Application of a chromium-based catalyst in the preparation of perfluorooctyl bromide, a key material for fluorocarbon nano-contrast agents, by catalytic bromination of perfluorooctyl iodide

Abstract

The present invention discloses an application of a chromium-based catalyst in the preparation of perfluorooctyl bromide, a key material for fluorocarbon nanocontrast agents, by catalytic bromination of perfluorooctyl iodide. The chromium-based catalyst is loaded into a fixed-bed reactor. After perfluorooctyl iodide and bromine are vaporized respectively and mixed with a carrier gas, they enter the fixed-bed reactor together, and the bromination reaction is carried out under normal pressure to obtain perfluorooctyl bromide. The chromium-based catalyst is prepared by sintering a chromium-based oxide precursor obtained by co-precipitation of a soluble chromium salt, a soluble metal salt auxiliary agent, a pore-forming agent and graphite, and then partially fluorinating it with a mixed gas of N2 / HF. The reaction raw materials of the present invention are easily available; the reaction conditions are mild, and materials and equipment matching the process can be found in the existing equipment and material system to support the long-term stable operation of production; through the efficient catalysis of the catalyst and the continuous progress of the reaction gas phase, the reaction efficiency is improved and the consumption of bromine is reduced, making the whole process have good atom economy and good application prospects.

Description

Technical Field

[0001] The present invention belongs to the technical field of pharmaceutical synthesis, and particularly relates to the application of a chromium-based catalyst in the catalytic bromination of perfluorooctyl iodide to prepare perfluorooctyl bromide. Background Art

[0002] As an indispensable diagnostic technology in modern medical diagnosis, ultrasonic imaging has the advantages of non-invasiveness, real-time, dynamic, repeatable examination, portability, wide application range and low examination cost in clinical applications, and has become the preferred technology for imaging diagnosis.

[0003] Liquid fluorocarbon nano-contrast emulsion has the advantages of being able to meet multiple imaging modalities such as ultrasound, CT, and magnetic resonance in molecular imaging. In clinical applications, it can avoid the need for patients to inject multiple types of contrast agents in a short period of time. On the one hand, it can save a large amount of money for patients, and on the other hand, it can greatly reduce the metabolic burden on the patient's body. As the key fluorocarbon material of the liquid fluorocarbon nano-contrast emulsion, perfluorooctyl bromide has the characteristic of not being penetrated by X-rays due to the presence of bromine atoms, and has become the most concerned fluorocarbon carrier with promising future application prospects.

[0004] The preparation methods of perfluorooctyl bromide reported in the current literature mainly include the following: First, EP519808 reported that perfluorooctanesulfonyl chloride and hydrogen bromide react under the catalysis of a catalyst and heating conditions to obtain perfluorooctyl bromide. On the one hand, the raw material of this method needs to use PFOS, which has been completely prohibited from production. On the other hand, the yield is not high. Second, CN112341310 reported that perfluoroiodooctane reacts with bromine under photocatalysis at 70 °C to obtain perfluorooctyl bromide. This method is carried out under liquid-phase conditions, the reaction temperature exceeds the boiling point of bromine, and the reaction is carried out under positive pressure, which is a very severe test for the sealing material of the equipment. It is difficult for the equipment to operate stably for a long time. In addition, due to the formation of iodine tribromide, a large amount of excessive bromine needs to be consumed, and the atom economy is not good. Third, US5455373 reported that silver perfluorononanoate reacts with bromine under heating to undergo decarboxylation reaction to obtain perfluorooctyl bromide. The raw material silver carbonate used in this method is a precious metal, and perfluorononanoic acid is also an uncommon industrial raw material, and the costs are both high and uneconomical. Fourth, RU2739762 reported that perfluorooctanesulfonyl fluoride and hydrazine hydrate produce perfluorooctanesulfonyl hydrazide and then react with bromine under heating to remove sulfur dioxide to obtain perfluorooctyl bromide. This method, like EP519808, needs to use the PFOS raw material that has been completely prohibited from production and does not have the feasibility of industrial production. Fifth, CN102992944 reported that perfluorooctanesulfonyl chloride and quaternary ammonium bromide undergo a substitution reaction to prepare perfluorooctyl bromide. This method, like EP519808, needs to use the PFOS raw material that has been completely prohibited from production and does not have the feasibility of industrial production. CN116903433 reported that perfluorooctane and bromine react under photocatalysis to obtain perfluorooctyl bromide. The perfluorooctane raw material used in this method is not easily available and the cost is high. JP2019014655 reported that perfluoroiodooctane and bromine undergo gas-phase bromination reaction at 350 °C to obtain perfluorooctyl bromide. This method has harsh conditions. At 350 °C, almost no materials can withstand the corrosion of bromine except for materials such as ceramics and quartz. However, the sealing of hard equipment such as ceramics and quartz has always been a difficult problem. Under such harsh conditions, there is almost no sealing material that can support the normal operation of the process, and it does not have the feasibility of mass production. JP60184033 reported that perfluoroiodooctane and bromine react under photocatalysis in a 1,1,2-trichloro-1,2,2-trifluoroethane solvent to obtain perfluorooctyl bromide, with a yield of 42%, too high cost, and a large amount of three wastes generated, and it does not have good technical economy. Summary of the Invention

[0005] In view of the disadvantages of the existing production methods of perfluorooctyl bromide, such as difficult availability of raw materials, harsh reaction conditions making it difficult to match suitable equipment and processes, large consumption of bromine, a large amount of three wastes, and high costs, the present invention aims to provide an application of a chromium-based catalyst in the catalytic bromination of perfluorooctyl iodide to prepare perfluorooctyl bromide, a key material for fluorocarbon nanocontrast agents. The reaction raw materials are easily available; the reaction conditions are mild, and materials and equipment that match the process can be found in the existing equipment and material systems to support the long-term stable operation of production; through the efficient catalysis of the chromium-based catalyst and the continuous gas phase of the reaction, the reaction efficiency is improved and the bromine consumption is reduced, making the whole process have good atom economy and good application prospects.

[0006] To achieve the above technical objectives, the technical solution adopted by the present invention is as follows:

[0007] An application of a chromium-based catalyst in the catalytic bromination of perfluorooctyl iodide to prepare perfluorooctyl bromide, a key material for fluorocarbon nanocontrast agents. The chromium-based catalyst is loaded into a fixed-bed reactor. Perfluorooctyl iodide and bromine are vaporized respectively and then mixed with a carrier gas and enter the fixed-bed reactor together, and a bromination reaction is carried out under normal pressure to obtain perfluorooctyl bromide; the chromium-based catalyst is prepared by sintering a chromium-based oxide precursor obtained by co-precipitation of a soluble chromium salt, a soluble metal salt auxiliary agent, a pore-forming agent and graphite, and then partially fluorinating with a N2 / HF mixed gas. The metal in the soluble metal salt auxiliary agent is selected from at least one of lanthanum, aluminum, manganese, cobalt, antimony and zinc.

