A method for synthesizing phosphorus pentafluoride using difluorophosphoric acid as a raw material

Abstract

The present invention belongs to the field of new energy and discloses a method for synthesizing phosphorus pentafluoride using difluorophosphoric acid as a raw material, which comprises the following steps: Step 1: Add difluorophosphoric acid into a reaction kettle and start stirring; Step 2: Introduce hydrogen fluoride gas into difluorophosphoric acid, and at the same time, dropwise add sulfur trioxide into difluorophosphoric acid, and collect the gas generated by the reaction; Condense the gas and collect phosphorus pentafluoride through a cold trap; The molar ratio of difluorophosphoric acid, sulfur trioxide, and hydrogen fluoride is 1:(3 - 6):(4 - 8), and the molar ratio of hydrogen fluoride to sulfur trioxide is less than or equal to 1.5. The method of the present invention uses difluorophosphoric acid as a raw material, sulfur trioxide as a water absorbent, and hydrogen fluoride as a fluorinating agent. The product is PF5 in gaseous form. After condensation for impurity removal and cold trap condensation, PF5 with a purity of not less than 99.7% can be obtained. The advantages of the present invention are: mild reaction conditions, PF5 can be directly obtained from difluorophosphoric acid, the process is simple, and the product purity is high.

Description

Technical Field

[0001] The present invention relates to the field of new energy, and particularly to a method for synthesizing phosphorus pentafluoride using difluorophosphoric acid as a raw material. Background Art

[0002] Phosphorus pentafluoride (PF5) is an important inorganic compound, which is widely used in the fields of electronics industry, organic synthesis, lithium battery electrolyte, etc.

[0003] In the prior art, phosphorus pentafluoride is usually prepared by reacting phosphorus pentachloride with hydrogen fluoride (PCl5 + 5HF → PF5 + 5HCl), but this method has problems such as harsh reaction conditions, low yield, and many by-products. In addition, direct reaction of elemental phosphorus (such as red phosphorus or yellow phosphorus) with fluorine gas to form phosphorus pentafluoride can obtain high-purity products, but the reaction is difficult to control and the production cost is relatively high. Reacting polyphosphoric acid with anhydrous hydrogen fluoride to form phosphorus pentafluoride has mild reaction conditions, but there are many by-products during the reaction process.

[0004] Therefore, developing an efficient, low-cost and environmentally friendly method for preparing phosphorus pentafluoride has important industrial significance. Summary of the Invention

[0005] The object of the present invention is to provide a method for synthesizing phosphorus pentafluoride using difluorophosphoric acid as a raw material. The method of the present invention uses difluorophosphoric acid as a raw material, sulfur trioxide as a water absorbent, and hydrogen fluoride as a fluorinating agent. The product is PF5 in gaseous form. After condensation and impurity removal and cold trap condensation, PF5 with a purity of not less than 99.7% can be obtained. The advantages of the present invention are: mild reaction conditions, PF5 can be directly obtained from difluorophosphoric acid, the process is simple, and the product purity is high.

[0006] The specific solution of the present invention is as follows:

[0007] A method for synthesizing phosphorus pentafluoride using difluorophosphoric acid as a raw material, the method comprising the following steps:

[0008] Step 1: Add difluorophosphoric acid to the reaction kettle and start stirring;

[0009] Step 2: Introduce hydrogen fluoride gas into difluorophosphoric acid, and at the same time, dropwise add sulfur trioxide to difluorophosphoric acid, control the reaction temperature at -10~0°C, and collect the gas generated by the reaction; condense the gas and collect phosphorus pentafluoride through a cold trap;

[0010] The molar ratio of the difluorophosphoric acid, sulfur trioxide, and hydrogen fluoride is 1:(3~6):(4~8), and the molar ratio of hydrogen fluoride and sulfur trioxide is less than or equal to 1.5.

