Process for the photocatalytic telomerization for the preparation of hexafluoropropylene oxide dimers and pentafluoropropionic acid

Abstract

The application provides a method for preparing hexafluoropropylene oxide dimer and pentafluoropropionic acid through photocatalytic telomerization, and relates to the technical field of organic synthesis, and comprises the following steps: performing photocatalytic reaction on a rectification residue of hexafluoropropylene oxide dimer to continuously pyrolyze and telomerize, so that hexafluoropropylene oxide dimer and pentafluoropropionic acid are obtained; the components of the rectification residue include hexafluoropropylene oxide polymer CF3CF2CF2O[CF(CF3)CF2O] n CF(CF3)COF, n=0~5; the catalyst used in the photocatalytic reaction comprises alkali metal fluoride loaded on activated carbon. The application resources the hexafluoropropylene oxide polymer to hexafluoropropylene oxide dimer and pentafluoropropionic acid through telomerization, so that by-product waste liquid is fully resourced, and the application has the advantages of green economy; meanwhile, the reaction condition is mild, the conversion rate is high, the yield is high, the time is short, and the application is easy to industrialize.

Description

Technical Field

[0001] This invention relates to the field of organic synthesis, and in particular to a method for photocatalytic telomerization to prepare hexafluoropropylene oxide dimer and pentafluoropropionic acid. Background Technology

[0002] Hexafluoropropylene oxide dimer (C6F) 12 O2 (CAS: 2062-98-8) is a colorless, transparent liquid with a boiling point of 54℃-56℃ and a density of approximately 1.61 g / cm³. 3 It is mainly used as a surfactant and in the synthesis of perfluoropropyl vinyl ether (PPVE); PPVE is further used to synthesize high-performance, market-promising soluble polytetrafluoroethylene materials. Hexafluoropropylene oxide trimer (C9F) 18 O3 (CAS: 2641-34-1) is a colorless, transparent liquid with a boiling point of 113℃-115℃ and a density of approximately 1.8 g / cm³. 3 Primarily used as a PVDF surfactant, its use has been restricted because nonacarbon-perfluorocarboxylic acid cannot be metabolized in vivo. Hexafluoropropylene oxide dimers, trimers, and other hexafluoropropylene oxide polymers are all derived from the self-polymerization of hexafluoropropylene oxide. Currently, industrial treatment of hexafluoropropylene oxide trimers mainly involves incineration, resulting in significant resource waste. Pentafluoropropionic acid (CF3CF2COOH, CAS: 422-64-0) is a colorless, transparent liquid, mainly obtained by the hydrolysis of pentafluoropropionic acid fluoride, and can be used in fluorinated pharmaceuticals, fluorinated pesticides, and other fields.

[0003] Existing technology CN 104016848 discloses the preparation of pentafluoropropionyl fluoride using hexafluoropropylene oxide oligomers as raw materials, amines as solvents, cesium fluoride as catalysts, at temperatures of 110℃-130℃ and pressures of 2-3 atm, for a reaction time of 2-4 hours. However, this method uses expensive raw materials, which is not conducive to industrial production. Existing technology CN105315150 discloses the preparation of pentafluoropropionyl fluoride using hexafluoropropylene oxide oligomers as raw materials, ethers as solvents, alkali metal fluorides as catalysts, at temperatures of 90℃-200℃, for a reaction time of 12 hours. A method for preparing pentafluoropropionyl fluoride in 72 hours has been proposed, but this method has a long reaction time, and all hexafluoropropylene oxide oligomers are pyrolyzed and liquefied into pentafluoropropionyl fluoride, resulting in high energy consumption. Existing technology US6211415 discloses a method using hexafluoropropylene oxide oligomers as raw materials, ethers as solvents, and alkali metal fluorides as catalysts, with temperatures above 100℃, to produce hexafluoropropylene oxide dimers, trimers, and tetramers, etc., but this method involves high reaction temperatures and is a batch reaction. Existing technology CN 112028747 discloses adding perfluoro-2-methyl-2,3-epoxypentane to a methoxy alkali metal salt alcohol solution to obtain hexafluoroisopropyl methyl ether and methyl pentafluoropropionate. The separated methyl pentafluoropropionate is hydrolyzed to pentafluoropropionic acid under acidic conditions, and then distilled to obtain pentafluoropropionic acid. However, this method is a co-production process with high raw material costs, complex reactions, low yields, and high requirements for distillation equipment.

