A method for cracking polystyrene

Abstract

The present invention relates to the field of catalyst preparation technology and plastic cracking and recycling catalytic path design, and specifically to a polystyrene cracking method. The present invention provides a polystyrene cracking method, comprising the following steps: polystyrene is cracked under the action of a catalyst and methanol; the catalyst is composed of a carrier and ruthenium loaded on the carrier. The polystyrene cracking method provided by the present invention is more economical by selecting methanol instead of hydrogen as a hydrogen source, and at the same time provides a suitable hydrogen partial pressure for the system, which has a higher conversion rate for the cracking of polystyrene, avoids benzene ring hydrogenation, reduces carbon deposition and improves aromatic selectivity. The present invention solves the problem that when hydrogen is used to crack polystyrene, the hydrogen partial pressure is too high, resulting in benzene ring hydrogenation and reduced aromatic selectivity; it also solves the problem that in the absence of hydrogen, polystyrene is easy to self-ring hydrogen production to form condensed ring aromatics, resulting in catalyst deactivation.

Description

Technical Field

[0001] The invention relates to the field of catalyst preparation technology and plastic cracking and recycling catalytic path design, and specifically to a polystyrene cracking method. Background Art

[0002] In recent years, waste plastics have spread around the world, and their long-term accumulation in the environment has led to serious environmental pollution and waste of energy resources. At present, most waste plastics are not recycled, causing serious harm to the ecological environment. As one of the five major plastics, polystyrene (PS) plastic has increased its production from 1.5 million tons in 1950 to 368 million tons in 2019, and is widely used in packaging for automotive parts, electronic products, food and medicine. However, less than 1% of PS is currently recycled, and a large amount of plastic waste ends up in landfills, accounting for about one-third of the global landfill volume. At the same time, the chemical inertness of PS causes it to degrade extremely slowly in nature, which may take thousands of years. Therefore, we urgently need a new route for recycling PS to achieve sustainable development.

[0003] Currently, the recycling of polystyrene plastics does not justify the cost of recycling, sorting and processing the plastics, and the performance of traditional mechanical methods for recycling plastics is poor and the cost is higher than that of plastics produced on a large scale in factories. So, although polymer plastic recycling is a necessary step in sustainable development, there are still some obstacles to overcome. For traditional pyrolysis of polystyrene, it is usually carried out at 450°C, which consumes a lot of energy and causes serious carbon deposition on the catalyst. In addition, there is another route, plasma-assisted rapid hydrogenolysis of PS. Although it can obtain a high ethylene yield, this method cannot recover the valuable benzene rings in PS and consumes a lot of hydrogen, which is still not economical. Therefore, we urgently need a sustainable and economically feasible method, that is, using a suitable catalyst under milder conditions than pyrolysis, and upgrading waste PS to valuable aromatics without consuming hydrogen or other solvents. Therefore, we need to innovate in catalyst preparation and reaction route design. Summary of the invention

[0004] In view of this, the technical problem to be solved by the present invention is to provide a method for cracking polystyrene. The method provided by the present invention utilizes methanol to crack polystyrene, has low cost and good catalytic activity and stability, while improving the selectivity of aromatics and having good economic benefits.

[0005] The present invention provides a polystyrene cracking method, comprising the following steps:

[0006] Under the action of catalyst and methanol, polystyrene is cracked;

[0007] The catalyst consists of a carrier and ruthenium supported on the carrier.

[0008] The catalyst used in the above method is composed of a carrier and ruthenium supported on the carrier. In certain embodiments of the present invention, the carrier is selected from SiO 2 、Al 2 O 3 or TiO 2 In one embodiment, the carrier is selected from SiO 2 、α-Al 2 O 3 , β-Al 2 O 3 or TiO 2 In one embodiment, the carrier is selected from silica. In one embodiment, the carrier is selected from fumed silica with a specific surface area of ​​200 m 2 / g~380m 2 / g, and its particle size is 7nm to 40nm. In certain embodiments of the present invention, the ruthenium loading is 1wt% to 2wt%. In one embodiment, the ruthenium loading is 1.4wt%. In one embodiment, the catalyst used in the above method is composed of silicon dioxide and ruthenium loaded on the silicon dioxide, that is, a ruthenium-based catalyst loaded on silicon dioxide, also known as Ru / SiO 2 catalyst.

