Method for improving the activity and stability of catalysts in the fluorochloride exchange reaction of HFC-23

Abstract

The present invention discloses a method for improving the activity and stability of a catalyst in the fluorine-chlorine exchange reaction of HFC-23. In the fluorine-chlorine exchange reaction, trifluoromethane and trichloromethane are used as raw materials to prepare difluorochloromethane (HCFC-22) and chlorodifluoromethane (HCFC-21). The catalyst is an aluminum-based catalyst. During the fluorine-chlorine exchange reaction, a fluorochloroaluminum adsorption active site of trifluoromethane is formed on the catalyst, and the fluorochloroaluminum adsorption active site of trifluoromethane accounts for 5-15% of the aluminum species on the catalyst surface. The present invention has an appropriate proportion of the fluorochloroaluminum adsorption active site of trifluoromethane, which can improve the catalytic activity of the catalyst, and further enhance the catalytic activity and catalytic stability of the catalyst by increasing the number of trifluoromethane adsorption active sites.

Description

Technical Field

[0001] The present invention relates to a fluorine-chlorine exchange reaction, and particularly to a method for improving the activity and stability of a catalyst in the fluorine-chlorine exchange reaction of trifluoromethane and trichloromethane. Background Art

[0002] HFC-23 (CHF3, trifluoromethane, R23) is an inevitable by-product in the industrial production of HCFC-22 (difluorochloromethane, R22 or CHClF2), and its GWP value is as high as 14800. The annual by-production of HFC-23 in China accounts for about 70% of the global production. Calculated based on an annual output of 15,000 tons, it is equivalent to 220 million tons of CO2 in terms of equivalent. According to international laws such as the Kigali Amendment, since January 1, 2020, HFC-23 must be destroyed as much as possible in the same year, and HFC-23 is generally treated by high-temperature incineration at 1200 °C. This method has large pollution and high operating costs. Therefore, the resource utilization of HFC-23 is an important topic in realizing energy conservation and emission reduction.

[0003] Currently, the methods for the resource utilization of HFC-23 include: pyrolysis to prepare TFE and hexafluoropropene (HFP), co-pyrolysis with CH4 to prepare vinylidene fluoride (VDF), fluorine-chlorine exchange with CHCl3 to prepare HCFC-22, preparation of CF3I, etc. Among them, the reaction of HFC-23 with CHCl3 to generate HCFC-22 and HCFC-21 by fluorine-chlorine exchange has mild conditions and high product selectivity, and is more suitable for industrial application.

[0004] Patent CN104628513A discloses a method for converting trifluoromethane and trichloromethane into HCFC-22 under the catalysis of a Lewis acid, but the catalyst has a short lifespan, which affects industrial application.

[0005] Patent CN109748775A continuously adds promoting gases such as Cl2, CCl4, H2, O2, CO2, O3, and NO x in the reaction of CHF3 and CHCl3, which improves the lifespan and stability of the catalyst, but the improvement is limited.

[0006] Patent CN112979410A regenerates by timely introducing a regeneration gas according to the characteristics shown by the reaction. For example, when the selectivity of HCFC-22 drops to 46% - 48%, a decarbonizing gas accounting for 0.5n% of the volume of the mixed gas is introduced for a duration of 10n hours, where n is the number of regeneration times and n ≤ 6. When n > 6, a decarbonizing gas accounting for 1% - 3% of the volume of the mixed gas is continuously introduced. Using this method of selectively introducing the decarbonizing gas, under the conditions of a reaction temperature of 310 °C, a pressure of 1 bar, and a residence time of 5 s, the catalyst lifespan reaches 2507 h, the conversion rate of trifluoromethane is 25.1%, and the selectivity of HCFC-22 is 43.3%.

[0007] Patent CN112973685A discloses that the Co / AlF3-supported noble metal composite catalyst continuously introduces H2 in situ to eliminate carbon deposition at a temperature of 330 °C, a pressure of 1 bar, and a residence time of 5 s. The catalyst life reaches 2145 h, the raw material conversion rate is 26.7%, and the selectivity for HCFC-22 is 44.6%.