[0004] In view of this, the present invention is hereby proposed. Summary of the Invention

[0005] The purpose of this invention is to provide a method for photocatalytic polymerization to prepare hexafluoropropylene oxide dimer and pentafluoropropionic acid. This method can convert hexafluoropropylene oxide polymer into hexafluoropropylene oxide dimer and pentafluoropropionic acid, making full use of by-product waste liquid. It has the advantages of being green and economical. At the same time, the reaction conditions are mild, the conversion rate is high, the yield is high, the reaction time is short, and it is easy to industrialize.

[0006] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted: A method for photocatalytic telomerization to prepare hexafluoropropylene oxide dimer and pentafluoropropionic acid includes the following steps: The distillation residue of hexafluoropropylene oxide dimer was subjected to a photocatalytic reaction, resulting in continuous pyrolysis and polymerization to obtain hexafluoropropylene oxide dimer and pentafluoropropionic acid. The distillation residue comprises hexafluoropropylene oxide polymers CF3CF2CF2O[CF(CF3)CF2O]. n CF(CF3)COF, n=0~5; The photocatalytic reaction uses alkali metal fluorides supported on activated carbon as the catalyst.

[0007] Furthermore, the content of hexafluoropropylene oxide dimer (n=0) in the distillation residue is less than 20 wt.%. Preferably, the content of hexafluoropropylene oxide trimer (n=1) in the distillation residue is less than 90 wt.%. Preferably, the content of hexafluoropropylene oxide tetramer (n=2) in the distillation residue is less than 40 wt.%. Preferably, the content of hexafluoropropylene oxide pentamer (n=3) in the distillation residue is less than 20 wt.%. Preferably, the content of hexafluoropropylene oxide hexamer (n=4) in the distillation residue is less than 10 wt.%. Preferably, the content of hexafluoropropylene oxide heptamer (n=5) in the distillation residue is less than 5 wt.%.

[0008] Furthermore, the alkali metal fluoride includes at least one of LiF, NaF, KF, RbF, CsF, MgF2, and CaF2; Preferably, the loading of the alkali metal fluoride on the activated carbon is 1%-5%.

[0009] Furthermore, the solvent used in the photocatalytic reaction includes at least one of diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and polyethylene glycol dimethyl ether. Preferably, the mass ratio of the solvent to the distillation residue is (0.5-2):1.

[0010] Furthermore, the photocatalytic reaction also employs an auxiliary agent; Preferably, the adjuvant includes at least one of 12-crown-4, 15-crown-5, and 18-crown-6.

[0011] Furthermore, the mass ratio of the auxiliary agent to the solvent is 1:(100-1000).

[0012] Furthermore, the photocatalytic reactor used in the photocatalytic reaction includes a horizontal cylindrical photocatalytic reactor; Preferably, the material of the photocatalytic reactor includes at least one of 316L, 904L, Monel, and Hastelloy.

[0013] Furthermore, the photocatalytic reactor is equipped with alkali metal fluorides supported on activated carbon as catalysts at both ends, and ultraviolet lamps are provided at the other two ends of the photocatalytic reactor. Preferably, the ultraviolet light irradiation wavelength of the ultraviolet lamp is 100nm-400nm, and more preferably 150nm-300nm; Preferably, the ultraviolet lamps and the catalyst are distributed crosswise along the inner surface of the reactor.

[0014] Furthermore, the photocatalytic reaction is carried out at a temperature of 50℃-100℃ for a time of 5h-30h.

[0015] Compared with the prior art, the present invention has at least the following beneficial effects: The method for preparing hexafluoropropylene oxide dimer and pentafluoropropionic acid by photocatalytic polymerization provided by this invention utilizes the high-boiling-point residual liquid from the distillation process during the synthesis of hexafluoropropylene oxide dimer to convert it into hexafluoropropylene oxide dimer and pentafluoropropionic acid through photocatalytic polymerization. This fully utilizes the by-product waste liquid, offering advantages of green economy. Furthermore, the reaction conditions are mild, the conversion rate is high, the yield is high, the reaction time is short, and it is easily industrialized. Attached Figure Description

[0016] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0017] Figure 1 A flowchart illustrating a method for photocatalytic telomerization to prepare hexafluoropropylene oxide dimer and pentafluoropropionic acid according to one embodiment of the present invention; Figure 2 A left view of a photocatalytic reactor provided according to one embodiment of the present invention; Figure 3 A front view of a photocatalytic reactor provided according to one embodiment of the present invention; Figure 4 The vapor phase spectrum of the high-boiling-point residue from the distillation of hexafluoropropylene oxide dimer.

