A platinum-tungsten bimetallic catalyst for PET depolymerization and a method for directional depolymerization of PET

Abstract

The present invention discloses a platinum-tungsten bimetallic catalyst for PET depolymerization and a method for directional depolymerization of PET, belonging to the technical field of solid waste recycling. The platinum-tungsten bimetallic catalyst for PET depolymerization is prepared by a deposition-precipitation method, with a mesoporous molecular sieve MCM-48 as a carrier to load the platinum-tungsten bimetallic, the molar ratio of platinum to tungsten elements being 1:0.5-2, and the platinum loading being 4wt%-6wt%. The platinum-tungsten bimetallic catalyst can be used to achieve a one-pot depolymerization of PET to produce p-toluic acid and p-xylene, and the platinum-tungsten bimetallic catalyst can be used multiple times through reduction regeneration, having high catalytic activity. The PET depolymerization method catalyzed by the platinum-tungsten bimetallic catalyst is suitable for recycling and upgrading various forms of PET waste, solving the technical problems of efficient depolymerization and directional conversion of waste PET, and providing a sustainable technical path for the resource recovery of PET.

Description

Technical Field

[0001] The invention belongs to the technical field of solid waste recycling and utilization, and particularly relates to a platinum-tungsten bimetallic catalyst for PET depolymerization and a method for directional depolymerization of PET. Background Art

[0002] Plastic waste pollution has now become the focus of environmental governance. Nowadays, the plastic with the highest annual output is polyethylene terephthalate (PET), which is widely used to make plastic packaging boxes, fiber textiles, etc. and has a short service life. The development of technologies for the refined and high-value utilization of waste PET is a key link in solving the environmental pollution caused by waste plastics. At present, the problem of resource waste in the industrial process of recycling, utilization and upgrading of waste PET needs to be solved urgently. The mechanical and physical "downgrading and recycling" method leads to the loss of plastic functions, the degree of biodegradation conversion is limited, and the application conditions are immature. Chemical upgrading and recycling to prepare important chemicals is one of the best ways to achieve refined and high-value utilization of waste PET, which can return it to petroleum products and realize the recycling of carbon resources at the molecular level.

[0003] The chemical recycling of waste PET to produce high-value chemicals generally involves two steps: 1. Depolymerization of the waste PET into small-molecule monomers; 2. Catalytic conversion of the small-molecule monomers into high-value chemicals. Currently, chemical depolymerization of waste PET primarily involves methanolysis, hydrolysis, glycolysis, and aminolysis, yielding small molecules such as dimethyl terephthalate, terephthalic acid, ethylene terephthalate, terephthalonitrile, and terephthalamide. These small molecules are then further converted and upgraded into higher-value products (see Chinese patent publications CN119118826A and CN117049962A, among others). While prior research has primarily focused on the targeted depolymerization of waste PET to produce oxygen-free light aromatic compounds such as benzene, toluene, and xylene, given the rich oxygen content of PET, the selective conversion of PET into high-value oxygen-containing small molecules is equally important. If we want to achieve the one-step targeted depolymerization of waste PET and upgrade it to produce high-value oxygen-containing chemicals, the development of high-performance catalysts is the technical key. It is also a new direction to break through the bottleneck of low-value and extensive "violent cracking" of waste PET chemical recycling in the future.

[0004] Para-toluic acid (p-TA) is an important oxygen-containing industrial raw material. Compared to other PET depolymerization products (such as benzene, toluene, and xylene), it is highly valuable and widely used in the manufacture of photosensitive materials, organic synthesis intermediates, and fungicides such as phosphamide. However, industrial production still relies on the oxidation of p-xylene (PX) with concentrated nitric acid, a reaction time of up to 30 hours, with a yield of only approximately 58%. Furthermore, the use of concentrated nitric acid as a reactant makes the synthesis process highly hazardous, requiring high equipment corrosion resistance and maintenance costs. Therefore, the development of green, simple, and low-cost routes to prepare p-toluic acid is urgently needed. Summary of the Invention

[0005] In order to address the deficiencies in the above-mentioned prior art, the present invention provides a platinum-tungsten bimetallic catalyst for PET depolymerization and a method for directional depolymerization of PET to produce p-toluic acid and p-xylene. The platinum-tungsten bimetallic catalyst is a heterogeneous solid catalyst that can be recycled and catalyzes the directional one-pot depolymerization and selective hydrogenation reaction of PET to obtain high-value oxygenated aromatic compound p-toluic acid and aromatic hydrocarbon p-xylene.