[0011] The reaction mechanism of the present invention is:

[0012] The reaction between difluorophosphoric acid and phosphorus oxyfluoride is a reversible reaction, and its chemical reaction equation is as follows:

[0013] ......Equation 1;

[0014] Although the reaction of Equation 1 above is a reversible reaction, the reaction from left to right is relatively easy, while the reaction from right to left is relatively difficult; in order to promote the above reaction from right to left, this invention considers using a water absorbent to absorb the water in the reaction process, thereby promoting the reaction to proceed to the left. After verification, it is found that in this reaction, conventional water absorbents, such as chlorine gas, have poor water absorption capacity, and ultimately the conversion rate of PF5 is very low; for example, fuming sulfuric acid has a certain effect, but the final yield is still not high; concentrated sulfuric acid also has the problem of low yield of PF5; and sulfur trioxide as a water absorbent has the best effect. However, simply using a water absorbent is not enough to limit the reaction of phosphorus oxyfluoride with water. To solve this problem, this application continuously introduces hydrogen fluoride and limits the amount of sulfur trioxide used, and at the same time controls the reaction temperature at -10~0°C, so that the phosphorus oxyfluoride generated by the leftward reaction of Equation 1 preferentially reacts with hydrogen fluoride to generate phosphorus pentafluoride and water. The specific reaction equation can refer to Equation 2:

[0015] POF3 + 2HF → PF5 + H2O......Equation 2.

[0016] Specifically, as can be seen from Equation 1 and Equation 2, for every 1 mole of phosphorus pentafluoride generated, 2 moles of water will be produced and 3 moles of hydrogen fluoride will be consumed. Therefore, control the molar amount of hydrogen fluoride not to be higher than 1.5 times the molar amount of sulfur trioxide to ensure that the generated water can be absorbed by sulfur trioxide as much as possible; through the above control of the amounts of sulfur trioxide and hydrogen fluoride, the leftward reaction of Equation 1 can be promoted. However, during this process, a chemical reaction as shown in Equation 3 will also occur:

[0017] POF3 + 3HF + H2O → HPF6·2H2O......Equation 3;

[0018] That is to say, if the molar amount of hydrogen fluoride exceeds 1.5 times the molar amount of sulfur trioxide, the possibility of the reaction shown in Equation 3 will increase. The reason is that after the molar amount of hydrogen fluoride exceeds 1.5 times the molar amount of sulfur trioxide, more hydrogen fluoride will react with difluorophosphoric acid and phosphorus oxyfluoride per unit time to generate more water. If the amount of sulfur trioxide used is too small, the generated phosphorus oxyfluoride will react with 3 moles of hydrogen fluoride and water to generate hexafluorophosphoric acid, reducing the direct production yield of phosphorus pentafluoride. Therefore, in order to obtain more direct production yield of phosphorus pentafluoride, it is very important to control the ratio of reactants. The inventor found that when the molar ratio of difluorophosphoric acid, sulfur trioxide, and hydrogen fluoride is 1:3~6:4~8, and the molar ratio of hydrogen fluoride to sulfur trioxide is less than 1.5, and at the same time control the reaction temperature at -10~0°C, more direct production yield of phosphorus pentafluoride can be obtained.

[0019] In some preferred embodiments of the present invention, the molar ratio of hydrogen fluoride to sulfur trioxide can be selected as 1.5, 1.4, 1.35, 1.3, 1.2, 1.1 or 1.0.

[0020] In the above method for synthesizing phosphorus pentafluoride using difluorophosphoric acid as a raw material, the addition time of sulfur trioxide and hydrogen fluoride lasts for 5 to 10 hours.

[0021] In some preferred embodiments of the present invention, the addition time of sulfur trioxide and hydrogen fluoride lasts for 5 hours, 6 hours, 7 hours, 8 hours, 9 hours or 10 hours.

[0022] In the above method for synthesizing phosphorus pentafluoride using difluorophosphoric acid as a raw material, the molar ratio of difluorophosphoric acid, sulfur trioxide, and hydrogen fluoride is 1:4.5 - 6:6 - 8, and the molar ratio of hydrogen fluoride to sulfur trioxide is 1 - 1.4, particularly preferably 1.25 - 1.33.

[0023] In the above method for synthesizing phosphorus pentafluoride using difluorophosphoric acid as a raw material, both sulfur trioxide and hydrogen fluoride are added to difluorophosphoric acid at a uniform speed.

