Method for synthesizing phosphorus pentafluoride by taking difluorophosphoric acid as raw material

Abstract

The invention belongs to the field of new energy, and discloses a method for synthesizing phosphorus pentafluoride by taking difluorophosphoric acid as a raw material, which comprises the following steps: step 1, adding difluorophosphoric acid into a reaction kettle, and starting stirring; 2, introducing hydrogen fluoride gas into the difluorophosphoric acid, dropwise adding sulfur trioxide into the difluorophosphoric acid at the same time, and collecting gas generated by reaction; condensing the gas, and collecting phosphorus pentafluoride through a cold trap; the molar ratio of difluorophosphoric acid to sulfur trioxide to 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. According to the method, difluorophosphoric acid is adopted as a raw material, sulfur trioxide is adopted as a water absorbent, hydrogen fluoride is adopted as a fluorinating agent, the product is PF5 in the form of gas, and after condensation impurity removal and cold trap condensation, PF5 with the purity not lower than 99.7% can be obtained, and the method has the advantages that reaction conditions are mild, 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 (PF 5 ) is an important inorganic compound and is widely used in the fields of electronics industry, organic synthesis, lithium battery electrolytes, etc.

[0003] In the prior art, phosphorus pentafluoride is usually prepared by reacting phosphorus pentachloride with hydrogen fluoride (PCl 5 + 5HF → PF 5 + 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 produce phosphorus pentafluoride can obtain high-purity products, but the reaction is difficult to control and the production cost is high. Reacting polyphosphoric acid with anhydrous hydrogen fluoride to produce 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 PF in gaseous form 5 , and after condensation for impurity removal and cold trap condensation, PF with a purity not lower than 99.7% can be obtained 5 . The advantages of the present invention are: the reaction conditions are mild, PF can be directly obtained from difluorophosphoric acid 5 , the process is simple, and the product purity is high.

[0006] The specific solution of the present invention is as follows: A method for synthesizing phosphorus pentafluoride using difluorophosphoric acid as a raw material, the method comprising the following steps: Step 1: Add difluorophosphoric acid to 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; condense the gas and 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.

[0007] The reaction mechanism of the present invention is: The reaction between difluorophosphoric acid and phosphorus oxyfluoride is a reversible reaction, and its chemical reaction equation is as follows: ......Equation 1; 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 to proceed from right to left, the present 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 finally the conversion rate of PF 5 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 PF 5 ; 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, the present application continuously introduces hydrogen fluoride and limits the amount of sulfur trioxide used, while controlling 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: POF 3 +2HF→PF 5 +H 2 O......Equation 2.

[0008] Specifically, it can be seen from Equation 1 and Equation 2 that 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, the molar amount of hydrogen fluoride is controlled 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; by controlling the amounts of sulfur trioxide and hydrogen fluoride as described above, the leftward progress of Equation 1 can be promoted. However, during this process, a chemical reaction as shown in Equation 3 will also occur: POF 3 +3HF+H 2 O→HPF 6 .2H 2 O......Equation 3; That is, if the molar amount of hydrogen fluoride exceeds 1.5 times the molar amount of sulfur trioxide, the possibility of the reaction shown in Formula 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 oxytrifluoride per unit time to produce more water. If the amount of sulfur trioxide used is too small, the phosphorus oxytrifluoride generated will react with 3 moles of hydrogen fluoride and water to form hexafluorophosphoric acid, reducing the direct production yield of phosphorus pentafluoride. Therefore, in order to obtain a higher direct production yield of phosphorus pentafluoride, it is very important to control the ratio of reactants. The inventors 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 the reaction temperature is controlled at -10 to 0 °C, a higher direct production yield of phosphorus pentafluoride can be obtained.

[0009] 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.

[0010] 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 5-10 h.

[0011] In some preferred embodiments of the present invention, the addition time of sulfur trioxide and hydrogen fluoride lasts for 5 h, 6 h, 7 h, 8 h, 9 h, or 10 h.

[0012] 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.

[0013] In the above method for synthesizing phosphorus pentafluoride using difluorophosphoric acid as a raw material, sulfur trioxide and hydrogen fluoride are both added to difluorophosphoric acid at a uniform speed.

[0014] 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. In order 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: 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, the heating temperature can be reduced to 100 °C and it is still feasible. In actual applications, the heating temperature can be flexibly controlled according to the amount of residues.

[0015] In the above method for synthesizing phosphorus pentafluoride using difluorophosphoric acid as a raw material, an exhaust pipe is connected to the top of the reaction kettle, and the exhaust pipe is successively connected to one or more condensers, a cold trap, and a tail gas absorption device.

[0016] In the above method for synthesizing phosphorus pentafluoride using difluorophosphoric acid as a raw material, a vertical shell-and-tube condenser is connected to the upper part of the reaction kettle, 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 sucking out the gas in the reaction kettle is connected to the exhaust pipe.