[0006] The specific technical solutions adopted are as follows:

[0007] A platinum-tungsten bimetallic catalyst for PET depolymerization is prepared by the following preparation method:

[0008] A mixture comprising chloroplatinic acid, sodium tungstate, mesoporous molecular sieve MCM-48, and deionized water is prepared, the mixture is naturally dried until mud cracks are formed, and then the temperature is increased to further dry the mixture. The dried solid is reduced at 400-500°C for 4-5 hours in a reducing gas atmosphere to obtain a platinum-tungsten bimetallic catalyst for PET depolymerization;

[0009] The carrier of the platinum-tungsten bimetallic catalyst for PET depolymerization is mesoporous molecular sieve MCM-48, platinum is in the form of zero-valent platinum, and tungsten is in the form of WO x The platinum is loaded on the mesoporous molecular sieve MCM-48 in the form of a mixture of pentavalent and hexavalent tungsten, with a molar ratio of platinum to tungsten of 1:0.5-2 and a platinum loading of 4wt%-6wt% (obtained by ICP test).

[0010] The mixture is naturally dried at room temperature until mud cracks are formed, which is conducive to the full loading of the platinum-tungsten bimetallic on the MCM-48 mesoporous molecular sieve, thereby ensuring the catalytic effect of the catalyst.

[0011] Preferably, the temperature is raised to 50-70° C. and further dried for 10-14 hours.

[0012] Preferably, the reducing gas atmosphere is a hydrogen-argon mixed atmosphere, and the heating rate during the reduction process is 1-3°C / min.

[0013] The present invention also provides a method for producing p-toluic acid and p-xylene through the targeted depolymerization of PET, utilizing the platinum-tungsten bimetallic catalyst for PET depolymerization. The platinum-tungsten bimetallic catalyst catalyzes the depolymerization of PET, promoting in-situ hydrodeoxygenation of the intermediate terephthalic acid, to selectively generate p-toluic acid and co-produce p-xylene. The platinum-tungsten bimetallic catalyst is relatively stable in a hydrothermal acidic system and can be regenerated through reduction to achieve multiple uses, maintaining good catalytic activity.

[0014] Specifically, the method for producing p-toluic acid and p-xylene by directional depolymerization of PET comprises the following steps:

[0015] S1. Constructing a reaction system using crushed PET, a platinum-tungsten bimetallic catalyst for PET depolymerization, and an acid solution, wherein the acid solution is a phosphotungstic acid aqueous solution;

[0016] S2. Place the reaction system constructed in step S1 under a hydrogen atmosphere at a temperature of 240-280° C. for a depolymerization reduction reaction for 8-16 hours. After the reaction is completed, quench and cool to obtain p-toluic acid and p-xylene.

[0017] The depolymerization reduction reaction equation can be expressed as:

[0018]

[0019] Preferably, the crushed PET is pre-cleaned and pre-crushed waste PET in the form of flakes, granules or powder. Waste PET includes but is not limited to PET bottles, PET films, PET trays, PET ribbons, PET ropes, PET non-woven fabrics, and blends of PET with cotton, spandex or other components. The pre-cleaning and pre-crushing operations can prevent other pollutants from interfering with and poisoning the catalyst, thereby ensuring the conversion effect of the waste PET. Of course, the corresponding pure PET chemicals can also be used to construct the reaction system.