[0008] In summary, in the prior art, either the original active sites are restored by carbon burning, or the anti-carbon deposition active sites are added to improve the activity and stability of the catalyst. These two methods only alleviate the reduction rate of active sites in the reaction and cannot increase the total number of trifluoromethane active sites on the catalyst surface. Eventually, the catalyst will gradually deactivate due to the coverage of the active sites on the surface. This not only cannot significantly improve the conversion rate of R23, but also cannot significantly increase the life of the catalyst.

[0009] Therefore, it is of great significance to improve the activity and stability of the catalyst by increasing the proportion and number of active sites on the catalyst surface. Summary of the Invention

[0010] To solve the above technical problems, the present invention proposes a method for improving the activity and stability of the catalyst in the fluorine-chlorine exchange reaction of HFC-23 by controlling the proportion of the fluorine-chlorine-aluminum adsorption active sites of trifluoromethane and increasing the number of the fluorine-chlorine-aluminum adsorption active sites of trifluoromethane.

[0011] The object of the present invention is achieved by the following technical solutions:

[0012] A method for improving the activity and stability of the catalyst in the fluorine-chlorine exchange reaction of HFC-23, wherein the fluorine-chlorine exchange reaction uses trifluoromethane and trichloromethane as raw materials to prepare dichlorofluoromethane (HCFC-22) and chlorodifluoromethane (HCFC-21). In particular: the catalyst includes an aluminum-based catalyst. During the fluorine-chlorine exchange reaction, the fluorine-chlorine-aluminum adsorption active sites of trifluoromethane are formed on the catalyst, and the fluorine-chlorine-aluminum adsorption active sites of trifluoromethane account for 1-20% of the aluminum species on the catalyst surface.

[0013] The catalyst is selected from at least one of Al2O3, AlF3, Cr2O3-Al2O3, Cr2O3-AlF3, MgO-Al2O3 or MgO-AlF3.

[0014] It has been found through research in the present invention that in the fluorine-chlorine exchange reaction of HFC-23, the catalyst will first adsorb chloroform to form a fluorine-chlorine-aluminum adsorption active site for trifluoromethane, which can solve the problem of difficult activation of trifluoromethane. The fluorine-chlorine-aluminum adsorption active site of trifluoromethane can be expressed as AlClxFy, where 0.01 ≤ x ≤ 0.5 and x + y = 3. When the proportion of the fluorine-chlorine-aluminum adsorption active site of trifluoromethane in the aluminum species on the catalyst surface is within a reasonable range, the catalyst has the maximum catalytic activity. For example, when the proportion of the trifluoromethane adsorption active site AlClxFy in the aluminum species on the catalyst surface < 1%, the conversion rate of R23 in the reaction is very low; when the proportion of the trifluoromethane adsorption active site AlClxFy in the total active sites of the catalyst > 20%, the catalyst is prone to deactivation.

[0015] The aluminum species on the catalyst surface are aluminum-containing coordination compounds formed during the preparation, pretreatment of the catalyst, and adsorption of the raw material gas, such as ACl3, AlF3, AlClxFy, AlOxCly, AlOxFy, etc.

[0016] Therefore, preferably, the proportion of the fluorine-chlorine-aluminum adsorption active site of trifluoromethane in the aluminum species on the catalyst surface is 1-20%, more preferably 5-15%;

[0017] Further research in the present invention has found that when trifluoromethane and chloroform come into contact with the catalyst on the surface of the catalyst bed, whether trifluoromethane and chloroform are evenly dispersed greatly affects the proportion of the fluorine-chlorine-aluminum adsorption active site of trifluoromethane. Therefore, the present invention controls the proportion of the fluorine-chlorine-aluminum adsorption active site of trifluoromethane by adjusting the degree of uniform mixing of the raw material gas on the catalyst surface. Specifically:

[0018] The catalyst is arranged in the reactor in the form of a catalyst bed, and a packing layer is provided on the catalyst bed. The height of the packing layer is 1-15 cm, and preferably the height of the packing layer is 5-8 cm.