[0024] Due to the existence of the reaction of formula 3, in the substrate of the final reaction kettle, sulfuric acid, sulfur trioxide, hexafluorophosphoric acid, and unreacted difluorophosphoric acid may exist. To further improve the yield of phosphorus pentafluoride, in the above method for synthesizing phosphorus pentafluoride using difluorophosphoric acid as a raw material, after the reaction in step 2, step 3 is further included:

[0025] The remaining reactants in the reaction kettle are heated to 120 - 140 °C to pyrolyze the hexafluorophosphoric acid in the reactants. The tail gas obtained by pyrolysis is condensed and phosphorus pentafluoride is collected through a cold trap. The pyrolysis of hexafluorophosphoric acid to produce phosphorus pentafluoride is a conventional technique in the art. In some embodiments, it is still feasible to lower the heating temperature to 100 °C. In practical applications, the heating temperature can be flexibly controlled according to the amount of the residue.

[0026] In the above method for synthesizing phosphorus pentafluoride using difluorophosphoric acid as a raw material, the top of the reaction kettle is connected to an exhaust pipe, and the exhaust pipe is sequentially connected to one or more condensers, a cold trap, and a tail gas absorption device.

[0027] In the above method for synthesizing phosphorus pentafluoride using difluorophosphoric acid as a raw material, the upper part of the reaction kettle is connected to a vertical shell and tube condenser, and the exhaust pipe is connected to the gas outlet position of the vertical shell and tube condenser; a pump for generating negative pressure and used to suck out the gas in the reaction kettle is connected to the exhaust pipe.

[0028] In actual production, it is also possible not to set up a pump. During the reaction process, due to the introduction of HF and the generation of phosphorus pentafluoride, pressure will be generated inside the reaction kettle. In this case, a reaction kettle that can withstand pressure should be selected. Within the optional pressure range, the reaction pressure can be further maintained at 1 to 5 standard atmospheres; by controlling the controllable release of the gas in the reaction kettle to maintain the pressure inside the reaction kettle. In the small-scale experiment, we further found that the reaction vessel is under pressure, and as the pressure increases, the reaction rate can be further increased, the reaction time can be shortened, and there is a slight contribution to the yield. Therefore, in the subsequent research process, the above process can be further optimized to improve production efficiency.

[0029] The beneficial effects of this application are:

[0030] The present invention uses difluorophosphoric acid as a raw material, sulfur trioxide as a water absorbent, and hydrogen fluoride as a fluorinating agent. The product is PF5 in gas form. After condensation and impurity removal and cold trap condensation, PF5 with a purity of 99.8% can be obtained. The advantages of the present invention are: mild reaction conditions, PF5 can be directly obtained from difluorophosphoric acid, the process is simple, and the product purity is high. Description of the Drawings

[0031] Figure 1 It is the equipment flow chart of the present invention. Detailed Embodiments

[0032] Next, the embodiments of the present invention will be combined to clearly and completely describe the present invention. In the description of the present invention, it should be noted that for those conditions not specified in the embodiments, they are carried out according to conventional conditions or the conditions recommended by the manufacturer. For reagents or instruments whose manufacturers are not specified, they are all conventional products that can be obtained through commercial purchase.

[0033] Before elaborating on the method of the present invention, the production system of the present invention will be introduced first;

[0034] Refer to Figure 1 , the production system of the present invention includes a reaction kettle 1, a vertical shell-and-tube condenser 2, a vacuum pump 3, a horizontal condenser 4, a cold trap 5, and a tail gas absorption device 6; the upper part of the reaction kettle 1 is connected to the vertical shell-and-tube condenser 2, and the vertical shell-and-tube condenser 2, the vacuum pump 3, the horizontal condenser 4, the cold trap 5, and the tail gas absorption device 6 are connected in sequence. The phosphorus pentafluoride generated by the reaction kettle 1 is condensed in sequence through the vertical shell-and-tube condenser 2, the horizontal condenser 4, and the cold trap 5, and is recovered in the cold trap 5; the condensation temperature of the vertical shell-and-tube condenser 2 is -10 to -20 °C; the condensation temperature of the horizontal condenser 4 is -20 to -30 °C; the condensation temperature of the cold trap 5 is -96 to -100 °C. The tail gas absorption device 6 is a tail gas absorption device 6 with water as the absorption medium.