[0020] The method of the invention is suitable for recycling and upgrading various forms of PET waste materials and has high efficiency and good adaptability.

[0021] Preferably, the platinum-tungsten bimetallic catalyst for PET depolymerization has a molar ratio of platinum to tungsten of 1:1.5, a platinum loading of 5 wt%, and a mass ratio of crushed PET to the platinum-tungsten bimetallic catalyst for PET depolymerization of 1:0.5-2. Under these conditions, the platinum-tungsten bimetallic catalyst for PET depolymerization exhibits excellent catalytic performance, capable of producing p-toluic acid and p-xylene in high yields, thus contributing to the efficient utilization of carbon resources in PET.

[0022] Preferably, the pH value of the phosphotungstic acid aqueous solution is 1-5.

[0023] More preferably, the pH value of the phosphotungstic acid aqueous solution is 2, and the usage ratio of the crushed PET and the phosphotungstic acid aqueous solution is 1 g:40-80 mL.

[0024] Preferably, the depolymerization reduction reaction is controlled to be carried out under a pressure of 1-5 MPa.

[0025] Preferably, the conditions for the depolymerization reduction reaction are 2 MPa, 260° C., and 12 h.

[0026] The present invention also provides a method for processing waste PET products, which utilizes the method for producing p-toluic acid and p-xylene by directional depolymerization of PET to process waste PET.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] (1) The platinum-tungsten bimetallic catalyst used in the present invention is a heterogeneous solid catalyst with the advantages of high catalytic efficiency, convenient recovery, and recyclability. It is beneficial to improve the selectivity of intermediate hydrogenation products and can achieve one-step directional depolymerization and selective hydrogenation of waste PET.

[0029] (2) The high-value PET conversion strategy developed in this invention has the advantages of short reaction time, simple steps, and high catalytic efficiency. It can produce the high-value oxygenated aromatic chemical p-toluic acid and p-xylene in a single step, breaking through the thermodynamic limit of PET depolymerization. Compared with the traditional petroleum production pathway for chemical monomers, this green catalytic system greatly reduces environmental impact and production costs, and to some extent alleviates the depletion of petroleum resources.

[0030] (3) The method of the present invention has a wide range of applications. The types of waste PET raw materials are varied. Various types of PET waste materials, such as PET water bottles, PET films, PET trays, PET ribbons, PET ropes, PET non-woven fabrics, and blended fabrics of PET and other components such as cotton and spandex, can all be efficiently upgraded and converted in a short period of time according to the method of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is the XRD characterization result diagram of PtW / MCM-48 catalyst (Pt loading is 5wt%, Pt / W molar ratio changes).

[0032] Figure 2 It is the NMR spectrum of p-toluic acid obtained in Example 1.

[0033] Figure 3 This is the NMR spectrum of p-xylene obtained in Example 1. DETAILED DESCRIPTION

[0034] In order to make the objects, features and advantages of the present invention more clearly understood, a detailed description is given below using specific embodiments. In the following description, many specific details are set forth to fully understand the present invention. However, the present invention can be implemented in many other ways than those described herein, and those skilled in the art can make similar improvements without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below. The technical features in the various embodiments of the present invention can be combined accordingly without conflicting with each other.

[0035] The procedures for the following examples, in which specific conditions are not specified, generally follow conventional conditions or those recommended by the manufacturer. Any material not described in detail in this specification belongs to the prior art known to those skilled in the art. The experimental materials used in the following examples, unless otherwise specified, can be purchased from conventional biochemical reagent companies.

[0036] In the following examples, the platinum-tungsten bimetallic catalyst (PtW / MCM-48 catalyst) for PET depolymerization was prepared according to the following deposition-precipitation method:

[0037] In a 100 mL beaker, chloroplatinic acid, sodium tungstate, 0.5 g of MCM-48 mesoporous molecular sieve, and a small amount (5-10 mL) of deionized water were added. Stirring was continued for 5 minutes, followed by ultrasonic treatment for 15 minutes. The resulting mixture was naturally dried at ambient temperature until visible mud cracks formed, and then dried in a 60°C oven for 12 hours. The dried solid was ground uniformly and reduced in a tube furnace at 450°C for 4 hours (reducing gas: 5% H2 / 95% Ar; heating rate: 2°C / min; gas flow rate: 30 mL / min) to obtain the PtW / MCM-48 catalyst.