[0018] Icons: 1-First catalyst loading position; 2-Second catalyst loading position; 3-First ultraviolet lamp; 4-Second ultraviolet lamp; 5-Rotary joint. Detailed Implementation

[0019] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] This invention provides a method for photocatalytic telomerization to prepare hexafluoropropylene oxide dimer and pentafluoropropionic acid, comprising the following steps: The distillation residue of hexafluoropropylene oxide dimer was subjected to a photocatalytic reaction, resulting in continuous pyrolysis and polymerization to obtain hexafluoropropylene oxide dimer and pentafluoropropionic acid. The distillation residue includes hexafluoropropylene oxide polymers CF3CF2CF2O[CF(CF3)CF2O]. n CF(CF3)COF, n=0~5; The catalysts used in the photocatalytic reaction include alkali metal fluorides supported on activated carbon.

[0021] The method of this invention converts the high-boiling-point residual liquid from the distillation of hexafluoropropylene oxide during the synthesis of hexafluoropropylene oxide dimer into hexafluoropropylene oxide dimer and pentafluoropropionic acid through photocatalytic polymerization. This fully utilizes the by-product waste liquid, has the advantages of being green and economical, and features mild reaction conditions, high conversion rate, high yield, short reaction time, and easy industrialization.

[0022] The boiling point of hexafluoropropylene oxide dimer is 54℃-56℃. The boiling point of hexafluoropropylene oxide trimer is 113℃-115℃. The boiling point of hexafluoropropylene oxide tetramer is 158℃-161℃. The boiling point of hexafluoropropylene oxide pentamer is 200℃-203℃. The boiling point of hexafluoropropylene oxide hexamer is 231℃-235℃. Compared to the hexafluoropropylene oxide dimer, the hexafluoropropylene oxide trimer, tetramer, pentamer, and hexamer are all high-boiling-point compounds.

[0023] In this invention, the content of hexafluoropropylene oxide dimer (n=0) in the distillation residue can be less than 20 wt.%; the content of hexafluoropropylene oxide trimer (n=1) in the distillation residue can be less than 90 wt.%; the content of hexafluoropropylene oxide tetramer (n=2) in the distillation residue can be less than 40 wt.%; the content of hexafluoropropylene oxide pentamer (n=3) in the distillation residue can be less than 20 wt.%; the content of hexafluoropropylene oxide hexamer (n=4) in the distillation residue can be less than 10 wt.%; the content of hexafluoropropylene oxide heptamer (n=5) in the distillation residue can be less than 5 wt.%; at the same time, the content of other impurities in the distillation residue can be less than 20 wt.%, and other impurities include the solvent, catalyst used in the self-polymerization reaction of hexafluoropropylene oxide, and hexafluoropropylene polymers in hexafluoropropylene oxide, etc.

[0024] In a preferred embodiment, the alkali metal fluoride includes, but is not limited to, at least one of LiF, NaF, KF, RbF, CsF, MgF2 and CaF2, which is beneficial to improve the conversion rate and result in a high yield of hexafluoropropylene oxide dimer and pentafluoropropionic acid.

[0025] In this invention, the loading of alkali metal fluorides on activated carbon can be 1%-5%, with typical but non-limiting loadings being, for example, 1%, 2%, 3%, 4%, and 5%.

[0026] In a preferred embodiment, the solvent used in the photocatalytic reaction includes, but is not limited to, at least one of diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and polyethylene glycol dimethyl ether. The solvent is a liquid phase carrier, which is beneficial for dispersing heat and avoiding local overheating.

[0027] The solvent can either enter the photocatalytic reactor first, or it can be mixed with the distillation residue of hexafluoropropylene oxide dimer before entering the photocatalytic reactor to carry out the photocatalytic reaction.

[0028] In a preferred embodiment, the mass ratio of solvent to distillation residue can be (0.5-2):1, with typical but non-limiting mass ratios such as 0.5:1, 1:1, 1.5:1, and 2:1.

[0029] In this invention, the photocatalytic reaction can also employ an auxiliary agent.

[0030] In a preferred embodiment, the adjuvant includes, but is not limited to, at least one of 12-crown-4, 15-crown-5, and 18-crown-6, which is beneficial for mass transfer and accelerates the reaction rate.

[0031] The additives can be mixed with the solvent first, and the mass ratio of the additives to the solvent can be 1:(100-1000).