[0008] Preferably, in the bromination reaction, the molar ratio of bromine to perfluorooctyl iodide is not less than 1, and more preferably 1.5 - 2.5.

[0009] Preferably, in the bromination reaction, the space velocity is 100 - 500 h -1 , and more preferably 150 - 300 h -1 .

[0010] Preferably, in the bromination reaction, the reaction temperature is 100 - 200 °C, and more preferably 150 - 170 °C.

[0011] Preferably, the soluble chromium salt is selected from one of chromium chloride and chromium nitrate; the soluble metal salt auxiliary agent is selected from one of their chlorides and nitrates; the pore-forming agent is selected from one of sodium silicate and silicon tetrachloride.

[0012] Preferably, the base added in the co-precipitation process is ammonia water, and the adjusted pH is 14; the sintering temperature is 350 - 450 °C, and the time is 6 - 12 h.

[0013] Preferably, in the fluorination process, a chromium-based oxide precursor is subjected to gradient fluorination using N2 / HF mixed gases with different concentrations. During the gradient fluorination process, the volume ratio of N2 to HF decreases in a gradient manner. For example, the volume ratios of N2 to HF are 20:1, 10:1, 5:1, 3:1, 1:1, and 0:1 in sequence.

[0014] Preferably, the molar ratio of chromium in the soluble chromium salt to the metal in the soluble metal salt auxiliary is 90 - 99:1 - 10; more preferably 92 - 97:3 - 8.

[0015] Preferably, the molar ratio of chromium in the soluble chromium salt to the pore-forming agent is 90 - 100:1; the molar ratio of chromium in the soluble chromium salt to graphite is 1:70 - 90.

[0016] Preferably, the metal in the soluble metal salt auxiliary is lanthanum and aluminum, where the molar ratio of lanthanum to aluminum is 1:1, and the total molar amount of lanthanum and aluminum to chromium is 5:95.

[0017] The advantages of the present invention are as follows:

[0018] (1) The present invention uses a metal-doped partially fluorinated chromium-based catalyst as a catalyst for the catalytic bromination of perfluorooctyl iodide to prepare perfluorooctyl bromide. The raw material used is perfluorooctyl iodide, which is currently produced on a large scale in industrialization, thus well solving the problem of difficult availability of raw materials.

[0019] (2) The present invention uses a metal-doped partially fluorinated chromium-based catalyst as a catalyst for the catalytic bromination of perfluorooctyl iodide to prepare perfluorooctyl bromide. The consumption of bromine is within 2 equivalents, and the total yield can be as high as 93% based on perfluorooctyl iodide, thus well solving the problem of poor atom economy caused by the need for a large excess of bromine due to the formation of iodine tribromide from bromine and iodine.

[0020] (3) The present invention uses a metal-doped partially fluorinated chromium-based catalyst as a catalyst for the catalytic bromination of perfluorooctyl iodide to prepare perfluorooctyl bromide, greatly reducing the difficulty of the bromination reaction. The reaction can proceed smoothly at about 165 °C optimally, with good conversion and yield. At this temperature, the mature PTFE-lined metal pipes widely used industrially can well withstand the corrosion of bromine, and the sealing problem can be solved by using PTFE gaskets between the pipelines, thus well solving the problem of the matching of equipment materials and equipment structure.

[0021] (4) The present invention uses a metal-doped partially fluorinated chromium-based catalyst as a catalyst for the catalytic bromination of perfluorooctyl iodide to prepare perfluorooctyl bromide. The entire reaction process can be continuously carried out under atmospheric pressure. A relatively small tubular continuous reaction equipment can be used for continuous production over a long period of time. Both the process safety and process stability are excellent, which well solves a series of problems such as continuous production process. Large-scale production can be carried out with very small equipment and little floor area, and it has good application prospects.

[0022] (5) The metal-doped partially fluorinated chromium-based catalyst of the present invention can also respectively obtain the corresponding perfluorobutyl bromide and perfluorohexyl bromide for raw materials such as perfluorobutyl iodide and perfluorohexyl iodide in the same series. Description of the Drawings

[0023] Figure 1 It is the gas chromatogram of the reaction material in Comparative Example 1 after being condensed and collected and washed with sodium hydroxide to remove bromine and then sampled.

[0024] Figure 2 It is the gas chromatogram of the reaction material in Comparative Example 2 after being condensed and collected and washed with sodium hydroxide to remove bromine and then sampled.

[0025] Figure 3 It is the gas chromatogram of the reaction material in Example 1 after being condensed and collected and washed with sodium hydroxide to remove bromine and then sampled.

[0026] Figure 4 It is the gas chromatogram of the reaction material in Example 1 after sampling and washing with sodium hydroxide to remove bromine after the reaction lasts for 200 h.

[0027] Figure 5 It is the gas chromatogram of the reaction material in Example 2 after being condensed and collected and washed with sodium hydroxide to remove bromine and then sampled.

[0028] Figure 6 It is the gas chromatogram of the reaction material in Example 3 after being condensed and collected and washed with sodium hydroxide to remove bromine and then sampled.

[0029] Figure 7 It is the gas chromatogram of the reaction material in Example 4 after being condensed and collected and washed with sodium hydroxide to remove bromine and then sampled.

[0030] Figure 8 It is the gas chromatogram of the reaction material in Example 5 after being condensed and collected and washed with sodium hydroxide to remove bromine and then sampled.

[0031] Figure 9 It is the gas chromatogram of the reaction material in Example 6 after being condensed and collected and washed with sodium hydroxide to remove bromine and then sampled.