[0035] The reactor 1 is externally provided with a temperature control jacket; a dropping pipe 7 for dropping sulfur trioxide is provided on the reactor 1; in addition, a pipe 8 for inputting hydrogen fluoride gas, which extends to the lower part of the reactor 1, is also provided on the reactor 1.

[0036] The following examples and comparative examples are all implemented with this system.

[0037] Example 1

[0038] A method for synthesizing phosphorus pentafluoride from difluorophosphoric acid as a raw material, the method comprising the following steps:

[0039] Step 1: Add 1 mol of difluorophosphoric acid to the reactor, start stirring, and the stirring speed is 60 rpm;

[0040] Step 2: Synchronously drop sulfur trioxide and inject hydrogen fluoride gas into the reactor, control the reaction temperature at -5 ± 2 °C, and the reaction time is 5 h;

[0041] During the whole reaction process, the addition of sulfur trioxide and hydrogen fluoride is uniform;

[0042] The molar ratio of difluorophosphoric acid, sulfur trioxide, and HF is 1:4:5; the molar ratio of HF to sulfur trioxide is 1.25;

[0043] During the reaction process, the gas generated by the reactor is condensed multiple times, phosphorus pentafluoride is collected, and the amount of phosphorus pentafluoride obtained in this step is counted;

[0044] Step 3: After the reaction is completed, heat the material at the bottom of the reactor to pyrolyze the residual hexafluorophosphoric acid; the heating temperature is 130 °C; the gas generated by the reactor is condensed multiple times, phosphorus pentafluoride is collected, and the amount of phosphorus pentafluoride obtained in this step is counted.

[0045] Example 2

[0046] It is generally the same as Example 1, the difference is that the molar ratio of difluorophosphoric acid, sulfur trioxide, and HF is 1:3:4.5; the molar ratio of HF to sulfur trioxide is 1.5.

[0047] Example 3

[0048] It is generally the same as Example 1, the difference is that the molar ratio of difluorophosphoric acid, sulfur trioxide, and HF is 1:6:8; the molar ratio of HF to sulfur trioxide is 1.33.

[0049] Example 4

[0050] It is generally the same as Example 1, the difference is that the molar ratio of difluorophosphoric acid, sulfur trioxide, and HF is 1:4.5:6; the molar ratio of HF to sulfur trioxide is 1.33.

[0051] Example 5

[0052] Basically the same as Example 1, except that the molar ratio of difluorophosphoric acid, sulfur trioxide, and HF is 1:5:7; the molar ratio of HF to sulfur trioxide is 1.4.

[0053] Example 6

[0054] Basically the same as Example 1, except that the reaction time is 8 h and the reaction temperature is -8 ± 2 °C.

[0055] Example 7

[0056] Basically the same as Example 1, except that the reaction time is 10 h and the reaction temperature is -2 ± 2 °C.

[0057] Comparative Example 1

[0058] Basically the same as Example 1, except that the molar ratio of difluorophosphoric acid, sulfur trioxide, and HF is 1:3:8; the molar ratio of HF to sulfur trioxide is 2.7.

[0059] Comparative Example 2

[0060] Basically the same as Example 1, except that the molar ratio of difluorophosphoric acid, sulfur trioxide, and HF is 1:6:10; the molar ratio of HF to sulfur trioxide is 1.7.

[0061] Comparative Example 3

[0062] Basically the same as Example 1, except that the molar ratio of difluorophosphoric acid, sulfur trioxide, and HF is 1:8:8; the molar ratio of HF to sulfur trioxide is 1.

[0063] Comparative Example 4

[0064] Basically the same as Example 1, except that the molar ratio of difluorophosphoric acid, sulfur trioxide, and HF is 1:6:3; the molar ratio of HF to sulfur trioxide is 0.5.

[0065] Comparative Example 5

[0066] Basically the same as Example 1, except that the molar ratio of difluorophosphoric acid, sulfur trioxide, and HF is 1:2:5; the molar ratio of HF to sulfur trioxide is 2.5.

[0067] Comparative Example 6

[0068] Basically the same as Example 1, except that the reaction temperature is controlled at 18 ± 2 °C.