[0032] In a preferred embodiment, the photocatalytic reactor used in the photocatalytic reaction includes, but is not limited to, a horizontal cylindrical photocatalytic reactor.

[0033] In this invention, the material of the photocatalytic reactor can be at least one of 316L, 904L, Monel, and Hastelloy.

[0034] In a preferred embodiment, the two ends of the photocatalytic reactor can be catalysts impregnated with activated carbon. The activated carbon is filled on the surface of the cylinder in a circular grid area. The diameter of the circular grid can be 0.1 mm to 1 mm. The size of the activated carbon is larger than the diameter of the circular grid to prevent the activated carbon from leaking out from the filling position.

[0035] In this invention, the amount of activated carbon packed in the cylinder can be 2, 4, 6, etc., evenly distributed.

[0036] In a preferred embodiment, ultraviolet lamps can be provided at the other two ends of the photocatalytic reactor, with an irradiation wavelength of 100nm-400nm, more preferably 150nm-300nm.

[0037] In this invention, the number of ultraviolet lamps can be 2, 4, 6, etc., and they can be distributed crosswise with the catalyst along the inner surface of the reactor. To ensure that the glass tube on the outer surface of the ultraviolet lamp is not corroded by the material, a corrosion-resistant and temperature-resistant film material can be attached to the outer surface of the glass tube, such as a transparent film made of PFA or Teflon AF.

[0038] In a preferred embodiment, the outer shell of the photocatalytic reactor is fixed, while the inner layer can rotate around the central axis. The feed and discharge ports are located at the central axis ports and are equipped with rotary joints to ensure continuous material feeding and discharging.

[0039] The inner layer of the photocatalytic reactor rotates around the central axis, which is beneficial to improve the mass transfer efficiency of the photocatalytic reaction. The rotation speed can be 30rpm-100rpm, for example, 30rpm, 40rpm, 50rpm, 60rpm, 70rpm, 80rpm, 90rpm, 100rpm, but is not limited to this.

[0040] In a preferred embodiment, the photocatalytic reactor can be electrically heated, and the reaction temperature can be set to 50°C-100°C. Typical but non-limiting temperatures include 50°C, 60°C, 70°C, 80°C, 90°C, and 100°C. The reaction temperature affects the telomerization product; the higher the temperature, the higher the yield of pentafluoropropionyl fluoride. The reaction time can be 5h-30h, for example, 5h, 10h, 15h, 20h, 25h, and 30h, but is not limited to these.

[0041] After the photocatalytic reaction, the resulting product is refluxed for gas-liquid separation at a temperature of -10℃ to 20℃. The uncondensed gas phase is pentafluoropropionyl fluoride, which is absorbed and hydrolyzed by pure water to obtain pentafluoropropionic acid. The absorption method can be primary, secondary, tertiary, or higher-level absorption. The pentafluoropropionic acid can be further purified by adsorption with sodium sulfate, magnesium sulfate, alumina, or by distillation to remove moisture, hydrofluoric acid, and other impurities, thus obtaining qualified pentafluoropropionic acid. The liquid phase is further separated into layers. The upper layer contains solvent and additives, which can be recycled. The lower layer is the fluorine phase, which has a significantly increased content of hexafluoropropylene oxide dimer and a decreased content of hexafluoropropylene oxide trimer and tetramer.

[0042] The present invention will be further illustrated below by way of examples. Unless otherwise specified, the materials in the examples are prepared according to existing methods or purchased directly from the market.

[0043] Example 1 A method for photocatalytic telomerization to prepare hexafluoropropylene oxide dimer and pentafluoropropionic acid, the flowchart of which is shown below. Figure 1 This includes the following steps: (1) Diethylene glycol dimethyl ether (solvent) containing 2 wt.‰ auxiliary agent 18-crown-6 and the high-boiling point residue of distillation of hexafluoropropylene oxide dimer (hexafluoropropylene oxide polymer) were respectively transported into the photocatalytic reactor by metering pumps, with the flow rates set to 600 g / h and 480 g / h respectively, to form the reaction system; The high-boiling-point residue from distillation contains 0.05 wt.% hexafluoropropylene oxide dimer, 65.43 wt.% hexafluoropropylene oxide trimer, 6.76 wt.% hexafluoropropylene oxide tetramer, 2.54 wt.% hexafluoropropylene oxide pentamer, and 1.17 wt.% hexafluoropropylene oxide hexamer. (2) The reaction system in step (1) is subjected to photocatalytic reaction under the catalysis of CsF supported on activated carbon. The loading amount of CsF on activated carbon is 2%. The photocatalytic reactor is rotated along the central axis with a rotation speed of 40 rpm, a temperature of 70 °C, and an ultraviolet wavelength of 200 nm. After the photocatalytic reaction, the product was refluxed and the gas and liquid phases were separated. The reflux temperature was 0℃. The gas phase was pentafluoropropionyl fluoride, which was absorbed and hydrolyzed by secondary water to obtain pentafluoropropionic acid with a yield (relative to the hexafluoropropionyl polymer in the high-boiling-point residue of the distillation of the hexafluoropropionyl dimer) of 40.4%. The liquid phase was further separated into liquid and liquid phases, and the solvent phase and fluorine phase were separated. The solvent phase can be recycled. The fluorine phase was purified by distillation to obtain the hexafluoropropionyl dimer with a yield of 54.1%. The distillation residue can be returned to the photocatalytic reaction.