[0038] The carrier of the PtW / MCM-48 catalyst is the mesoporous molecular sieve MCM-48. The loaded Pt / W molar ratio and the platinum loading can be regulated by changing the feeding amounts of chloroplatinic acid and sodium tungstate. The Pt / W ratio is generally controlled to be 1:0.5 to 1:2, and the platinum loading is generally controlled to be 4wt% to 6wt%. Experiments have shown that the best depolymerization reaction catalytic performance can be achieved when the Pt / W molar ratio is 1:1.5 and the platinum loading is 5wt%.

[0039] The XRD characterization results of PtW / MCM-48 catalyst (Pt loading 5 wt%, Pt / W molar ratio 1:2, 1:1.8, 1:1.5, 1:1.3, 1:0.5), mesoporous molecular sieve MCM-48 and corresponding active metals are shown in Figure 2. Figure 1 As shown, it is proved that platinum is in the form of zero-valent platinum and tungsten is in the form of WO x (tungsten is a mixture of pentavalent and hexavalent tungsten) and was successfully loaded on the mesoporous molecular sieve MCM-48.

[0040] Among them, MCM-48 mesoporous molecular sieve can be synthesized according to the method described in the prior art. Specifically, it can be synthesized by hydrothermal method using hexadecyltrimethylammonium bromide (CTAB) as a template. First, add 600g of deionized water to a beaker, then dissolve 9.1g of NaOH under stirring to prepare an alkaline solution, then add sodium fluoride (NaF) and CTAB, and magnetically stir the solution in a constant temperature oil bath for 2h. Finally, 101.2g of tetraethyl orthosilicate (TEOS) is added dropwise to the mixture. The molar ratio of the above added materials is TEOS:H2O:NaOH:NaF:CTAB=1:68.608:0.468:0.093:0.466. The resulting solution is transferred to a hydrothermal reactor and crystallized at 110°C for 48h. Hydrothermal exchange is performed at 80°C. The solid after vacuum filtration is rinsed with deionized water and ethanol 2-3 times, and then preliminarily dried at 110°C. The dried solid was ground evenly and calcined in a muffle furnace at 550° C. for 15 h to remove the CTAB template, ultimately obtaining a white powdery mesoporous molecular sieve MCM-48.

[0041] Example 1

[0042] In an autoclave reactor equipped with an electromagnetic stirrer, a thermocouple, and a programmable temperature controller, 0.05g of PET powder, 0.025g of PtW / MCM-48 catalyst (Pt loading of 5wt%, Pt / W=1:1.5), and 3mL of phosphotungstic acid aqueous solution (pH=2) were added. After sealing the autoclave reactor, the gas in the autoclave was replaced with hydrogen three times, and then hydrogen was pressurized to 2MPa at ambient temperature. Stirring and heating were turned on to heat the reaction system to 260°C and react at this temperature for 12h. After the reaction was completed, the autoclave was quenched in an ice-water bath to ambient temperature. The liquid phase was extracted three times with chloroform, using trioxane as an internal standard. The extract was mixed with tritiated chloroform and then analyzed by nuclear magnetic resonance spectroscopy (NMR). The results showed that the PET conversion rate of this embodiment was 100%, and p-toluic acid (NMR spectrum as shown in FIG. 1 ) was 100%. Figure 2 As shown, 1 H NMR (600 MHz, CDCl3) δ = 8.01 (d, J = 8.0 Hz, 2H), 7.28 (d, J = 8.0 Hz, 2H), 2.43 (s, 3H).) The yield was 52.8%, and p-xylene (NMR spectrum as shown Figure 3 As shown, 1 H NMR (600 MHz, CDCl 3 ) δ=7.05 (s, 4H), 2.30 (s, 6H). The yield was 37%.