[0019] The packing in the packing layer is selected from at least one of ceramic, silicon carbide or nickel-based alloy random packing, where the nickel-based alloy includes at least one of Inconel alloy, Hastelloy alloy or Monel alloy, and the packing is preferably ceramic packing.

[0020] In addition, a gas mixer can also be set so that trifluoromethane and chloroform are mixed through the gas mixer and then contact the catalyst for reaction.

[0021] Of course, setting the packing layer and the gas mixer can both adjust the proportion of the fluorine-chlorine-aluminum adsorption active site of trifluoromethane, and one of them can be selected; in order to achieve better results, it is preferred to set the packing layer and the gas mixer at the same time.

[0022] On the premise of a specific proportion of trifluoromethane adsorption active sites, by increasing the number of trifluoromethane adsorption active sites, the catalytic activity and stability of the catalyst can be further improved. Therefore:

[0023] The catalyst of the present invention is obtained by mixing an aluminum-based catalyst and an auxiliary agent and subjecting it to fluorination pretreatment. The auxiliary agent is selected from at least one of activated carbon, silicon powder, or silica powder.

[0024] The activated carbon is selected from at least one of coconut shell activated carbon, wood activated carbon, fruit shell activated carbon, or coal-based activated carbon. The activated carbon has a high specific surface area (400 - 800 m 2 / g), and is resistant to hydrogen fluoride corrosion. After being mixed with the aluminum-based catalyst, it can become the pore framework of the aluminum-based catalyst, increasing the number of pores in the catalyst and exposing more active sites of the catalyst.

[0025] When the auxiliary agent is silicon powder or silica powder, it can react with the hydrogen fluoride atmosphere to generate gas overflow during the fluorination pretreatment, creating more pores in the aluminum-based catalyst, thereby exposing more active sites. When the auxiliary agent is silicon powder, silicon tetrafluoride gas and hydrogen escape during the fluorination process to create pores; when the auxiliary agent is silica powder, silicon tetrafluoride gas and moisture escape during the fluorination process to create pores.

[0026] The auxiliary agent accounts for 0.01 - 20.0 wt% of the total mass of the catalyst.

[0027] When the auxiliary agent is activated carbon, the addition amount of the activated carbon accounts for 1.0 - 10.0 wt% of the total mass of the catalyst; when the auxiliary agent is silicon powder or silica powder, its addition amount accounts for 5.0 - 20.0 wt% of the total mass of the catalyst.

[0028] The fluorination pretreatment process includes the following steps:

[0029] (1) Under a mixed atmosphere of 1 - 20% hydrogen fluoride and 80% - 99% nitrogen, fluorination treatment is carried out at 250 - 450 °C for 1 - 6 h;

[0030] (2) Under a hydrogen fluoride atmosphere, treatment is carried out at 300 - 500 °C for 2 - 10 hours;

[0031] (3) Purge and cool with nitrogen.

[0032] As described above, the fluorination pretreatment process of the present invention is not only for the fluorination treatment of the catalyst, but also can react with silicon powder or silica powder to create more pores for the catalyst, effectively increasing the specific surface area of the catalyst, increasing the number of trifluoromethane adsorption active sites generated by the catalyst in the HFC - 23 fluorochloride exchange reaction, and further improving the catalytic activity and stability of the catalyst.

[0033] In the above-mentioned HFC-23 fluorochloride exchange reaction process of the present invention, the molar ratio of trifluoromethane to trichloromethane is: 1 to (1:3), the reaction temperature is 250 to 400 °C, the reaction pressure is 0.1 to 3 bar, and the residence time in the catalyst bed is 4 to 50 s.

[0034] Preferably, the molar ratio of trifluoromethane to trichloromethane is: 1 to (1:2), the reaction temperature is 300 to 360 °C, the reaction pressure is 0.1 to 2 bar, and the residence time in the catalyst bed is 4 to 12 s.

[0035] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0036] 1. By setting the packing layer and / or the raw material gas mixer on the surface of the catalyst bed, the present invention realizes the adjustment of the mixing uniformity of the raw material gas on the surface of the catalyst, and controls the fluorochloroaluminum adsorption active sites of trifluoromethane to account for 1 to 20% of the aluminum species on the surface of the catalyst, which can greatly improve the catalytic activity of the catalyst.