[0017] In actual production, the pump can also not be set. 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, and within the optional pressure range, the reaction pressure can be further maintained at 1 to 5 standard atmospheric pressures; by controlling the controllable release of the reaction kettle gas to maintain the internal pressure of 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.

[0018] The beneficial effects of this application are: 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 PF in gaseous form. 5 After condensation for impurity removal and cold trap condensation, PF with a purity of 99.8% can be obtained. 5 The advantages of the present invention are: the reaction conditions are mild, PF can be directly obtained from difluorophosphoric acid. 5 The process is simple and the product purity is high. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a process flow diagram of the equipment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0020] The following will combine the embodiments of the present invention to clearly and completely describe the present invention. In the description of the present invention, it should be noted that for those not specified in the embodiments, they are carried out according to the conventional conditions or the conditions recommended by the manufacturer. For the reagents or instruments not specified in the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0021] Before elaborating on the method of the present invention, the production system of the present invention will be introduced first; Refer to Figure 1, the production system of the present invention includes a reaction kettle 1, a vertical shell-and-tube condenser 2, an air extraction 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 air extraction 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 then 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.

[0022] A temperature control jacket is provided outside the reaction kettle 1; a dropping pipe 7 for dropping sulfur trioxide is provided on the reaction kettle 1; in addition, a pipe 8 for inputting hydrogen fluoride gas, which extends to the lower part of the reaction kettle 1, is also provided on the reaction kettle 1.

[0023] The following examples and comparative examples are all implemented with this system.

[0024] Example 1 A method for synthesizing phosphorus pentafluoride from difluorophosphoric acid as a raw material, the method comprising the following steps: Step 1: Add 1 mol of difluorophosphoric acid to the reaction kettle, start stirring, and the stirring speed is 60 rpm; Step 2: Synchronously drop sulfur trioxide and inject hydrogen fluoride gas into the reaction kettle, control the reaction temperature to be -5 ± 2 °C, and the reaction time to be 5 h; During the whole reaction process, the addition of sulfur trioxide and hydrogen fluoride is uniform; The molar ratio of difluorophosphoric acid, sulfur trioxide, and HF is 1:4:5; the molar ratio of HF and sulfur trioxide is 1.25; During the reaction process, the gas generated by the reaction kettle is condensed multiple times, and phosphorus pentafluoride is collected, and the amount of phosphorus pentafluoride obtained in this step is counted; Step 3: After the reaction is completed, heat the material at the bottom of the reaction kettle to pyrolyze the residual hexafluorophosphoric acid; the heating temperature is 130 °C; the gas generated by the reaction kettle is condensed multiple times, and phosphorus pentafluoride is collected, and the amount of phosphorus pentafluoride obtained in this step is counted.

[0025] Example 2 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 and sulfur trioxide is 1.5.

[0026] Example 3 Basically the same as Example 1, except 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.

[0027] Example 4 Basically the same as Example 1, except 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.

[0028] Example 5 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.

[0029] Example 6 Basically the same as Example 1, except that the reaction time is 8 h and the reaction temperature is -8 ± 2°C.

[0030] Example 7 Basically the same as Example 1, except that the reaction time is 10 h and the reaction temperature is -2 ± 2°C.

[0031] Comparative Example 1 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.

[0032] Comparative Example 2 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.

[0033] Comparative Example 3 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.

[0034] Comparative Example 4 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.

[0035] Comparative Example 5 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.

[0036] Comparative Example 6 Basically the same as Example 1, except that the reaction temperature is controlled at 18 ± 2°C.

[0037] Comparative Example 7 The process is substantially the same as in Example 1, except that the reaction temperature is controlled at -25±2°C.

[0038] Comparative Example 8 The method is substantially the same as Example 1, except that an equal molar amount of chlorine is used to replace sulfur trioxide.

[0039] Comparative Example 9 The method is substantially the same as Example 1, except that concentrated sulfuric acid (concentration ≥ 70 wt %) is used instead of sulfur trioxide, and the molar amount of the concentrated sulfuric acid is the same as the molar amount of sulfur trioxide.

[0040] Performance Testing The purity test method of phosphorus pentafluoride is: Infrared spectroscopy Principle: Phosphorus pentafluoride molecules have a specific infrared absorption spectrum. By measuring its infrared spectrum and comparing it with the standard spectrum, its purity can be qualitatively determined.

[0041] Steps: Phosphorus pentafluoride samples are prepared as thin sheets or gas cell samples.

[0042] Use an infrared spectrometer to scan the sample and record its infrared spectrum.

[0043] The measured spectrum is compared with the infrared spectrum of standard phosphorus pentafluoride, and the position and intensity of its absorption peak are analyzed to determine whether there is an impurity absorption peak, thereby evaluating the purity.

