL PFA container. HF in the gas component was collected in the same container by cooling the condenser to -60°C. The generated phosphorus pentafluoride was collected as lithium hexafluorophosphate in a trap containing approximately 500 mL of LiF / EMC solution containing 32.0 g (1.23 mol) of LiF (reaction temperature: 25°C). The collected liquid was circulated within the reactor for 3 hours, yielding 0.69 mol (62% yield) of lithium hexafluorophosphate. The lithium hexafluorophosphate production rate was 35 g / hour. After circulating the mixture in the apparatus for 10 hours, 1.02 mol (yield: 92%) of lithium hexafluorophosphate was finally obtained.

[0079] Example 4: 167.5 g (1.45 mol) of 85% by weight phosphoric acid (water: 1.40 mol) was introduced into a 1000 mL PFA vessel, and 816.3 g (8.16 mol, water: 0.91 mol) of 98% by weight sulfuric acid (water: 0.91 mol) was added dropwise over 10 minutes at room temperature. Next, while cooling to -10°C in an ice bath, 524.4 g (26.21 mol) of anhydrous HF was added dropwise over 120 minutes. The prepared solution was introduced into a 10 m ¼-inch PFA tube preheated to 100°C in an oil bath at 30 mL / min (approximately 56 g / min) using a plunger pump. The preheated reaction solution was introduced into a 120 mL SiC multi-channel reactor (Kobe Eco-Solutions Co., Ltd., SMCR (registered trademark)) heated with oil at 200°C (residence time: 4.0 min). After heating, the mixture was separated into gas and liquid, and the liquid component was collected in a 1000 mL PFA container. The HF in the gas component was collected in the same container by cooling the condenser to -60°C. The generated phosphorus pentafluoride was collected as lithium hexafluorophosphate in a trap containing approximately 500 mL of LiF / EMC solution containing 40.3 g (1.55 mol) of LiF (reaction temperature: 25°C). The collected liquid was circulated within the device for 3 hours, resulting in 0.57 mol (40% yield) of lithium hexafluorophosphate. The lithium hexafluorophosphate production rate at this time was 29 g / hour. After a final 11-hour circulation within the device, 1.24 mol (86% yield) of lithium hexafluorophosphate was obtained.

[0080] Example 5 First step: Concentration of a mixed solution of 85% by mass phosphoric acid and 98% by mass sulfuric acid 900.1 g (7.81 mol, water: 7.50 mol) of 85% by mass phosphoric acid and 3,949.7 g (39.46 mol, water: 4.39 mol) of 98% by mass sulfuric acid were transferred to a 5 L three-necked glass flask, the pressure was reduced to 6 hPa using a diaphragm pump, and the mixture was heated in an oil bath at an internal temperature of 200° C. for 7 hours. The water concentration of the residue was measured using a Karl Fischer moisture meter and found to have been reduced to 1% by mass or less.

[0081] Second Step: 6533 g of a mixed solution (containing 1167 g, 11.9 mol of phosphoric acid, 5287 g, 54.0 mol of sulfuric acid, and 79 g, 4.4 mol of water) prepared in the same manner as in the first step was introduced into a 5 L PFA vessel. Next, while cooling to -10°C in an ice bath, 3564 g (178.13 mol) of anhydrous HF was added dropwise over 120 minutes. The prepared solution was transferred to a 20 L PFA-lined reactor. The reactor was depressurized to -0.03 MPaG using a vacuum pump. This solution was introduced at 800 mL / min using a diaphragm pump into a 5 m ¼-inch PFA tube heated to 150°C in an oil bath. After heating, the mixture was separated into gas and liquid, and the liquid component was returned to the reactor. The HF in the gaseous components was liquefied by cooling the condenser to -60°C and then returned to the reactor along with the liquid components. The generated phosphorus pentafluoride passed through the condenser and was then collected as lithium hexafluorophosphate in a trap containing approximately 4 L of LiF / EMC solution containing 250 g (9.65 mol) of LiF (reaction temperature: 25°C). The liquid in the reactor was circulated between the heated PFA tube and the reactor for 3 hours, yielding 518 g (3.42 mol, 28.8% yield). The lithium hexafluorophosphate production rate at this time was 172.5 g / hour. Finally, after 19 hours of operation, 10.0 mol (84% yield) of lithium hexafluorophosphate was obtained.

[0082] Example 6 7145 g of a mixed solution (containing 1191 g, 12.2 mol of phosphoric acid, 5883 g, 60.0 mol of sulfuric acid, and 72 g, 4.0 mol of water) prepared in the same manner as in the first step of Example 5 was introduced into a 5 L PFA vessel. Next, while cooling to -10°C in an ice bath, 3666 g (182.23 mol) of anhydrous HF was added dropwise over 120 minutes. The prepared solution was transferred to a 20 L PFA-lined reactor. This solution was introduced at 800 mL / min using a diaphragm pump into a 2 m ¼-inch PFA tube heated to 150°C in an oil bath. After heating, the mixture was separated into gas and liquid, and the liquid component was returned to the reactor. The HF in the gas component was liquefied by cooling the condenser to -60°C, and then returned to the reactor along with the liquid component. The generated phosphorus pentafluoride passed through a condenser and was then collected as lithium hexafluorophosphate in a trap containing approximately 5 L of LiF / EMC solution containing 320 g (12.34 mol) of LiF (reaction temperature: 25°C). The liquid in the reactor was circulated between the heated PFA tube and the reactor for 3 hours, resulting in 130 g (0.86 mol, 7% yield) of lithium hexafluorophosphate. The lithium hexafluorophosphate production rate at this time was 43.6 g / hour. Finally, after 31 hours of operation, 9.6 mol (79% yield) of lithium hexafluorophosphate was obtained.