[0032] Figure 10 It is the gas chromatogram analysis chart of the reaction materials in Example 7 after being condensed and collected and washed with sodium hydroxide to remove bromine and then sampled. Detailed implementation mode

[0033] Comparative Example 1

[0034] Perfluorooctyl iodide was added to the perfluorooctyl iodide vaporization tank from the perfluorooctyl iodide feeding port at a speed of 2.5 g / min by a peristaltic pump for vaporization. The temperature of the perfluorooctyl iodide vaporization tank was controlled at 180 °C. Nitrogen was introduced into the perfluorooctyl iodide vaporization tank at a speed of 75 ml / min to be mixed with perfluorooctyl iodide, and then the mixed gas entered the mixing tank to be mixed with bromine; bromine was added to the bromine vaporization tank from the bromine feeding port at a speed of 1.37 g / min by an injection pump for vaporization. The temperature of the bromine vaporization tank was controlled at 180 °C. Nitrogen was introduced into the bromine vaporization tank at a speed of 75 ml / min to be mixed with bromine, and then the mixed gas entered the mixing tank to be mixed with perfluorooctyl iodide; the temperature of the vaporization tank was controlled at 165 °C. The perfluorooctyl iodide, bromine and nitrogen mixed evenly in the vaporization tank entered the inner-lined tetrafluoro fixed-bed reactor for reaction without adding a catalyst. The temperature in the reactor was 165 °C, the pressure was normal pressure, and the overall space velocity was 266 h -1 , after the reaction materials were condensed and collected and washed with sodium hydroxide to remove bromine, samples were taken for gas chromatographic analysis, and the results were as Figure 1 shown. The content of perfluorooctyl bromide was 2.2%, and almost all the remaining components were unreacted perfluorooctyl iodide raw materials.

[0035] Comparative Example 2

[0036] 100 g of chromium trichloride, 0.77 g of sodium silicate and 640 g of graphite powder with a particle size of 400 mesh were added to 5 L of pure water, and then ammonia water was slowly added thereto until the pH value reached 14. Stirring reaction was continued for 30 min, followed by filtration. The filter cake was washed with pure water until neutral to obtain an oxide catalyst precursor powder mainly composed of chromium oxide. Granulation was carried out with a granulator to obtain chromium oxide catalyst precursor particles. The obtained chromium catalyst precursor particles were placed in a muffle furnace and sintered at 400 °C for 8 h and then taken out and cooled for standby.

[0037] Place the obtained 105 ml of chromium oxide catalyst precursor particles in a fixed-bed reactor lined with polytetrafluoroethylene with an inner diameter of 1.5 cm and a heating zone length of 60 cm. First, pass nitrogen with a flow rate of 100 ml / min, turn on the heating, and dry the catalyst precursor. First, heat it to 100 °C and hold it at 100 °C for 30 min, then raise the temperature to 200 °C and hold it at 200 °C for 30 min, and then raise the temperature to 300 °C to dry the chromium oxide catalyst precursor particles. After 12 h, the drying of the chromium oxide catalyst precursor particles is completed. Cool down to 165 °C. First, pass a mixed gas with a volume ratio of N2 / HF of 20:1 and a gas flow rate of 100 ml / min to partially fluorinate the chromium oxide catalyst precursor for 2 h; then adjust the volume ratio of N2 / HF to 10:1 and still maintain a gas flow rate of 100 ml / min to continue to partially fluorinate the chromium oxide catalyst precursor for 2 h; then adjust the volume ratio of N2 / HF to 5:1 and still maintain a gas flow rate of 100 ml / min to continue to partially fluorinate the chromium oxide catalyst precursor for 2 h; then adjust the volume ratio of N2 / HF to 3:1 and still maintain a gas flow rate of 100 ml / min to continue to partially fluorinate the chromium oxide catalyst precursor for 2 h; then adjust the volume ratio of N2 / HF to 1:1 and still maintain a gas flow rate of 100 ml / min to continue to partially fluorinate the chromium oxide catalyst precursor for 2 h; then adjust the volume ratio of N2 / HF to 0:1 and still maintain a gas flow rate of 100 ml / min to continue to partially fluorinate the chromium oxide catalyst precursor for 2 h; after the fluorination is completed, a partially fluorinated and doped chromium oxyfluoride catalyst is obtained. After the catalyst activation is completed, the reactor temperature is maintained at 165 °C and the nitrogen flow rate is maintained at 100 ml / min, waiting for feeding.

[0038] Perfluorooctyl iodide is added to the perfluorooctyl iodide vaporization tank from the perfluorooctyl iodide feeding port by a peristaltic pump at a speed of 2.5 g / min for vaporization. The temperature of the perfluorooctyl iodide vaporization tank is controlled at 180 °C. Nitrogen is introduced into the perfluorooctyl iodide vaporization tank at a speed of 75 ml / min to mix with perfluorooctyl iodide, and then the mixed gas enters the mixing tank to mix with bromine; bromine is added to the bromine vaporization tank from the bromine feeding port by an injection pump at a speed of 1.37 g / min for vaporization. The temperature of the bromine vaporization tank is controlled at 180 °C. Nitrogen is introduced into the bromine vaporization tank at a speed of 75 ml / min to mix with bromine, and then the mixed gas enters the mixing tank to mix with perfluorooctyl iodide; the temperature of the vaporization tank is controlled at 165 °C. The perfluorooctyl iodide, bromine, and nitrogen that are uniformly mixed in the vaporization tank enter a fixed-bed reactor lined with polytetrafluoroethylene with an inner diameter of 1.5 cm and a heating zone length of 60 cm filled with a rising catalyst for reaction. The temperature in the reactor is 165 °C, the pressure is atmospheric pressure, and the overall space velocity is 266 h -1, after the reaction materials were condensed and collected and washed with sodium hydroxide to remove bromine, samples were taken for gas chromatography analysis, and the results were as Figure 2 shown. The content of perfluorooctyl bromide was 61.81%, and 37.62% of the raw material perfluorooctyl iodide had not been completely converted.

[0039] Example 1

[0040] 100 g of chromium trichloride, 2.2 g of aluminum chloride, 5.4 g of lanthanum nitrate, 0.77 g of sodium silicate and 640 g of graphite powder with a particle size of 400 mesh were added to 5 L of pure water, and then ammonia water was slowly added to it until the pH value reached 14. The mixture was continuously stirred and reacted for 30 min, filtered, and the filter cake was washed with pure water until neutral to obtain an oxide catalyst precursor powder mainly composed of chromium oxide. The powder was granulated with a granulator to obtain chromium-aluminum-lanthanum oxide catalyst precursor particles. The obtained chromium-aluminum-lanthanum oxide catalyst precursor particles were placed in a muffle furnace and sintered at 400 °C for 8 h, and then taken out and cooled for use.