[0044] In this embodiment, the photocatalytic reactor is shown below. Figure 2 and Figure 3 Its diameter is 60mm, its length is 3m, and its material is Hastelloy; the cylinder filled with catalyst in the photocatalytic reactor has a radius of 10mm, and the circular mesh on its surface has a diameter of 0.5mm and a quantity of 2, see Figure 2 Activated carbon-supported KF was filled into two cylindrical catalyst regions within the photocatalytic reactor, designated as catalyst loading position 1 and catalyst loading position 2, for a total filling of 1000g. The ultraviolet lamps in the photocatalytic reactor had a diameter of 10mm, and their outer surface was covered with a PFA film; two lamps were used. (See...) Figure 2 The first ultraviolet lamp 3 and the second ultraviolet lamp 4 are respectively; the photocatalytic reactor is equipped with a rotary joint 5, the outer layer of which is fixed to ensure continuous material feeding and discharging.

[0045] The vapor phase spectrum of the high-boiling-point residue from the distillation of hexafluoropropylene oxide dimer is shown below. Figure 4 .

[0046] Example 2 The only difference between this embodiment and Embodiment 1 is that the loading of CsF on the activated carbon is 1%; The rest were the same as in Example 1, with a yield of 35.1% for pentafluoropropionic acid and a yield of 50.7% for hexafluoropropylene oxide dimer.

[0047] Example 3 The only difference between this embodiment and Embodiment 1 is that the loading of CsF on the activated carbon is 3%; Everything else was the same as in Example 1, with a yield of 44% for pentafluoropropionic acid and 53.5% for hexafluoropropylene oxide dimer.

[0048] Example 4 The only difference between this embodiment and Embodiment 1 is that the loading of CsF on the activated carbon is 5%; The rest were the same as in Example 1, with a yield of 47.9% for pentafluoropropionic acid and 49.0% for hexafluoropropylene oxide dimer.

[0049] Example 5 The only difference between this embodiment and Example 1 is that the photocatalytic reaction is carried out under the catalysis of KF supported on activated carbon; the loading amount of KF on the activated carbon is 2%. The rest were the same as in Example 1, with a yield of 39.8% for pentafluoropropionic acid and 52.8% for hexafluoropropylene oxide dimer.

[0050] Example 6 The only difference between this embodiment and Embodiment 1 is that the ultraviolet light wavelength is 150nm; The rest were the same as in Example 1, with a yield of 34.8% for pentafluoropropionic acid and a yield of 50.1% for hexafluoropropylene oxide dimer.

[0051] Example 7 The only difference between this embodiment and Embodiment 1 is that the ultraviolet light wavelength is 300nm; The rest were the same as in Example 1, with a yield of 41.9% for pentafluoropropionic acid and 52.8% for hexafluoropropylene oxide dimer.

[0052] Example 8 The only difference between this embodiment and Embodiment 1 is that the temperature of the photocatalytic reaction is 50°C. Everything else was the same as in Example 1, with a yield of 30.9% for pentafluoropropionic acid and 48.0% for hexafluoropropylene oxide dimer.

[0053] Example 9 The only difference between this embodiment and Example 1 is that the temperature of the photocatalytic reaction is 100°C. The rest were the same as in Example 1, with a yield of 46.1% for pentafluoropropionic acid and 49.7% for hexafluoropropylene oxide dimer.