[0037] 2. By adding additives, the present invention increases the number of catalyst pores, effectively increases the specific surface area of the catalyst, and increases the number of fluorochloroaluminum adsorption active sites of trifluoromethane generated by the catalyst in the HFC-23 fluorochloride exchange reaction on the premise of controlling the proportion of trifluoromethane adsorption active sites, thereby improving the catalytic activity and stability of the catalyst. Specific Embodiments

[0038] The present invention will be further described below with reference to specific embodiments, but the present invention is not limited to these specific embodiments. Those skilled in the art should recognize that the present invention covers all alternative solutions, improvement solutions, and equivalent solutions that may be included within the scope of the claims.

[0039] Example 1

[0040] This example provides a method for improving the activity and stability of the catalyst in the HFC-23 fluorochloride exchange reaction, which is achieved through the following steps:

[0041] S1. Take 20 g of AlF3 and put it into a ball mill. The rotation speed of the ball mill is 400 r / min. After ball milling for 4 h with both revolution and rotation, the ground sample is pressed and sieved to 10 - 20 mesh and then loaded into the reactor to form a catalyst bed;

[0042] S2. Fluorination pretreatment: 1) Under a mixed atmosphere of 10% hydrogen fluoride and 90% nitrogen, fluorination treatment is carried out at 250 °C for 2 hours; 2) Under a hydrogen fluoride atmosphere, treatment is carried out at 300 °C for 5 hours; 3) Purge and cool with nitrogen.

[0043] S3. Fluorine-chlorine exchange reaction: Chloroform is transported by a liquid pump to a vaporizer for vaporization, and then fully mixed with trifluoromethane in a gas mixer and enters a reactor. The reaction conditions are as follows: the molar ratio of HFC-23 to chloroform is 1:1.2, the reaction temperature is 340 °C, the reaction pressure is 0.1 bar, and the residence time is 6 s.

[0044] During the fluorine-chlorine exchange reaction, after 5 hours of reaction, the proportion of trifluoromethane adsorbed on the active sites, the catalytic reaction results, and the stable operation time of the catalyst are shown in Table 1 below.

[0045] Example 2

[0046] The operation of this example is the same as that of Example 1, except that: Inconel alloy random packing is added above the catalyst bed, and the packing height is 5 cm, and other conditions remain unchanged.

[0047] Example 3

[0048] The operation of this example is the same as that of Example 2, except that: The packing of the catalyst bed is changed from Inconel alloy random packing to ceramic packing, and others remain unchanged.

[0049] Example 4

[0050] The operation of this example is the same as that of Example 2, except that: The packing of the catalyst bed is changed from Inconel alloy random packing to silicon carbide packing, and others remain unchanged.

[0051] Example 5

[0052] The operation of this example is the same as that of Example 3, except that: The height of the ceramic random packing is reduced from 5 cm to 2 cm, and others remain unchanged.

[0053] Example 6

[0054] The operation of this example is the same as that of Example 3, except that: The height of the ceramic random packing is increased from 5 cm to 8 cm, and others remain unchanged.

[0055] Example 7

[0056] The operation of this example is the same as that of Example 1, except that: The catalyst is changed from AlF3 to coconut shell activated carbon / AlF3, where the coconut shell activated carbon accounts for 3.0% wt of the total mass of the catalyst, and others remain unchanged.

[0057] Example 8

[0058] The operation of this example is the same as that of Example 1, except that: The catalyst is changed from AlF3 to wood activated carbon / AlF3, where the wood activated carbon accounts for 3.0% wt of the total mass of the catalyst, and others remain unchanged.

[0059] Example 9

[0060] The operation of this example is the same as that of Example 1, except that: the catalyst is changed from AlF3 to coal-based activated carbon / AlF3, where the coal-based activated carbon accounts for 3.0% wt of the total mass of the catalyst, and the others remain unchanged.