[0044] Note: When preparing samples, be careful to avoid sample contamination or decomposition, and ensure that the light source, detector and other components of the spectrometer work properly to obtain accurate spectral data.

[0045] The yield of phosphorus pentafluoride can be seen in Table 1 below; Table 1 Purity and yield 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

[0046] The purity of phosphorus pentafluoride in the above-mentioned embodiments and comparative examples all reached above 99.7%.

[0047] Result analysis: 1. It can be seen from Examples 1 to 5 that by controlling the amount and ratio of sulfur trioxide and hydrogen fluoride, the overall yield can reach more than 88%. Among them, Examples 3 to 4 not only have high yields, but also have relatively low yields in step 3. This shows that the molar ratio of hydrogen fluoride to sulfur trioxide is further controlled within 1.4, preferably 1.33, and the amount of hydrogen fluoride is increased as much as possible, which can improve the yield of direct production of phosphorus pentafluoride in step 2; the whole process is stable and controllable, and there is no need to significantly increase the amount of supercooled brine.

[0048] 2. As can be seen from Examples 6 to 7, the extension of the reaction time and the reasonable adjustment of the temperature only slightly increase the yield, and have no obvious impact on the overall system.

[0049] 3. As can be seen from Comparative Examples 1 to 5, under different ratios, completely different results will be shown. Among them, although the molar dosage of Comparative Example 1 is within the optional range of the present invention, the molar amount of HF is more than 2 times the molar amount of sulfur trioxide, which will cause the water generated by Formula 1 and Formula 2 during the reaction to not be immediately and fully absorbed by sulfur trioxide, and then combine with phosphorus oxyfluoride to generate hexafluorophosphoric acid, ultimately resulting in a relatively high yield of phosphorus pentafluoride in Step 3.

[0050] The situation of Comparative Example 2 is similar to that of Comparative Example 1. Its further increase in the dosage of hydrogen fluoride and simultaneous increase in the dosage of sulfur trioxide will cause a relatively large amount of water 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.

[0051] In Comparative Example 3, the dosage of hydrogen fluoride is relatively large and the increase in sulfur trioxide leads to obvious heat release, making the reaction temperature control difficult; through the comparison between Comparative Example 3 and Example 3, it can be seen that although the ratio of sulfur trioxide in Comparative Example 3 increases and the dosage 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 leads to a decrease in temperature controllability. Therefore, in actual production, it is not recommended that the dosage of sulfur trioxide exceed 6 times the dosage of difluorophosphoric acid.

[0052] In Comparative Example 4, the dosage of hydrogen fluoride is too small, resulting in a relatively obvious decrease in the yield compared to Example 2.

[0053] In Comparative Example 5, the dosage of sulfur trioxide is too small, resulting in an increase in the yield of hexafluorophosphoric acid in Step 3 and the generation of hexafluorophosphoric acid consuming too much hydrogen fluoride, leading to a serious reduction in the overall output; in Comparative Examples 6 and 7, by adjusting the reaction temperature, Comparative Example 6 increases the reaction temperature, and its reaction controllability becomes relatively poor. Comparative Example 7 reduces the reaction temperature, and its yield decreases. It can be seen that when the temperature exceeds the recommended range, either the reaction is difficult to control or the output is too low.

[0054] 4. As can be seen from Comparative Examples 8 and 9, when using other dehydrating agents, most of the difluorophosphoric acid cannot achieve equilibrium conversion.

[0055] In summary, the reaction conditions of the present invention are mild, and PF can be directly produced from difluorophosphoric acid at least partially 5 , 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 comprises the following steps: Step 1: Add difluorophosphoric acid into the reaction kettle and start stirring; Step 2: passing hydrogen fluoride gas into difluorophosphoric acid, and simultaneously dropping sulfur trioxide into the difluorophosphoric acid, controlling the reaction temperature to -10~0°C, collecting the gas generated by the reaction; condensing the gas and collecting 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.

2. The method according to claim 1, characterized in that The sulfur trioxide and hydrogen fluoride are added for 5 to 10 hours.

3. The method according to claim 2, characterized in that 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.

4. The method according to claim 1, characterized in that 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 of step 2 is completed, step 3 is also included: The reactants remaining in the reactor are heated to 120-140° C. to thermally decompose the hexafluorophosphoric acid in the reactants, and the tail gas obtained by the thermal decomposition is condensed and then phosphorus pentafluoride is collected by a cold trap.

6. The method according to claim 1, characterized in that The top of the reactor is connected with an exhaust pipe, and the exhaust pipe is sequentially connected with a primary or multi-stage condenser, a cold trap, and a tail gas absorption device.

7. The method according to claim 6, characterized in that The upper part of the reactor is connected to a vertical shell and tube condenser, and the exhaust pipe is connected to the gas outlet of the vertical shell and tube condenser; the exhaust pipe is connected to a pump for sucking out the gas in the reactor and generating negative pressure.

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