[0083] Comparative Example 1 74.0 g (3.70 mol) of anhydrous HF was placed in a 1000 mL PFA vessel and cooled to −10° C. in an ice bath, and 20.0 g (0.17 mol, water: 0.17 mol) of 85% by mass phosphoric acid was added dropwise over 60 minutes. After heating to room temperature (25° C.) and stirring, fuming sulfuric acid (60% by mass SO ３ , Fujifilm Wako Pure Chemical Industries, Ltd.) 81.2 g (SO ３ :0.61mol,H ２ SO ４A solution of 0.33 mol of fumed sulfuric acid (LiF) was added dropwise over 60 minutes. After the addition was completed, the temperature inside the reaction vessel was gradually raised to approximately 150°C over 8.5 hours using an aluminum block heater. The generated phosphorus pentafluoride was collected as lithium hexafluorophosphate in a trap containing approximately 200 mL of LiF / DMC solution containing 5.1 g of LiF. As a result, lithium hexafluorophosphate was obtained with a yield of 91%. The amount of phosphorus pentafluoride generated was equivalent to 19.9 g. The amount of sulfuric acid waste was 96.7 g of 95% by mass sulfuric acid, which included the amount of fuming sulfuric acid used, phosphoric acid, and water derived from the reaction. This means that 4.0 g of sulfuric acid was generated as waste per 1 g of phosphorus pentafluoride produced.

[0084] [Comparative Example 2] 70.1 g of anhydrous HF was introduced into a 1000 mL PFA vessel and cooled to -10 ° C. in an ice bath. 20.0 g (0.17 mol) of 85% by weight phosphoric acid (0.17 mol, water: 0.17 mol) was added dropwise over 60 minutes. After heating to room temperature (25 ° C.) and stirring, 88.4 g (0.87 mol) of fluorosulfonic acid was added dropwise over 60 minutes. After completion of the addition, the temperature inside the reaction vessel was gradually raised to approximately 150 ° C. over 7 hours using an aluminum block heater. The HF distilled from the reaction system was returned to the reaction vessel as much as possible by cooling the condenser to -20 ° C. The generated phosphorus pentafluoride was collected as lithium hexafluorophosphate in a trap containing approximately 200 mL of LiF / DMC solution containing 4.5 g (0.17 mol) of LiF. As a result, lithium hexafluorophosphate was obtained in a yield of 97%. The amount of phosphorus pentafluoride generated is equivalent to 21.2 g. The amount of sulfuric acid waste is 103.9 g of 100% sulfuric acid, which contains phosphoric acid and sulfuric acid generated from fluorosulfonic acid hydrolyzed by water from the reaction. This means that 4.9 g of sulfuric acid is generated as waste per 1 g of phosphorus pentafluoride produced.

[0085] As described above, the method for producing phosphorus pentafluoride of the present invention is not only economical and enables the recycling of hydrogen fluoride and sulfuric acid by using sulfuric acid to dehydrate hexafluorophosphate hydrate, but also a sustainable, clean method that reduces emissions of substances that have an environmental impact because it produces almost no sulfuric acid waste.

[0086] According to the present invention, not only can hydrogen chloride waste be reduced, but sulfuric acid waste can also be effectively reduced compared to conventional techniques, and phosphorus pentafluoride can be produced in a clean manner. The method of the present invention is also economical in that it uses inexpensive and relatively easy-to-handle materials. Furthermore, hexafluorophosphate can be produced industrially and advantageously using phosphorus pentafluoride obtained by this method.

Claims

1. A method for producing phosphorus pentafluoride, comprising a dehydration step of dehydrating hexafluorophosphoric acid hydrate using sulfuric acid having a sulfur trioxide content of less than 5% by mass.

2. The method for producing phosphorus pentafluoride according to claim 1, wherein the dehydration step is carried out in the presence of hydrogen fluoride.

3. The method for producing phosphorus pentafluoride according to claim 1, wherein the dehydration step is carried out under atmospheric pressure or reduced pressure.

4. The method for producing phosphorus pentafluoride according to claim 1, wherein the dehydration step is carried out under a pressure of -0.07 to 0.10 MPaG.

5. The method for producing phosphorus pentafluoride according to claim 1 or 2, comprising the following steps a) and b): a) A step of concentrating the sulfuric acid that has undergone the dehydration step. b) A step of reusing the sulfuric acid concentrated in step a) for dehydrating hexafluorophosphoric acid hydrate, thereby reusing the sulfuric acid in the production of phosphorus pentafluoride.

6. The method for producing phosphorus pentafluoride according to claim 5, wherein the dehydration step is carried out in the presence of hydrogen fluoride, and before step a), the following step c) is included: c) A step of recovering hydrogen fluoride that has undergone the dehydration step.

7. The method for producing phosphorus pentafluoride according to claim 5, wherein the concentration of the sulfuric acid concentrated in step a) is 80% to 100% by mass.

8. As the hexafluorophosphoric acid hydrate, H ｘ PO ｙ F ｚ (where 0 ≤ x ≤ 3, 0 ≤ y ≤ 4, 0 ≤ z ≤ 6, and P is pentavalent). The method for producing phosphorus pentafluoride according to claim 1 or 2, which uses a hexafluorophosphoric acid hydrate obtained by reacting one or more phosphorus compounds selected from the compounds represented by the formula with hydrogen fluoride.

9. A method for producing hexafluorophosphate, comprising a step of reacting phosphorus pentafluoride produced by the method according to claim 1 or 2 with a fluoride salt in an organic solvent or a hydrogen fluoride solvent according to the following formula (1). PF ５ + MF → MPF ６ ... (1) (In formula (1), M is one or more selected from the group consisting of Li, Na, K, Rb, Cs, Ag, and NH ４ .)