[0009] The present invention provides a method for preparing the above-mentioned catalyst, comprising: heating a ruthenium source and a carrier for reaction, and then reducing and passivating to obtain the above-mentioned catalyst. Specifically, the present invention mixes the ruthenium source and the carrier in water for reaction, and then reduces and passivates to obtain the above-mentioned catalyst. In certain embodiments of the present invention, the present invention mixes the ruthenium source and the carrier in water, heats and reacts while stirring, and then evaporates and dries, grinds, reduces and passivates to obtain the above-mentioned catalyst.

[0010] In certain embodiments of the present invention, the temperature of the heating reaction is 75°C to 95°C; the time of the heating reaction is 10h to 12h. In one embodiment, the temperature of the heating reaction is 80°C; the time of the heating reaction is 12h. In one embodiment, the stirring rate is 400 to 600 rpm. In one embodiment, the stirring rate is 500 rpm.

[0011] In some embodiments of the present invention, the reduction is carried out in a hydrogen atmosphere. In some embodiments of the present invention, the reduction temperature is 380°C to 420°C, and the heating rate is 5°C min -1The reduction time is 110 min to 130 min. In one embodiment, the reduction temperature is 400° C., and the reduction time is 120 min.

[0012] In certain embodiments of the present invention, the passivation is at 1 vol% O 2 In some embodiments of the present invention, the passivation time is 50 min to 70 min. In one embodiment, the passivation time is 60 min.

[0013] The present invention obtains the Ru / SiO 2 In certain embodiments of the present invention, the present invention adopts an impregnation method to prepare Ru / SiO 2 catalyst; specifically, RuCl 3 ·xH 2 O and amorphous SiO 2 Deionized water was added; the mixture was vigorously stirred under heating conditions in a water bath, and then evaporated to dryness, ground, reduced and passivated to obtain Ru / SiO 2 Catalyst: RuCl 3 ·xH 2 The mass of O is 40 mg, and the amorphous SiO 2 The mass of is 1.00 g, and the volume of the deionized water is 50 mL.

[0014] The inventors of the present application creatively discovered that polystyrene can be cracked under the action of the above catalyst and methanol, and the method can improve the selectivity of aromatic hydrocarbons and has good catalytic activity and stability. The method has mild reaction conditions, is environmentally friendly, more economical and has high product value.

[0015] The polystyrene cracking method provided by the present invention cracks the polystyrene under the action of the above catalyst and methanol. Specifically, the present invention mixes the above catalyst, methanol and polystyrene, and cracks the polystyrene under a protective gas atmosphere.

[0016] In certain embodiments of the present invention, the mass ratio of the polystyrene, methanol and the catalyst is 2.5: (1.0-2.0): 0.25. In one embodiment, the mass ratio of the polystyrene, methanol and the catalyst is 2.5: 1.5: 0.25. In one embodiment, the catalyst is Ru / SiO 2 Catalyst, the polystyrene, methanol and Ru / SiO 2 The mass ratio of the catalyst is 2.5 g: (1.0 g to 2.0 g): 0.25 g. In one embodiment, the polystyrene, methanol and Ru / SiO 2The mass ratio of the catalyst is 2.5g:1.5g:0.25g.

[0017] The present invention provides a new route for cracking and upgrading waste polystyrene plastics to obtain high-value aromatic hydrocarbons by in-situ hydrogen production through methanol decomposition. In certain embodiments of the present invention, the weight average molecular weight of the polystyrene is 1000Da to 500000Da. In one embodiment, the weight average molecular weight of the polystyrene is 250000Da. In one embodiment, the polystyrene is waste polystyrene plastic. In certain embodiments of the present invention, the particle size of the polystyrene plastic is 10 to 100 meshes. In one embodiment, the polystyrene plastic and Ru / SiO 2 The mass ratio of the catalyst is 10:0.9-1.1; in a specific embodiment, the polystyrene plastic and Ru / SiO 2 The mass ratio of the catalyst is 10:1.0.

[0018] In certain embodiments of the present invention, the pyrolysis is carried out at a temperature of 250°C to 300°C, and the pyrolysis time is 0.5h to 12h. In one embodiment, the pyrolysis is carried out at a temperature of 280°C, and the pyrolysis time is 6h.