[0069] Comparative Example 7

[0070] Basically the same as Example 1, except that the reaction temperature is controlled at -25 ± 2 °C.

[0071] Comparative Example 8

[0072] It is generally the same as Example 1, except that an equimolar amount of chlorine gas is used to replace sulfur trioxide.

[0073] Comparative Example 9

[0074] It is generally the same as Example 1, except that concentrated sulfuric acid (concentration ≥ 70 wt%) is used to replace sulfur trioxide, and the molar amount of concentrated sulfuric acid is the same as that of sulfur trioxide.

[0075] Performance detection

[0076] The purity detection method of phosphorus pentafluoride is as follows:

[0077] Infrared spectroscopy

[0078] Principle: Phosphorus pentafluoride molecules have a specific infrared absorption spectrum. By measuring its infrared spectrum and comparing it with the standard spectrum, the purity can be qualitatively judged.

[0079] Operating steps:

[0080] Make the phosphorus pentafluoride sample into a thin slice or a gas cell sample.

[0081] Use an infrared spectrometer to scan the sample and record its infrared spectrum.

[0082] Compare the measured spectrum with the infrared spectrum of standard phosphorus pentafluoride, analyze the position and intensity of its absorption peaks, and judge whether there are impurity absorption peaks, so as to evaluate the purity.

[0083] Precautions: Pay attention to avoiding contamination or decomposition of the sample during sample preparation, and ensure the normal operation of components such as the light source and detector of the spectrometer to obtain accurate spectral data.

[0084] The yield of phosphorus pentafluoride can be seen in Table 1 below;

[0085] Table 1 Purity and yield table

[0086] Yield % of Step 2 Yield % of Step 3 Total Yield % Example 1 65.3 25.0 90.3 Example 2 61.1 27.4 88.5 Example 3 72.4 21.3 93.7 Example 4 69.2 23.3 92.5 Example 5 67.5 25.8 93.3 Example 6 65.8 25.3 91.1 Example 7 66.1 25.4 91.5 Comparative Example 1 47.3 40.4 87.7 Comparative Example 2 43.2 47.9 91.1 Comparative Example 3 66.2 25.5 91.7 Comparative Example 4 58.7 22.1 80.8 Comparative Example 5 21.4 21.2 42.6 Comparative Example 6 63.8 22.6 86.4 Comparative Example 7 43.9 12.4 56.3 Comparative Example 8 8.2 8.6 16.8 Comparative Example 9 9.4 13.2 22.6

[0087] The purity of phosphorus pentafluoride in the above-mentioned examples and comparative examples all reaches over 99.7%.

[0088] Result analysis:

[0089] 1. It can be seen from Examples 1 to 5 that by controlling the amounts and ratios of sulfur trioxide and hydrogen fluoride, the overall yield can reach over 88%. Among them, in Examples 3 to 4, not only is the overall yield high, but the yield of Step 3 is relatively low. This shows that by further controlling the molar ratio of hydrogen fluoride to sulfur trioxide within 1.4, preferably within 1.33, and increasing the amount of hydrogen fluoride as much as possible, the direct yield of phosphorus pentafluoride in Step 2 can be increased; the whole process is stable and controllable, without the need to significantly increase the amount of chilled brine additionally.

[0090] 2. It can be seen from Examples 6 to 7 that the extension of the reaction time and the reasonable adjustment of the temperature only slightly increase the yield, and the overall impact on the system is not obvious.

[0091] 3. It can be seen from Comparative Examples 1 to 5 that under different ratios, completely different results will be shown. Among them, although the molar amount used in Comparative Example 1 is within the optional range of the present invention, the molar amount of HF is more than twice the molar amount of sulfur trioxide. This will cause the water generated by Formula 1 and Formula 2 during the reaction not to be immediately and fully absorbed by sulfur trioxide, and then combine with phosphorus oxyfluoride to form hexafluorophosphoric acid, ultimately resulting in a relatively high yield of phosphorus pentafluoride in Step 3.