[0061] Example 10

[0062] The operation of this example is the same as that of Example 1, except that: the catalyst is changed from AlF3 to a silica / AlF3 mixture, where the silica accounts for 10.0% wt of the total mass of the catalyst, and the others remain unchanged.

[0063] Example 11

[0064] The operation of this example is the same as that of Example 1, except that: the catalyst is changed from AlF3 to a silicon powder / AlF3 mixture, where the silicon powder accounts for 10.0% wt of the total mass of the catalyst, and the others remain unchanged.

[0065] Example 12

[0066] The operation of this example is the same as that of Example 10, except that: 5 cm high ceramic random packing is added above the catalyst bed, and the others remain unchanged.

[0067] Example 13

[0068] The operation of this example is the same as that of Example 7, except that: 5 cm high ceramic random packing is added above the catalyst bed, and the others remain unchanged.

[0069] Example 14

[0070] The operation of this example is the same as that of Example 12, except that: the catalyst is changed to a silica / 10% Cr2O3 - AlF3 mixture catalyst, where the mass ratio of Cr2O3 to AlF3 is 1:9, and after ball milling and mixing for 4 h, the silica accounts for 10.0% wt of the total mass of the catalyst, and the others remain unchanged.

[0071] Example 15

[0072] The operation of this example is the same as that of Example 12, except that: the catalyst is changed to a silica / 10% MgO - AlF3 mixture catalyst, where the mass ratio of MgO to AlF3 is 1:9, the silica accounts for 10.0% wt of the total mass of the catalyst, and after ball milling and mixing for 4 h, the others remain unchanged.

[0073] Example 16

[0074] The operation of this example is the same as that of Example 12, with the only difference being that the catalyst is changed to a silica / 10% Cr2O3 - Al2O3 mixture catalyst, where the mass ratio of Cr2O3 to Al2O3 is 1:9, and silica accounts for 10.0% wt of the total mass of the catalyst. After ball milling and mixing for 4 h, the others remain unchanged.

[0075] Comparative Example 1

[0076] Take 20 g of AlF3 catalyst and put it into a ball mill. The rotation speed of the ball mill is 400 r / min. After ball milling for 4 h with both revolution and rotation, press the ground sample into tablets, sieve them to 10 - 20 mesh, and load them into the reactor for pretreatment. The pretreatment process includes: 1) fluorination treatment at 250 °C for 2 h in a mixed atmosphere of 10% hydrogen fluoride and 90% nitrogen; 2) treatment at 300 °C for 5 h in a hydrogen fluoride atmosphere; 3) purge and cool with nitrogen. Start the reaction after the pretreatment is completed.

[0077] Chloroform is transported to the vaporizer by a liquid pump for vaporization, and then mixed with trifluoromethane in the pipeline and enters the reaction tube for reaction. The molar ratio of HFC - 23 to chloroform is 1:1.2, the reaction temperature is 340 °C, the reaction pressure is 0.1 bar, and the residence time is 6 s.

[0078] Comparative Example 2

[0079] Take 20 g of AlF3 catalyst and put it into a ball mill. The rotation speed of the ball mill is 400 r / min. After ball milling for 4 h with both revolution and rotation, press the ground sample into tablets, sieve them to 10 - 20 mesh, and load them into the reactor for pretreatment. The pretreatment process includes: 1) fluorination treatment at 250 °C for 2 h in a mixed atmosphere of 10% hydrogen fluoride and 90% nitrogen; 2) treatment at 300 °C for 5 h in a hydrogen fluoride atmosphere; 3) purge and cool with nitrogen. Start the reaction after the pretreatment is completed.

[0080] Chloroform is transported to the vaporizer by a liquid pump for vaporization, and then mixed with trifluoromethane in the pipeline and enters the reaction tube for reaction. At the same time, oxygen is continuously introduced as a reaction - promoting gas. The molar ratio of HFC - 23, chloroform, and oxygen is 1:1.2:0.2, the reaction temperature is 340 °C, the reaction pressure is 0.1 bar, and the residence time is 6 s.