[0019] In certain embodiments of the present invention, the pyrolysis is carried out at a pressure of 0.5 MPa to 1.5 MPa. In one embodiment, the pyrolysis is carried out at a pressure of 1 MPa. The present invention controls the pressure of the reaction system by controlling the amount of protective gas introduced into the reaction system. The protective gas of the present invention is selected from N 2 , He or Ar, preferably selected from N 2 In some embodiments of the present invention, the pressure of the protective gas is 0.5 MPa to 1.5 MPa, preferably 1 MPa. In one embodiment, the protective gas of the present invention is 5 vol% N 2 / He, the N 2 The pressure of He is 0.5 MPa to 1.5 MPa. In one embodiment, the protective gas of the present invention is 5 vol% N 2 / He, the N 2 / The pressure of He is 1MPa.

[0020] In one embodiment, the present invention combines polystyrene plastic, methanol and Ru / SiO 2 The catalyst was placed in a Hastelloy alloy slurry bed reactor, and after removing the residual air in the reactor, 5 vol% N was charged into the reactor at room temperature. 2 / He gas, so that the polystyrene plastic, methanol and Ru / SiO 2 The catalyst was in 5 vol% N2 / He atmosphere to crack the polystyrene; after the reaction, the reactor was cooled to room temperature and the gas and liquid products were collected and quantified; the volume of the Hastelloy alloy slurry bed reactor was 50 mL; the residual air in the reactor consisted of 5 vol% N 2 / He gas was removed after 8 to 12 washes; in another embodiment, the residual air in the reactor consisted of 5 vol% N 2 / He gas was removed after 10 times of washing; the polystyrene plastic, methanol and Ru / SiO 2 The dosage ratio of the catalyst is the same as above and will not be repeated here; the temperature, time and pressure of the cracking are the same as above and will not be repeated here.

[0021] The present invention is the application of the above catalyst in the thermal catalytic methanol-assisted cracking of polystyrene. Under the above reaction conditions, methanol is decomposed into CO and active hydrogen, and the active hydrogen cracks the polystyrene chain to generate monomers (such as toluene, ethylbenzene, isopropylbenzene) and dimers (such as diphenyl substituted alkanes). Compared with the cracking of polystyrene by adding hydrogen (i.e., hydrogenolysis of PS), this route not only has a suitable hydrogen partial pressure to avoid further hydrogenation of aromatic products, but is also more economical. In addition, compared with the condition without hydrogen (i.e., the thermal decomposition of PS), since both methanol decomposition and PS aromatization can release hydrogen, they belong to two competitive processes. Therefore, when methanol is introduced as the main hydrogen supply, PS is mainly cracked by the hydrogen active species of methanol decomposition and generates monocyclic aromatic hydrocarbons and diphenyl substituted alkanes, avoiding the occurrence of aromatization side reactions of PS chains to generate byproducts such as polycyclic aromatic hydrocarbons. The present invention proposes a new reaction route to crack and upgrade polystyrene, and adjusts the reaction temperature and reaction time to further optimize the reaction activity and product selectivity.

[0022] Experiments show that the present invention uses Ru / SiO 2 Catalyst, filled with 1MPa 5vol%N 2 / He, 280 ° C for 6 hours, the hydrogenolysis of polystyrene and the pyrolysis of polystyrene were compared with the methanol-assisted polystyrene cracking of the present invention. It was found that the conversion rate of polystyrene in the thermal catalytic polystyrene cracking followed a trend of methanol-assisted polystyrene cracking > polystyrene pyrolysis > polystyrene hydrogenolysis. The selectivity of aromatics in the product followed a trend of methanol-assisted polystyrene cracking > polystyrene pyrolysis > polystyrene hydrogenolysis. The liquid product yield of methanol-assisted polystyrene cracking was 118.1 mmol carbon g catal -1 The mass of the polystyrene added is as high as 93.2wt% and the total selectivity of the target products, monocyclic aromatic hydrocarbons and diphenylalkanes, which are of higher value, is as high as 84.3%.