[0054] Example 10 The only difference between this embodiment and Example 1 is that the distillation high-boiling point residues containing 2 wt.‰ of the auxiliary agent 18-crown-6 diethylene glycol dimethyl ether and hexafluoropropylene oxide dimer are respectively transported into the photocatalytic reactor by metering pumps, with flow rates set to 480 g / h and 600 g / h respectively, to form the reaction system; The rest were the same as in Example 1, with a yield of 32.5% for pentafluoropropionic acid and 47.4% for hexafluoropropylene oxide dimer.

[0055] Example 11 The only difference between this embodiment and Example 1 is that the distillation high-boiling point residue containing 2 wt.‰ of the auxiliary agent 18-crown-6 diethylene glycol dimethyl ether and hexafluoropropylene oxide dimer is respectively transported into the photocatalytic reactor by metering pumps, with the flow rates set to 480 g / h and 360 g / h respectively, to form the reaction system; Everything else was the same as in Example 1, with a yield of 37.2% for pentafluoropropionic acid and 51.3% for hexafluoropropylene oxide dimer.

[0056] Comparative Example 1 The only difference between this comparative example and Example 1 is that no additives were added; The rest were the same as in Example 1, with a yield of 31.9% for pentafluoropropionic acid and 47.0% for hexafluoropropylene oxide dimer.

[0057] Compared to Example 1, the drawback of this comparative example is that the yields of both pentafluoropropionic acid and hexafluoropropylene oxide dimers are reduced.

[0058] Comparative Example 2 The only difference between this comparative example and Example 1 is that ultraviolet light irradiation was not used; The rest were the same as in Example 1, with a yield of 22.3% for pentafluoropropionic acid and a yield of 30.7% for hexafluoropropylene oxide dimer.

[0059] Compared to Example 1, the drawback of this comparative example is that the yields of both pentafluoropropionic acid and hexafluoropropylene oxide dimers are significantly reduced.

[0060] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for photocatalytic telomerization to prepare hexafluoropropylene oxide dimer and pentafluoropropionic acid, characterized in that, Includes the following steps: The distillation residue of hexafluoropropylene oxide dimer was subjected to a photocatalytic reaction, resulting in continuous pyrolysis and polymerization to obtain hexafluoropropylene oxide dimer and pentafluoropropionic acid. The distillation residue comprises hexafluoropropylene oxide polymers CF3CF2CF2O[CF(CF3)CF2O]. n CF(CF3)COF, n=0~5; The photocatalytic reaction uses alkali metal fluorides supported on activated carbon as the catalyst.

2. The method according to claim 1, characterized in that, The content of hexafluoropropylene oxide dimer in the distillation residue was less than 20 wt.%. The content of hexafluoropropylene oxide trimer in the distillation residue was less than 90 wt.%. The content of hexafluoropropylene oxide tetramer in the distillation residue was less than 40 wt.%. The content of hexafluoropropylene oxide pentamer in the distillation residue was less than 20 wt.%. The content of hexafluoropropylene oxide hexamer in the distillation residue was less than 10 wt.%. The content of hexafluoropropylene oxide heptamer in the distillation residue is less than 5 wt.%.

3. The method according to claim 1, characterized in that, The alkali metal fluorides include at least one of LiF, NaF, KF, RbF, CsF, MgF2, and CaF2; The loading of the alkali metal fluoride on the activated carbon is 1%-5%.

4. The method according to claim 1, characterized in that, The solvent used in the photocatalytic reaction includes at least one of diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and polyethylene glycol dimethyl ether; The mass ratio of the solvent to the distillation residue is (0.5-2):

1.

5. The method according to claim 4, characterized in that, The photocatalytic reaction also employs an auxiliary agent; The adjuvant includes at least one of 12-crown-4, 15-crown-5, and 18-crown-6.

6. The method according to claim 5, characterized in that, The mass ratio of the additive to the solvent is 1:(100-1000).

7. The method according to any one of claims 1-6, characterized in that, The photocatalytic reaction uses a horizontal cylindrical photocatalytic reactor.

8. The method according to claim 7, characterized in that, The photocatalytic reactor is made of at least one of 316L, 904L, Monel, and Hastelloy.

9. The method according to claim 7, characterized in that, The photocatalytic reactor has alkali metal fluorides supported on activated carbon as catalysts at both ends, and ultraviolet lamps at the other two ends. The ultraviolet light irradiation wavelength of the ultraviolet lamp is 100nm-400nm; The ultraviolet lamps and catalysts are distributed crosswise along the inner surface of the reactor.

10. The method according to any one of claims 1-6, characterized in that, The photocatalytic reaction is carried out at a temperature of 50℃-100℃ for a time of 5h-30h.

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