[0041] Place 105 ml of the obtained chromium-aluminum-lanthanum oxide catalyst precursor particles in a fixed-bed reactor lined with tetrafluoroethylene with an inner diameter of 1.5 cm and a heating zone length of 60 cm. First, pass nitrogen with a flow rate of 100 ml / min, turn on the heating, and dry the catalyst precursor. First, heat it to 100 °C and maintain it at 100 °C for 30 min, then raise the temperature to 200 °C and maintain it at 200 °C for 30 min, and then raise the temperature to 300 °C to dry the chromium-aluminum-lanthanum oxide catalyst precursor particles. After 12 h, the drying of the chromium-aluminum-lanthanum oxide catalyst precursor particles is completed. Cool down to 165 °C. First, pass a mixed gas with a volume ratio of N2 / HF of 20:1 and a gas flow rate of 100 ml / min to partially fluorinate the chromium-aluminum-lanthanum oxide catalyst precursor for 2 h; then adjust the volume ratio of N2 / HF to 10:1 and still maintain the gas flow rate at 100 ml / min to continue to partially fluorinate the chromium-aluminum-lanthanum oxide catalyst precursor for 2 h; then adjust the volume ratio of N2 / HF to 5:1 and still maintain the gas flow rate at 100 ml / min to continue to partially fluorinate the chromium-aluminum-lanthanum oxide catalyst precursor for 2 h; then adjust the volume ratio of N2 / HF to 3:1 and still maintain the gas flow rate at 100 ml / min to continue to partially fluorinate the chromium-aluminum-lanthanum oxide catalyst precursor for 2 h; then adjust the volume ratio of N2 / HF to 1:1 and still maintain the gas flow rate at 100 ml / min to continue to partially fluorinate the chromium-aluminum-lanthanum oxide catalyst precursor for 2 h; then adjust the volume ratio of N2 / HF to 0:1 and still maintain the gas flow rate at 100 ml / min to continue to partially fluorinate the chromium-aluminum-lanthanum oxide catalyst precursor for 2 h; after the fluorination is completed, a partially fluorinated, aluminum-lanthanum-doped chromium oxyfluoride catalyst is obtained. After the catalyst activation is completed, the reactor temperature is maintained at 165 °C and the nitrogen flow rate is maintained at 100 ml / min, waiting for feeding.

[0042] Perfluorooctyl iodide was added to the perfluorooctyl iodide vaporization tank from the perfluorooctyl iodide feeding port at a rate of 2.5 g / min by a peristaltic pump for vaporization. The temperature of the perfluorooctyl iodide vaporization tank was controlled at 180 °C. Nitrogen was introduced into the perfluorooctyl iodide vaporization tank at a rate of 75 ml / min to be mixed with perfluorooctyl iodide, and then the mixed gas entered the mixing tank to be mixed with bromine. Bromine was added to the bromine vaporization tank from the bromine feeding port at a rate of 1.37 g / min by an injection pump for vaporization. The temperature of the bromine vaporization tank was controlled at 180 °C. Nitrogen was introduced into the bromine vaporization tank at a rate of 75 ml / min to be mixed with bromine, and then the mixed gas entered the mixing tank to be mixed with perfluorooctyl iodide. The temperature of the vaporization tank was controlled at 165 °C. The perfluorooctyl iodide, bromine and nitrogen that were uniformly mixed in the vaporization tank entered a lined tetrafluoro fixed bed reactor with an inner diameter of 1.5 cm and a heating zone length of 60 cm filled with a catalyst for reaction. The temperature in the reactor was 165 °C, the pressure was atmospheric pressure, and the overall space velocity was 266 h -1 , after the reaction materials were condensed and collected and washed with sodium hydroxide to remove bromine, samples were taken for gas chromatography analysis. The results were as Figure 3 shown. The content of perfluorooctyl bromide reached 97.38%, and all the raw material perfluorooctyl iodide was converted.

[0043] In order to investigate the catalyst life and accumulate and separate and purify the materials, the whole reaction lasted for 200 h. After 200 h, samples were taken, washed with sodium hydroxide to remove bromine, and then subjected to gas chromatography analysis. The results were as Figure 4 shown. The content of perfluorooctyl bromide was still 97.2%, and all the raw material perfluorooctyl iodide was converted. A total of 30 Kg of raw materials were consumed in the whole process. The crude perfluorooctyl bromide obtained was rectified and purified after washing the bromine with sodium hydroxide, and 25.25 Kg of perfluorooctyl bromide product with a purity of 99.5% was obtained, and the total yield was 93%.

[0044] Example 2

[0045] 105 ml of the chromium-aluminum-lanthanum oxide catalyst precursor particles obtained in Example 1 were placed in a fixed-bed reactor lined with polytetrafluoroethylene with an inner diameter of 1.5 cm and a heating zone length of 60 cm. First, nitrogen was passed through at a flow rate of 100 ml / min, and heating was started to dry the catalyst precursor. It was first heated to 100 °C and maintained at 100 °C for 30 min, then heated to 200 °C and maintained at 200 °C for 30 min, and then heated to 300 °C to dry the chromium-aluminum-lanthanum oxide catalyst precursor particles. After 12 h, the drying of the chromium-aluminum-lanthanum oxide catalyst precursor particles was completed. The temperature was lowered to 165 °C. First, a mixed gas with a volume ratio of N2 / HF of 20:1 was passed through at a gas flow rate of 100 ml / min to partially fluorinate the chromium-aluminum-lanthanum oxide catalyst precursor for 2 h; then the volume ratio of N2 / HF was adjusted to 10:1 and the gas flow rate remained 100 ml / min to continue to partially fluorinate the chromium-aluminum-lanthanum oxide catalyst precursor for 2 h; then the volume ratio of N2 / HF was adjusted to 5:1 and the gas flow rate remained 100 ml / min to continue to partially fluorinate the chromium-aluminum-lanthanum oxide catalyst precursor for 2 h; then the volume ratio of N2 / HF was adjusted to 3:1 and the gas flow rate remained 100 ml / min to continue to partially fluorinate the chromium-aluminum-lanthanum oxide catalyst precursor for 2 h; then the volume ratio of N2 / HF was adjusted to 1:1 and the gas flow rate remained 100 ml / min to continue to partially fluorinate the chromium-aluminum-lanthanum oxide catalyst precursor for 2 h; then the volume ratio of N2 / HF was adjusted to 0:1 and the gas flow rate remained 100 ml / min to continue to partially fluorinate the chromium-aluminum-lanthanum oxide catalyst precursor for 2 h; after the fluorination was completed, a partially fluorinated, aluminum-lanthanum-doped chromium oxyfluoride catalyst was obtained. After the catalyst activation was completed, the reactor temperature was maintained at 165 °C and the nitrogen flow rate was maintained at 100 ml / min, waiting for feeding.