[0023] Under the same other conditions, the present invention is tested at 1h, 2h, 3h, 6h and 12h. By adjusting the reaction temperature, it is found that as the reaction time increases, the conversion of polystyrene is more thorough, the yields of toluene, ethylbenzene and isopropylbenzene increase significantly, and the yields of diphenylalkanes and liquid residues gradually decrease.

[0024] The present invention is tested at reaction temperatures of 250° C., 280° C. and 300° C. respectively under the same other conditions. By adjusting the reaction temperature, it is found that when the reaction temperature is reduced to 250° C., the degree of cracking of polystyrene is reduced, and when the reaction temperature is increased to 300° C., polystyrene is almost completely cracked, indicating that the increase in temperature accelerates the reaction kinetics.

[0025] The present invention provides a method for cracking polystyrene, comprising the following steps: cracking polystyrene under the action of a catalyst and methanol; the catalyst is composed of a carrier and ruthenium loaded on the carrier. The method for cracking polystyrene provided by the present invention is more economical by selecting methanol instead of hydrogen as a hydrogen source, and at the same time provides a suitable hydrogen partial pressure for the system, which has a higher conversion rate for cracking polystyrene, avoids benzene ring hydrogenation, reduces carbon deposition and improves aromatic selectivity. The present invention solves the problem that when hydrogen is used to crack polystyrene, the hydrogen partial pressure is too high, resulting in benzene ring hydrogenation and reduced aromatic selectivity; it also solves the problem that in the absence of hydrogen, polystyrene is easy to self-ring and produce hydrogen, forming condensed ring aromatics and causing catalyst deactivation. It can be seen that the method provided by the present invention has mild reaction conditions, is environmentally friendly, more economical and has high product value. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 is the X-ray diffraction pattern of the silica-supported ruthenium-based catalyst prepared in Example 1;

[0027] Figure 2 is a transmission electron microscopy image of the silica-supported ruthenium-based catalyst prepared in Example 1;

[0028] Figure 3 The activity and selectivity test results of the thermal catalytic methanol-assisted polystyrene cracking, hydrogenolysis of polystyrene and thermal decomposition of polystyrene over the ruthenium-based catalyst supported on silica are shown in FIG.

[0029] Figure 4 This is a graph showing the time-dependent evolution test results of thermal catalytic methanol-assisted polystyrene cracking over a ruthenium-based catalyst supported on silica;

[0030] Figure 5 This is a graph showing the results of temperature-dependent testing of thermal catalytic methanol-assisted polystyrene cracking using a silica-supported ruthenium-based catalyst. DETAILED DESCRIPTION

[0031] The present invention discloses a method for cracking polystyrene. Those skilled in the art can refer to the content of this article and appropriately improve the process parameters to achieve the same. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included in the present invention. The method and application of the present invention have been described through preferred embodiments, and relevant personnel can obviously modify or appropriately change and combine the method and application of this article without departing from the content, spirit and scope of the present invention to implement and apply the technology of the present invention.

[0032] The present invention will be further described below in conjunction with embodiments:

[0033] Example 1

[0034] The catalyst Ru / SiO was prepared by impregnation method. 2 The catalyst Ru / SiO 2 It is a ruthenium-based catalyst supported by silicon dioxide, with an average ruthenium loading of 1.4wt%. The synthesis method is as follows:

[0035] 40 mg of RuCl 3 ·xH 2 O and 1.00 g amorphous SiO 2 50 mL of deionized water was added to obtain a mixed solution, the mixed solution was heated in a water bath at 80 degrees Celsius and stirred vigorously for 12 hours, evaporated to dryness and then ground, the obtained sample was filled into a quartz reaction tube, the quartz tube containing the sample was placed in a reactor and reduced at 400 degrees Celsius for 2 hours under a hydrogen atmosphere at a heating rate of 5 degrees Celsius / min, and after the hydrogen pre-reduction was completed, it was cooled to room temperature under an argon atmosphere and heated to 1 vol% O 2 / Ar atmosphere for 1 hour to form a passivation layer on the sample surface, and obtain a ruthenium-based catalyst supported by silica. Finally, the obtained catalyst was vacuumed and stored until the catalytic test. Figure 1 As shown, Figure 1 is the X-ray diffraction pattern of the ruthenium-based catalyst supported on silica prepared in Example 1; Figure 2 As shown, Figure 2 This is a transmission electron microscopy image of the silica-supported ruthenium-based catalyst prepared in Example 1.