[0043] Example 2

[0044] In an autoclave reactor equipped with an electromagnetic stirrer, thermocouple, and programmable temperature controller, 0.05g of PET powder, 0.05g of PtW / MCM-48 catalyst (Pt loading of 5wt%, Pt / W = 1:1.5), and 3mL of phosphotungstic acid aqueous solution (pH = 2) were added. After sealing the autoclave, the atmosphere was replaced with hydrogen three times, and then pressurized with hydrogen to 2MPa at ambient temperature. Stirring and heating were activated, and the reaction system was heated to 260°C and allowed to react at this temperature for 12 hours. After completion of the reaction, the autoclave was quenched in an ice-water bath to ambient temperature. The liquid phase was extracted three times with chloroform using trioxane as an internal standard. The extract was mixed with tritiated chloroform and analyzed by nuclear magnetic resonance spectroscopy (NMR). The results showed that the PET conversion rate of this example was 100%, the p-toluic acid yield was 53.4%, and the p-xylene yield was 36.4%.

[0045] To test the recyclability of the PtW / MCM-48 catalyst, a recycling experiment was conducted on the post-reaction catalyst. The solid catalyst was recovered by filtration, washed with deionized water and ethanol, dried at 60°C, calcined in a muffle furnace at 450°C for 4 hours, and then reduced in a tube furnace at 450°C for 4 hours (reducing gas: 5% H2 / 95% Ar; heating rate: 2°C / min; gas flow rate: 30 mL / min). The treated catalyst was then used in the next cycle. The test results showed that the PtW / MCM-48 catalyst could be reused in the next cycle after hydrogen reduction with virtually unchanged yield. However, if the catalyst was reused without hydrogen reduction, the yield decreased. After six reuses, the PtW / MCM-48 catalyst maintained high catalytic efficiency, with yields of 51.2% for p-toluic acid and 34.3% for p-xylene, respectively.

[0046] Example 3

[0047] Waste PET (Coca-Cola bottles) was crushed beforehand. 0.05 g of PET flakes, 0.05 g of PtW / MCM-48 catalyst (Pt loading 5 wt%, Pt / W = 1:1.5), and 3 mL of phosphotungstic acid aqueous solution (pH = 2) were added to an autoclave reactor equipped with an electromagnetic stirrer, thermocouple, and programmable temperature controller. The autoclave was sealed and the atmosphere was replaced with hydrogen three times. Then, hydrogen was pressurized to 2 MPa at ambient temperature. Stirring and heating were initiated, and the reaction system was heated to 260°C and allowed to react at this temperature for 12 hours. After the reaction, the autoclave was quenched in an ice-water bath to ambient temperature. The liquid phase was extracted three times with chloroform using trioxane as an internal standard. The extract was mixed with tritiated chloroform and analyzed by nuclear magnetic resonance spectroscopy (NMR). The results showed that the PET conversion rate of this embodiment was 94.1%, the p-toluic acid yield was 53.8%, and the p-xylene yield was 28.7%.

[0048] Example 4

[0049] Waste PET (colored plastic ribbon) was pre-crushed. 0.05 g of the shredded colored ribbon, 0.05 g of PtW / MCM-48 catalyst (Pt loading 5 wt%, Pt / W ratio 1:1.5), and 3 mL of phosphotungstic acid (pH 2) were added to an autoclave reactor equipped with an electromagnetic stirrer, thermocouple, and programmable temperature controller. The autoclave was sealed and the atmosphere was replaced with hydrogen three times. Then, hydrogen was pressurized to 2 MPa at ambient temperature. Stirring and heating were initiated, and the reaction system was heated to 260°C and allowed to react at this temperature for 12 hours. After the reaction, the autoclave was quenched in an ice-water bath to ambient temperature. The liquid phase was extracted three times with chloroform using trioxane as an internal standard. The extract was mixed with tritiated chloroform and analyzed by nuclear magnetic resonance spectroscopy (NMR). The results showed that the PET conversion rate of this embodiment was 92.9%, the p-toluic acid yield was 53.9%, and the p-xylene yield was 25.4%.