[0092] The situation of Comparative Example 2 is similar to that of Comparative Example 1. It further increases the amount of hydrogen fluoride and at the same time increases the amount of sulfur trioxide. This will cause a relatively large amount of water to be generated per unit time. At this time, not only will the amount of hexafluorophosphoric acid in Step 3 increase, but also the difficulty of temperature control will increase.

[0093] In Comparative Example 3, the amount of hydrogen fluoride used is relatively large and the increase in sulfur trioxide leads to obvious heat release, increasing the difficulty of reaction temperature control; it can be seen from the comparison between Comparative Example 3 and Example 3 that although the ratio of sulfur trioxide in Comparative Example 3 increases and the amount of hydrogen fluoride is the same as that in Example 3, the overly active reaction system has a negative effect on the product yield, and its yield is still 2% lower than that of Example 3; the excessive increase in sulfur trioxide not only leads to a decrease in yield but also a decrease in temperature controllability. Therefore, in actual production, it is not recommended that the amount of sulfur trioxide exceed 6 times the amount of difluorophosphoric acid.

[0094] In Comparative Example 4, the amount of hydrogen fluoride used is too small, resulting in a relatively obvious decrease in the yield compared to Example 2.

[0095] The amount of sulfur trioxide in Comparative Example 5 was too small, resulting in an increase in the yield of hexafluorophosphoric acid in Step 3 and the consumption of too much hydrogen fluoride in the production of hexafluorophosphoric acid, leading to a serious reduction in the overall yield. In Comparative Examples 6 and 7, by adjusting the reaction temperature, the reaction temperature was increased in Comparative Example 6, and its reaction controllability became relatively poor. The reaction temperature was decreased in Comparative Example 7, and its yield decreased. It can be seen that when the temperature exceeds the recommended range, either the reaction is difficult to control or the yield is too low.

[0096] As can be seen from Comparative Examples 8 and 9, when other dehydrating agents are used, most of the difluorophosphoric acid cannot achieve equilibrium conversion.

[0097] In summary, the reaction conditions of the present invention are mild, PF5 can be directly produced at least partially from difluorophosphoric acid, the process is simple, and the product purity is high.

Claims

1. A method for synthesizing phosphorus pentafluoride using difluorophosphoric acid as a raw material, characterized in that, The method includes the following steps: Step 1: Add difluorophosphoric acid into the reaction kettle and start stirring; Step 2: Introduce hydrogen fluoride gas into difluorophosphoric acid, and at the same time, drip sulfur trioxide into difluorophosphoric acid. Control the reaction temperature at -10~0°C, and collect the gas generated by the reaction; after condensing the gas, collect phosphorus pentafluoride through a cold trap; The molar ratio of the difluorophosphoric acid, sulfur trioxide, and hydrogen fluoride is 1:(3~6):(4~8), and the molar ratio of hydrogen fluoride and sulfur trioxide is less than or equal to 1.

5.

2. The method according to claim 1, wherein The addition time of the sulfur trioxide and hydrogen fluoride lasts for 5~10 h.

3. The method according to claim 2, characterized in that, The molar ratio of the difluorophosphoric acid, sulfur trioxide, and hydrogen fluoride is 1:4.5~6:6~8, and the molar ratio of hydrogen fluoride and sulfur trioxide is 1~1.

4.

4. The method according to claim 1, wherein Both the sulfur trioxide and hydrogen fluoride are added into the difluorophosphoric acid at a uniform speed.

5. The method according to claim 1, characterized in that After the reaction in Step 2, it further includes Step 3: Heat the remaining reactants in the reaction kettle to 120~140°C to pyrolyze hexafluorophosphoric acid in the reactants, and after condensing the tail gas generated by the pyrolysis, collect phosphorus pentafluoride through a cold trap.

6. The method according to claim 1, characterized in that, The top of the reaction kettle is connected with an exhaust pipe, and the exhaust pipe is successively connected with one or more stages of condensers, a cold trap, and a tail gas absorption device.

7. The method according to claim 6, wherein The upper part of the reaction kettle is connected with a vertical shell-and-tube condenser, and the exhaust pipe is connected to the gas outlet position of the vertical shell-and-tube condenser; a pump for generating negative pressure and used to suck out the gas in the reaction kettle is connected to the exhaust pipe.

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