[0036] Example 2

[0037] (A) The silica-supported ruthenium-based catalyst prepared in Example 1 was used for thermal catalytic methanol-assisted polystyrene cracking, and the specific steps were as follows:

[0038] 250 mg of the silica-supported ruthenium-based catalyst prepared in Example 1, 2.5 g of polystyrene plastic and 1.5 g of methanol were added to a 50 mL Hastelloy reaction kettle and heated to 40 °C with 5 vol% N 2 / He was used to wash the reactor 10 times, and the reactor was filled with 1MPa 5vol%N 2 / He, react at 280 degrees Celsius for 6 hours. After the reaction is completed, the reactor is immediately placed in cold water to cool for more than 1 hour, and the gas phase product is detected by GC. After that, 3mL of cyclohexane is added to the kettle as an internal standard and mixed evenly with the liquid phase product. The supernatant and precipitate are collected after centrifugation. The centrifugal speed is 13000 rpm, and the centrifugal time is 5 minutes. The supernatant is quantitatively analyzed by GC and qualitatively analyzed by GC-MS. The precipitate obtained by centrifugation is dried in an oven at 80 degrees Celsius overnight to obtain a solid residue. The solid residue is weighed with an analytical balance, and its mass minus the mass of the catalyst added before the reaction can obtain the mass of the remaining undegraded plastic.

[0039] (B) The silica-supported ruthenium-based catalyst prepared in Example 1 was used for hydrogenolysis of polystyrene, and the specific steps were as follows:

[0040] 250 mg of the silica-supported ruthenium-based catalyst prepared in Example 1 and 2.5 g of polystyrene plastic were added to a 50 mL Hastelloy alloy reactor and heated to 40 °C with 5 vol% N 2 / H 2 Wash the reactor 10 times and fill the reactor with 4MPa 5vol%N 2 / H 2 , react at 280 degrees Celsius for 6 hours. After the reaction is completed, the reactor is immediately placed in cold water to cool for more than 1 hour, and the gas phase product is detected by GC. After that, 3 mL of cyclohexane is added to the kettle as an internal standard and mixed evenly with the liquid phase product. The supernatant and precipitate are collected after centrifugation. The centrifugal speed is 13000 rpm, and the centrifugal time is 5 minutes. The supernatant is quantitatively analyzed by GC and qualitatively analyzed by GC-MS. The precipitate obtained by centrifugation is dried in an oven at 80 degrees Celsius overnight to obtain a solid residue. The solid residue is weighed with an analytical balance, and the mass of the remaining undegraded plastic is obtained by subtracting the mass of the catalyst added before the reaction.

[0041] (C) The silica-supported ruthenium-based catalyst prepared in Example 1 was used for the pyrolysis of polystyrene, and the specific steps were as follows:

[0042] 250 mg of the silica-supported ruthenium-based catalyst prepared in Example 1 and 2.5 g of polystyrene plastic were added to a 50 mL Hastelloy alloy reactor and heated to 40 °C with 5 vol% N 2 / He wash the reactor 10 times, and fill the reactor with 1MPa5vol%N 2 / He, react at 280 degrees Celsius for 6 hours. After the reaction is completed, the reactor is immediately placed in cold water to cool for more than 1 hour, and the gas phase product is detected by GC. After that, 3mL of cyclohexane is added to the kettle as an internal standard and mixed evenly with the liquid phase product. The supernatant and precipitate are collected after centrifugation. The centrifugal speed is 13000 rpm, and the centrifugal time is 5 minutes. The supernatant is quantitatively analyzed by GC and qualitatively analyzed by GC-MS. The precipitate obtained by centrifugation is dried in an oven at 80 degrees Celsius overnight to obtain a solid residue. The solid residue is weighed with an analytical balance, and its mass minus the mass of the catalyst added before the reaction can obtain the mass of the remaining undegraded plastic.