[0046] Perfluorooctyl iodide was added into the perfluorooctyl iodide vaporization tank from the perfluorooctyl iodide feeding port at a speed of 3.5 g / min by a peristaltic pump for vaporization. The temperature of the perfluorooctyl iodide vaporization tank was controlled at 180 °C. Nitrogen was introduced into the perfluorooctyl iodide vaporization tank at a speed of 105 ml / min to be mixed with perfluorooctyl iodide. Then the mixed gas entered the mixing tank to be mixed with bromine. Bromine was added into the bromine vaporization tank from the bromine feeding port at a speed of 1.92 g / min by an injection pump for vaporization. The temperature of the bromine vaporization tank was controlled at 180 °C. Nitrogen was introduced into the bromine vaporization tank at a speed of 105 ml / min to be mixed with bromine. Then the mixed gas entered the mixing tank to be mixed with perfluorooctyl iodide. The temperature of the vaporization tank was controlled at 165 °C. The perfluorooctyl iodide, bromine and nitrogen that were uniformly mixed in the vaporization tank entered a lined tetrafluoro fixed-bed reactor with an inner diameter of 1.5 cm and a heating zone length of 60 cm filled with a catalyst for reaction. The temperature in the reactor was 165 °C, the pressure was atmospheric pressure, and the overall space velocity was 372.4 h -1 . After the reaction materials were condensed and collected and washed with sodium hydroxide to remove bromine, samples were taken for gas chromatography analysis. The results were as Figure 5 shown. The content of perfluorooctyl bromide reached 94.48%, and 4.32% of the raw material perfluorooctyl iodide was not completely converted.

[0047] Example 3

[0048] 105 ml of the chromium-aluminum-lanthanum oxide catalyst precursor particles obtained in Example 1 were placed in a fixed-bed reactor lined with polytetrafluoroethylene with an inner diameter of 1.5 cm and a heating zone length of 60 cm. First, nitrogen was passed through at a nitrogen flow rate of 100 ml / min, and heating was started to dry the catalyst precursor. It was first heated to 100 °C and maintained at 100 °C for 30 min, then heated to 200 °C and maintained at 200 °C for 30 min, and then heated to 300 °C to dry the chromium-aluminum-lanthanum oxide catalyst precursor particles. After 12 h, the drying of the chromium-aluminum-lanthanum oxide catalyst precursor particles was completed. The temperature was lowered to 165 °C. First, a mixed gas with a volume ratio of N2 / HF of 20:1 and a gas flow rate of 100 ml / min was passed through to partially fluorinate the chromium-aluminum-lanthanum oxide catalyst precursor for 2 h; then the volume ratio of N2 / HF was adjusted to 10:1 and the gas flow rate remained 100 ml / min to continue to partially fluorinate the chromium-aluminum-lanthanum oxide catalyst precursor for 2 h; then the volume ratio of N2 / HF was adjusted to 5:1 and the gas flow rate remained 100 ml / min to continue to partially fluorinate the chromium-aluminum-lanthanum oxide catalyst precursor for 2 h; then the volume ratio of N2 / HF was adjusted to 3:1 and the gas flow rate remained 100 ml / min to continue to partially fluorinate the chromium-aluminum-lanthanum oxide catalyst precursor for 2 h; then the volume ratio of N2 / HF was adjusted to 1:1 and the gas flow rate remained 100 ml / min to continue to partially fluorinate the chromium-aluminum-lanthanum oxide catalyst precursor for 2 h; then the volume ratio of N2 / HF was adjusted to 0:1 and the gas flow rate remained 100 ml / min to continue to partially fluorinate the chromium-aluminum-lanthanum oxide catalyst precursor for 2 h; after the fluorination was completed, a partially fluorinated, aluminum-lanthanum-doped chromium oxyfluoride catalyst was obtained. After the catalyst activation was completed, the reactor temperature was maintained at 165 °C and the nitrogen flow rate was maintained at 100 ml / min, waiting for feeding.

[0049] Perfluorooctyl iodide was added into the perfluorooctyl iodide vaporization tank from the perfluorooctyl iodide feeding port at a rate of 1.5 g / min by a peristaltic pump for vaporization. The temperature of the perfluorooctyl iodide vaporization tank was controlled at 180 °C. Nitrogen was introduced into the perfluorooctyl iodide vaporization tank at a rate of 45 ml / min to be mixed with perfluorooctyl iodide, and then the mixed gas entered the mixing tank to be mixed with bromine. Bromine was added into the bromine vaporization tank from the bromine feeding port at a rate of 0.82 g / min by an injection pump for vaporization. The temperature of the bromine vaporization tank was controlled at 180 °C. Nitrogen was introduced into the bromine vaporization tank at a rate of 45 ml / min to be mixed with bromine, and then the mixed gas entered the mixing tank to be mixed with perfluorooctyl iodide. The temperature of the vaporization tank was controlled at 165 °C. The perfluorooctyl iodide, bromine and nitrogen that were uniformly mixed in the vaporization tank entered a lined tetrafluoro fixed-bed reactor with an inner diameter of 1.5 cm and a heating zone length of 60 cm filled with a catalyst for reaction. The temperature in the reactor was 165 °C, the pressure was atmospheric pressure, and the overall space velocity was 159.6 h -1 . After the reaction materials were condensed and collected and washed with sodium hydroxide to remove bromine, samples were taken for gas chromatography analysis. The results are as Figure 6 shown. The content of perfluorooctyl bromide reached 97.76%, and the raw material perfluorooctyl iodide was completely converted.