[0081] After reacting for 5 h, the percentage of trifluoromethane - adsorbed active sites in the total surface aluminum species in the catalyst is obtained by XPS characterization analysis. The specific catalyst evaluation and characterization results are shown in Table 1 below:

[0082] Table 1 Evaluation of the fluorine - chlorine exchange reaction results of trifluoromethane

[0083]

[0084]

Claims

1. A method for improving the activity and stability of a catalyst in the fluorine-chlorine exchange reaction of HFC-23. The fluorine-chlorine exchange reaction uses trifluoromethane and trichloromethane as raw materials to prepare dichlorofluoromethane (HCFC-22) and chlorodifluoromethane (HCFC-21), and is characterized in that: The catalyst is obtained by mixing an aluminum-based catalyst and an auxiliary agent and then subjecting the mixture to fluorination pretreatment. The auxiliary agent is selected from at least one of activated carbon, silicon powder, or silica powder; Trifluoromethane and trichloromethane are mixed through a gas mixer and then brought into contact with the catalyst for reaction; During the fluorine-chlorine exchange reaction, fluorine-chlorine-aluminum adsorption active sites for trifluoromethane are formed on the catalyst, and the fluorine-chlorine-aluminum adsorption active sites for trifluoromethane account for 5.1 - 15% of the aluminum species on the catalyst surface; The catalyst is arranged in the reactor in the form of a catalyst bed layer, and a packing layer is provided on the catalyst bed layer, and the height of the packing layer is 1 - 15 cm.

2. The method for improving the activity and stability of a catalyst in the fluorine-chlorine exchange reaction of HFC-23 according to claim 1, characterized in that: The aluminum-based catalyst is selected from at least one of Al2O3, AlF3, Cr2O3 - Al2O3, Cr2O3 - AlF3, MgO - Al2O3, or MgO - AlF3.

3. The method for improving the activity and stability of a catalyst in the fluorine-chlorine exchange reaction of HFC-23 according to claim 1, characterized in that: The packing in the packing layer is selected from at least one of ceramic, silicon carbide, or nickel-based alloy random packings, and the nickel-based alloy includes at least one of Inconel alloy, Hastelloy alloy, or Monel alloy.

4. The method for improving the activity and stability of a catalyst in the fluorine-chlorine exchange reaction of HFC-23 according to claim 1, characterized in that: The auxiliary agent accounts for 0.01 - 20.0 wt% of the total mass of the catalyst.

5. The method for improving the activity and stability of a catalyst in the fluorine-chlorine exchange reaction of HFC-23 according to claim 4, characterized in that: When the auxiliary agent is activated carbon, the addition amount of the activated carbon accounts for 1.0 - 10.0 wt% of the total mass of the catalyst; when the auxiliary agent is silicon powder or silica powder, its addition amount accounts for 5.0 - 20.0 wt% of the total mass of the catalyst.

6. The method for improving the activity and stability of a catalyst in the fluorine-chlorine exchange reaction of HFC-23 according to claim 1, characterized in that: The fluorination pretreatment process includes the following steps: (1) Under a mixed atmosphere of 1 - 20% hydrogen fluoride and 80% - 99% nitrogen, perform fluorination treatment at 250 - 450 °C for 1 - 6 h; (2) Under a hydrogen fluoride atmosphere, treat at 300 - 500 °C for 2 - 10 hours; (3) Purge and cool with nitrogen.

7. The method for improving the activity and stability of a catalyst in the fluorine-chlorine exchange reaction of HFC-23 according to any one of claims 1-6, characterized in that: During the fluorine-chlorine exchange reaction, the molar ratio of trifluoromethane to trichloromethane is: 1 - (1:3), the reaction temperature is 250 - 400 °C, the reaction pressure is 0.1 - 3 bar, and the residence time in the catalyst bed layer is 4 - 50 s.

8. The method for improving the activity and stability of a catalyst in the fluorine-chlorine exchange reaction of HFC-23 according to claim 7, characterized in that: During the fluorine-chlorine exchange reaction, the molar ratio of trifluoromethane to trichloromethane is: 1 - (1:2), the reaction temperature is 300 - 360 °C, the reaction pressure is 0.1 - 2 bar, and the residence time in the catalyst bed layer is 4 - 12 s.

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