[0043] The present application compares the performance of thermal catalytic methanol-assisted polystyrene cracking with hydrogenolysis of polystyrene and thermal decomposition of polystyrene on a silica-supported ruthenium-based catalyst. The activity and selectivity test results are shown in FIG. Figure 3 As shown, Figure 3 The activity and selectivity test results of the thermal catalytic methanol-assisted polystyrene cracking, hydrogenolysis of polystyrene and thermal decomposition of polystyrene over ruthenium-based catalysts supported on silica are shown in Figure 2. Figure 3 It can be seen that the conversion rate of polystyrene in thermal catalytic polystyrene cracking follows a trend: methanol-assisted polystyrene cracking > polystyrene thermal decomposition > polystyrene hydrogenolysis. The selectivity of aromatics in the product follows a trend: methanol-assisted polystyrene cracking > polystyrene thermal decomposition > polystyrene hydrogenolysis. The liquid phase product yield of methanol-assisted polystyrene cracking is 118.1 mmol carbon g catal -1 , its mass is as high as 93.2wt% of the input polystyrene, and the total selectivity of the target products with higher value, monocyclic aromatic hydrocarbons and diphenylalkanes, is as high as 84.3%. The comparison of these three reaction routes shows that the methanol-assisted polystyrene cracking proposed in the present invention is innovative in the design of the reaction route, which improves the conversion rate of polystyrene and the product value.

[0044] Example 3

[0045] Testing of the trend of thermal catalytic methanol-assisted polystyrene cracking over silica-supported ruthenium-based catalysts as a function of reaction time:

[0046] Under the reaction conditions of Example 2 (A), only the reaction time was changed to obtain the catalytic performance after 1 hour, 2 hours, 3 hours, 6 hours and 12 hours of reaction. Figure 4 As shown, Figure 4 This is a graph showing the time-dependent evolution test results of the thermal catalytic methanol-assisted polystyrene cracking over a ruthenium-based catalyst supported on silica. Figure 4 It can be seen that: with the extension of reaction time, the conversion rate of polystyrene material continues to increase, and the yield of aromatic hydrocarbons and diphenylalkane products also continues to increase.

[0047] Example 4

[0048] Testing of the variation trend of thermal catalytic methanol-assisted polystyrene cracking over silica-supported ruthenium-based catalysts with reaction temperature:

[0049] Under the reaction conditions of Example 2 (A), only the reaction temperature was changed, and the catalytic performance at 250°C, 280°C and 300°C was obtained respectively. Figure 5 As shown, Figure 5 This is a graph showing the results of temperature-dependent testing of thermal catalytic methanol-assisted polystyrene cracking using a ruthenium-based catalyst supported on silica. Figure 5 It can be seen that when the reaction temperature is reduced to 250°C, the degree of cracking of polystyrene decreases, and when the reaction temperature is increased to 300°C, polystyrene is almost completely cracked, indicating that the increase in temperature accelerates the reaction kinetics.

[0050] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A method for cracking polystyrene, It is characterized in that The following steps are involved: Under the action of a catalyst and methanol, polystyrene is cracked; the cracking is performed at a pressure of 0.5 MPa to 1.5 MPa; the mass ratio of polystyrene, methanol and catalyst is 2.5:(1.0 to 2.0):0.25; the cracking is performed at a temperature of 250°C to 300°C; The catalyst is composed of a carrier and ruthenium supported on the carrier; The preparation method of the catalyst comprises: heating a ruthenium source and a carrier for reaction, and then reducing and passivating the ruthenium source and obtaining the catalyst.

2. The lysis method according to claim 1, It is characterized in that The loading amount of ruthenium is 1 wt%~2 wt%.

3. The lysis method according to claim 2, It is characterized in that The loading amount of ruthenium is 1.4 wt%.

4. The lysis method according to claim 1, It is characterized in that The carrier is selected from silicon dioxide, Al 2 O 3 or TiO 2 .

5. The lysis method according to any one of claims 1 to 4, It is characterized in that The lysis time is 0.5 h to 12 h.

6. The lysis method according to claim 5, It is characterized in that The pyrolysis is carried out at a temperature of 280°C; The lysis time is 6 h.

7. The lysis method according to any one of claims 1 to 4, It is characterized in that The weight average molecular weight of the polystyrene is 1000 Da to 500000 Da.

8. The lysis method according to any one of claims 1 to 4, It is characterized in that The polystyrene is waste polystyrene plastic.

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