[0050] Example 4

[0051] 490 ml of the chromium-aluminum-lanthanum oxide catalyst precursor particles obtained in Example 1 were placed in a fixed-bed reactor lined with polytetrafluoroethylene with an inner diameter of 2.5 cm and a heating zone length of 100 cm. First, nitrogen was passed through at a nitrogen flow rate of 500 ml / min, and heating was started to dry the catalyst precursor. First, it was heated to 100 °C and maintained at 100 °C for 30 min, then heated to 200 °C and maintained at 200 °C for 30 min, and then heated to 300 °C to dry the chromium-aluminum-lanthanum oxide catalyst precursor particles. After 12 h, the drying of the chromium-aluminum-lanthanum oxide catalyst precursor particles was completed. The temperature was lowered to 165 °C. First, a mixed gas with a volume ratio of N2 / HF of 20:1 and a gas flow rate of 500 ml / min was passed through to partially fluorinate the chromium-aluminum-lanthanum oxide catalyst precursor for 2 h; then the volume ratio of N2 / HF was adjusted to 10:1 and the gas flow rate remained 500 ml / min to continue the partial fluorination of the chromium-aluminum-lanthanum oxide catalyst precursor for 2 h; then the volume ratio of N2 / HF was adjusted to 5:1 and the gas flow rate remained 500 ml / min to continue the partial fluorination of the chromium-aluminum-lanthanum oxide catalyst precursor for 2 h; then the volume ratio of N2 / HF was adjusted to 3:1 and the gas flow rate remained 500 ml / min to continue the partial fluorination of the chromium-aluminum-lanthanum oxide catalyst precursor for 2 h; then the volume ratio of N2 / HF was adjusted to 1:1 and the gas flow rate remained 500 ml / min to continue the partial fluorination of the chromium-aluminum-lanthanum oxide catalyst precursor for 2 h; then the volume ratio of N2 / HF was adjusted to 0:1 and the gas flow rate remained 500 ml / min to continue the partial fluorination of the chromium-aluminum-lanthanum oxide catalyst precursor for 2 h; after the fluorination was completed, a partially fluorinated, aluminum-lanthanum-doped chromium oxyfluoride catalyst was obtained. After the catalyst activation was completed, the reactor temperature was maintained at 165 °C and the nitrogen flow rate was maintained at 100 ml / min, waiting for feeding.

[0052] Perfluorooctyl iodide was added to the perfluorooctyl iodide vaporization tank from the perfluorooctyl iodide feeding port at a rate of 11.6 g / min by a peristaltic pump for vaporization. The temperature of the perfluorooctyl iodide vaporization tank was controlled at 180 °C. Nitrogen was introduced into the perfluorooctyl iodide vaporization tank at a rate of 345 ml / min to be mixed with perfluorooctyl iodide, and then the mixed gas entered the mixing tank to be mixed with bromine. Bromine was added to the bromine vaporization tank from the bromine feeding port at a rate of 6.35 g / min by an injection pump for vaporization. The temperature of the bromine vaporization tank was controlled at 180 °C. Nitrogen was introduced into the bromine vaporization tank at a rate of 345 ml / min to be mixed with bromine, and then the mixed gas entered the mixing tank to be mixed with perfluorooctyl iodide. The temperature of the vaporization tank was controlled at 165 °C. The perfluorooctyl iodide, bromine and nitrogen that were uniformly mixed in the vaporization tank entered a lined tetrafluoro fixed-bed reactor with an inner diameter of 2.5 cm and a heating zone length of 100 cm filled with a catalyst for reaction. The temperature in the reactor was 165 °C, the pressure was atmospheric pressure, and the overall space velocity was 266 h -1 , after the reaction materials were condensed and collected and washed with sodium hydroxide to remove bromine, samples were taken for gas chromatography analysis, and the results were as Figure 7 shown. The content of perfluorooctyl bromide was less than 97.52%, and all perfluorooctyl iodide was converted.

[0053] Example 5

[0054] 100 g of chromium trichloride, 5.4 g of lanthanum nitrate, 0.77 g of sodium silicate and 640 g of graphite powder with a particle size of 400 mesh were added to 5 L of pure water, and then ammonia water was slowly added thereto until the pH value reached 14. Stirring reaction was continued for 30 min, followed by filtration. The filter cake was washed with pure water until neutral to obtain a chromium lanthanum oxide catalyst precursor powder mainly composed of chromium oxide. Granulation was carried out with a granulator to obtain chromium aluminum lanthanum oxide catalyst precursor particles. The obtained chromium aluminum lanthanum oxide catalyst precursor particles were placed in a muffle furnace and sintered at 400 °C for 8 h, and then taken out and cooled for standby.

[0055] Place 105 ml of the obtained chromium lanthanum oxide catalyst precursor particles in a fixed-bed reactor lined with polytetrafluoroethylene with an inner diameter of 1.5 cm and a heating zone length of 60 cm. First, pass nitrogen with a flow rate of 100 ml / min, turn on the heating, and dry the catalyst precursor. First, heat to 100 °C and hold at 100 °C for 30 min, then raise the temperature to 200 °C and hold at 200 °C for 30 min, and then raise the temperature to 300 °C to dry the chromium lanthanum oxide catalyst precursor particles. After 12 h, the drying of the chromium aluminum lanthanum oxide catalyst precursor particles is completed. Cool down to 165 °C. First, pass a mixed gas with a volume ratio of N2 / HF of 20:1 and a gas flow rate of 100 ml / min to partially fluorinate the chromium lanthanum oxide catalyst precursor for 2 h; then adjust the volume ratio of N2 / HF to 10:1 and still maintain the gas flow rate at 100 ml / min to continue to partially fluorinate the chromium lanthanum oxide catalyst precursor for 2 h; then adjust the volume ratio of N2 / HF to 5:1 and still maintain the gas flow rate at 100 ml / min to continue to partially fluorinate the chromium aluminum lanthanum oxide catalyst precursor for 2 h; then adjust the volume ratio of N2 / HF to 3:1 and still maintain the gas flow rate at 100 ml / min to continue to partially fluorinate the chromium lanthanum oxide catalyst precursor for 2 h; then adjust the volume ratio of N2 / HF to 1:1 and still maintain the gas flow rate at 100 ml / min to continue to partially fluorinate the chromium lanthanum oxide catalyst precursor for 2 h; then adjust the volume ratio of N2 / HF to 0:1 and still maintain the gas flow rate at 100 ml / min to continue to partially fluorinate the chromium lanthanum oxide catalyst precursor for 2 h; after the fluorination is completed, a partially fluorinated and lanthanum-doped chromium oxyfluoride catalyst is obtained. After the catalyst activation is completed, the reactor temperature is maintained at 165 °C and the nitrogen flow rate is maintained at 100 ml / min, waiting for feeding.

[0056] Perfluorooctyl iodide is added to the perfluorooctyl iodide vaporization tank through a peristaltic pump at a speed of 2.5 g / min from the perfluorooctyl iodide feeding port for vaporization. The temperature of the perfluorooctyl iodide vaporization tank is controlled at 180 °C, and nitrogen is introduced into the perfluorooctyl iodide vaporization tank at a speed of 75 ml / min to mix with perfluorooctyl iodide, and then the mixed gas enters the mixing tank to mix with bromine; bromine is added to the bromine vaporization tank through an injection pump at a speed of 1.37 g / min from the bromine feeding port for vaporization. The temperature of the bromine vaporization tank is controlled at 180 °C, and nitrogen is introduced into the bromine vaporization tank at a speed of 75 ml / min to mix with bromine, and then the mixed gas enters the mixing tank to mix with perfluorooctyl iodide; the temperature of the vaporization tank is controlled at 165 °C. The perfluorooctyl iodide, bromine, and nitrogen that are uniformly mixed in the vaporization tank enter the above-mentioned fixed-bed reactor lined with polytetrafluoroethylene with an inner diameter of 1.5 cm and a heating zone length of 60 cm filled with the catalyst for reaction. The temperature in the reactor is 165 °C, the pressure is atmospheric pressure, and the overall space velocity is 266 h-1 , after the reaction materials were condensed and collected and washed with sodium hydroxide to remove bromine, samples were taken for gas chromatography analysis. The results are as Figure 8 shown. The content of perfluorooctyl bromide reached 91.35%, and 7.74% of the raw material perfluorooctyl iodide was not completely converted.

[0057] Example 6

[0058] 10 g of chromium trichloride, 0.226 g of zinc chloride, 0.298 g of manganese nitrate, 0.081 g of sodium silicate and 60 g of graphite powder with a particle size of 400 mesh were added to 1 L of pure water. Then, ammonia water was slowly added to it until the pH value reached 14, and the reaction was continued with stirring for 30 min. After filtration, the filter cake was washed with pure water until neutral to obtain a chromium-manganese-zinc oxide catalyst precursor powder mainly composed of chromium oxide. The powder was granulated with a granulator to obtain chromium-manganese-zinc oxide catalyst precursor particles. The obtained chromium-aluminum-lanthanum oxide catalyst precursor particles were placed in a muffle furnace and sintered at 400 °C for 8 h, and then taken out and cooled for use.

[0059] Place 105 ml of the obtained chromium manganese zinc oxide catalyst precursor particles into a fixed-bed reactor lined with polytetrafluoroethylene with an inner diameter of 1.5 cm and a heating zone length of 60 cm. First, pass nitrogen with a flow rate of 100 ml / min, turn on the heating, and dry the catalyst precursor. First, heat it to 100 °C and maintain it at 100 °C for 30 min, then raise the temperature to 200 °C and maintain it at 200 °C for 30 min, and then raise the temperature to 300 °C to dry the chromium manganese zinc oxide catalyst precursor particles. After 12 h, the drying of the chromium manganese zinc oxide catalyst precursor particles is completed. Cool down to 165 °C. First, pass a mixed gas with a volume ratio of N2 / HF of 20:1 and a gas flow rate of 100 ml / min to partially fluorinate the chromium manganese zinc oxide catalyst precursor for 2 h; then adjust the volume ratio of N2 / HF to 10:1 and still maintain the gas flow rate at 100 ml / min to continue to partially fluorinate the chromium manganese zinc oxide catalyst precursor for 2 h; then adjust the volume ratio of N2 / HF to 5:1 and still maintain the gas flow rate at 100 ml / min to continue to partially fluorinate the chromium manganese zinc oxide catalyst precursor for 2 h; then adjust the volume ratio of N2 / HF to 3:1 and still maintain the gas flow rate at 100 ml / min to continue to partially fluorinate the chromium manganese zinc oxide catalyst precursor for 2 h; then adjust the volume ratio of N2 / HF to 1:1 and still maintain the gas flow rate at 100 ml / min to continue to partially fluorinate the chromium manganese zinc oxide catalyst precursor for 2 h; then adjust the volume ratio of N2 / HF to 0:1 and still maintain the gas flow rate at 100 ml / min to continue to partially fluorinate the chromium manganese zinc oxide catalyst precursor for 2 h; after the fluorination is completed, a partially fluorinated, manganese zinc-doped chromium oxyfluoride catalyst is obtained. After the catalyst activation is completed, the reactor temperature is maintained at 165 °C and the nitrogen flow rate is maintained at 100 ml / min, waiting for feeding.

[0060] Perfluorooctyl iodide was added into the perfluorooctyl iodide vaporization tank from the perfluorooctyl iodide feeding port at a speed of 2.5 g / min by a peristaltic pump for vaporization. The temperature of the perfluorooctyl iodide vaporization tank was controlled at 180 °C. Nitrogen was introduced into the perfluorooctyl iodide vaporization tank at a speed of 75 ml / min to be mixed with perfluorooctyl iodide, and then the mixed gas entered the mixing tank to be mixed with bromine. Bromine was added into the bromine vaporization tank from the bromine feeding port at a speed of 1.37 g / min by an injection pump for vaporization. The temperature of the bromine vaporization tank was controlled at 180 °C. Nitrogen was introduced into the bromine vaporization tank at a speed of 75 ml / min to be mixed with bromine, and then the mixed gas entered the mixing tank to be mixed with perfluorooctyl iodide. The temperature of the vaporization tank was controlled at 165 °C. The perfluorooctyl iodide, bromine and nitrogen that were uniformly mixed in the vaporization tank entered a lined tetrafluoro fixed bed reactor with an inner diameter of 1.5 cm and a heating zone length of 60 cm filled with a catalyst for reaction. The temperature in the reactor was 165 °C, the pressure was atmospheric pressure, and the overall space velocity was 266 h -1 , after the reaction materials were condensed and collected and washed with sodium hydroxide to remove bromine, samples were taken for gas chromatography analysis, and the results were as Figure 9 shown. The content of perfluorooctyl bromide reached 86.99%, and 12.27% of the raw material perfluorooctyl iodide was not completely converted.

[0061] Example 7

[0062] 10 g of chromium trichloride, 0.216 g of cobalt chloride, 0.515 g of antimony nitrate, 0.081 g of sodium silicate and 60 g of graphite powder with a particle size of 400 mesh were added to 1 L of pure water, and then ammonia water was slowly added thereto until the pH value reached 14. The mixture was continuously stirred and reacted for 30 min, filtered, and the filter cake was washed with pure water until neutral to obtain a chromium cobalt antimony oxide catalyst precursor powder mainly composed of chromium oxide. The powder was granulated by a granulator to obtain chromium cobalt antimony oxide catalyst precursor particles. The obtained chromium cobalt antimony oxide catalyst precursor particles were placed in a muffle furnace and sintered at 400 °C for 8 h, and then taken out and cooled for use.

[0063] Place 105 ml of the obtained chromium-manganese-zinc oxide catalyst precursor particles in a fixed-bed reactor lined with polytetrafluoroethylene with an inner diameter of 1.5 cm and a heating zone length of 60 cm. First, pass nitrogen with a flow rate of 100 ml / min, turn on the heating, and dry the catalyst precursor. First, heat it to 100 °C and hold it at 100 °C for 30 min, then raise the temperature to 200 °C and hold it at 200 °C for 30 min, and then raise the temperature to 300 °C to dry the chromium-cobalt-antimony oxide catalyst precursor particles. After 12 h, the drying of the chromium-cobalt-antimony oxide catalyst precursor particles is completed. Cool down to 165 °C. First, pass a mixed gas with a volume ratio of N2 / HF of 20:1 and a gas flow rate of 100 ml / min to partially fluorinate the chromium-cobalt-antimony oxide catalyst precursor for 2 h; then adjust the volume ratio of N2 / HF to 10:1 and still maintain the gas flow rate at 100 ml / min to continue to partially fluorinate the chromium-cobalt-antimony oxide catalyst precursor for 2 h; then adjust the volume ratio of N2 / HF to 5:1 and still maintain the gas flow rate at 100 ml / min to continue to partially fluorinate the chromium-cobalt-antimony oxide catalyst precursor for 2 h; then adjust the volume ratio of N2 / HF to 3:1 and still maintain the gas flow rate at 100 ml / min to continue to partially fluorinate the chromium-cobalt-antimony oxide catalyst precursor for 2 h; then adjust the volume ratio of N2 / HF to 1:1 and still maintain the gas flow rate at 100 ml / min to continue to partially fluorinate the chromium-cobalt-antimony oxide catalyst precursor for 2 h; then adjust the volume ratio of N2 / HF to 0:1 and still maintain the gas flow rate at 100 ml / min to continue to partially fluorinate the chromium-cobalt-antimony oxide catalyst precursor for 2 h; after the fluorination is completed, a partially fluorinated, cobalt-antimony-doped chromium oxyfluoride catalyst is obtained. After the catalyst activation is completed, the reactor temperature is maintained at 165 °C and the nitrogen flow rate is maintained at 100 ml / min, waiting for feeding.

[0064] Perfluorooctyl iodide was added into the perfluorooctyl iodide vaporization tank through the perfluorooctyl iodide feeding port by a peristaltic pump at a rate of 2.5 g / min for vaporization. The temperature of the perfluorooctyl iodide vaporization tank was controlled at 180 °C. Nitrogen was introduced into the perfluorooctyl iodide vaporization tank at a rate of 75 ml / min to be mixed with perfluorooctyl iodide, and then the mixed gas entered the mixing tank to be mixed with bromine. Bromine was added into the bromine vaporization tank through the bromine feeding port by an injection pump at a rate of 1.37 g / min for vaporization. The temperature of the bromine vaporization tank was controlled at 180 °C. Nitrogen was introduced into the bromine vaporization tank at a rate of 75 ml / min to be mixed with bromine, and then the mixed gas entered the mixing tank to be mixed with perfluorooctyl iodide. The temperature of the vaporization tank was controlled at 165 °C. The perfluorooctyl iodide, bromine and nitrogen that were uniformly mixed in the vaporization tank entered a lined tetrafluoro fixed-bed reactor with an inner diameter of 1.5 cm and a heating zone length of 60 cm filled with a catalyst for reaction. The temperature in the reactor was 165 °C, the pressure was atmospheric pressure, and the overall space velocity was 266 h -1 . After the reaction materials were condensed and collected and washed with sodium hydroxide to remove bromine, samples were taken for gas chromatography analysis. The results were as Figure 10 shown. The content of perfluorooctyl bromide reached 92.17%, and 6.52% of the raw material perfluorooctyl iodide was not completely converted.

Claims

1. Application of a chromium-based catalyst in the preparation of perfluorooctyl bromide, a key material for fluorocarbon nano-contrast agents, by catalytic bromination of perfluorooctyl iodide, characterized in that: A chromium-based catalyst is loaded into a fixed-bed reactor. Perfluorooctyl iodide and bromine are vaporized respectively and then mixed with a carrier gas and enter the fixed-bed reactor together, and a bromination reaction is carried out under normal pressure to obtain perfluorooctyl bromide; the chromium-based catalyst is prepared by sintering a chromium-based oxide precursor obtained by co-precipitation of a soluble chromium salt, a soluble metal salt promoter, a pore-forming agent and graphite, and then partially fluorinating with a N2 / HF mixed gas. The metal in the soluble metal salt promoter is selected from lanthanum; or lanthanum and aluminum; or zinc and manganese; or cobalt and antimony.

2. The application according to claim 1, wherein: In the bromination reaction, the molar ratio of bromine to perfluorooctyl iodide is not less than 1; the space velocity is 100 - 500 h -1 ; the reaction temperature is 100 - 200 °C.

3. The application according to claim 2, characterized in that: In the bromination reaction, the molar ratio of bromine to perfluorooctyl iodide is 1.5 to 2.5; the space velocity is 150 to 300 h -1 ; the reaction temperature is 150 to 170 °C.

4. The application according to claim 1, characterized in that: The soluble chromium salt is selected from one of chromium chloride and chromium nitrate; the soluble metal salt promoter is selected from one of their chlorides and nitrates; the pore-forming agent is selected from one of sodium silicate and silicon tetrachloride.

5. The application according to claim 1, wherein: The alkali added during the co-precipitation process is ammonia water, and the adjusted pH is 14; the sintering temperature is 350-450 °C and the time is 6-12 h.

6. The application according to claim 1, wherein: The fluorination process is to carry out gradient fluorination of the chromium-based oxide precursor with N2 / HF mixed gases of different concentrations. During the gradient fluorination process, the volume ratio of N2 to HF decreases gradually in a gradient manner.

7. The application according to claim 1, characterized in that: The molar ratio of chromium in the soluble chromium salt to the metal in the soluble metal salt promoter is 90-99:1-10.

8. The application according to claim 7, characterized in that: The molar ratio of chromium in the soluble chromium salt to the metal in the soluble metal salt promoter is 92-97:3-8.

9. The application according to claim 1, characterized in that: The molar ratio of chromium in the soluble chromium salt to the pore-forming agent is 90-100:1; the molar ratio of chromium in the soluble chromium salt to graphite is 1:70-90.

10. The application according to any one of claims 1-9, characterized in that: The metal in the soluble metal salt promoter is lanthanum and aluminum, and the molar ratio of lanthanum to aluminum is 1:1, and the total molar amount of lanthanum and aluminum to chromium is 5:95.

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