[0050] Example 5

[0051] A discarded PET blend (white polyester / cotton, 65% PET + 35% cotton) was pre-crushed. Then, in an autoclave equipped with an electromagnetic stirrer, thermocouple, and programmable temperature controller, 0.05 g of the crushed white polyester / cotton, 0.05 g of PtW / MCM-48 catalyst (Pt loading of 5 wt%, Pt / W = 1:1.5), and 3 mL of phosphotungstic acid aqueous solution (pH = 2) were added. After sealing the autoclave, the atmosphere was replaced with hydrogen three times. Then, hydrogen was pressurized to 2 MPa at ambient temperature. Stirring and heating were initiated, and the reaction system was heated to 260°C and allowed to react at this temperature for 12 hours. After completion of the reaction, the autoclave was quenched in an ice-water bath to ambient temperature. The liquid phase was extracted three times with chloroform using trioxane as an internal standard. The extract was mixed with tritiated chloroform and analyzed by nuclear magnetic resonance spectroscopy (NMR). The results showed that the PET conversion rate of this embodiment was 93.6%, the p-toluic acid yield was 48.4%, and the p-xylene yield was 30.4%.

[0052] Example 6

[0053] A discarded PET blend (black polyester / cotton, 85% PET + 15% cotton) was pre-crushed. Then, in an autoclave equipped with an electromagnetic stirrer, thermocouple, and programmable temperature controller, 0.05 g of the crushed black polyester / cotton, 0.05 g of PtW / MCM-48 catalyst (Pt loading 5 wt%, Pt / W = 1:1.5), and 3 mL of phosphotungstic acid aqueous solution (pH = 2) were added. After sealing the autoclave, the atmosphere was replaced with hydrogen three times. Then, hydrogen was pressurized to 2 MPa at ambient temperature. Stirring and heating were initiated, and the reaction system was heated to 260°C and allowed to react at this temperature for 12 hours. After completion of the reaction, the autoclave was quenched in an ice-water bath to ambient temperature. The liquid phase was extracted three times with chloroform using trioxane as an internal standard. The extract was mixed with tritiated chloroform and analyzed by nuclear magnetic resonance spectroscopy (NMR). The results showed that the PET conversion rate of this embodiment was 99.9%, the p-toluic acid yield was 52.2%, and the p-xylene yield was 22.5%.

[0054] Example 7

[0055] In an autoclave reactor equipped with an electromagnetic stirrer, thermocouple, and programmable temperature controller, 0.05g of PET powder, 0.05g of PtW / MCM-48 catalyst (Pt loading of 5wt%, Pt / W = 1:1.4), and 3mL of phosphotungstic acid aqueous solution (pH = 2) were added. After sealing the autoclave, the atmosphere was replaced with hydrogen three times, and then pressurized with hydrogen to 2MPa at ambient temperature. Stirring and heating were activated, and the reaction system was heated to 260°C and allowed to react at this temperature for 12 hours. After completion of the reaction, the autoclave was quenched in an ice-water bath to ambient temperature. The liquid phase was extracted three times with chloroform using trioxane as an internal standard. The extract was mixed with tritiated chloroform and analyzed by nuclear magnetic resonance spectroscopy (NMR). The results showed that the PET conversion rate of this example was 90.6%, the p-toluic acid yield was 44.0%, and the p-xylene yield was 20.4%.

[0056] Example 8

[0057] In an autoclave reactor equipped with an electromagnetic stirrer, thermocouple, and programmable temperature controller, 0.05g of PET powder, 0.05g of PtW / MCM-48 catalyst (Pt loading of 5wt%, Pt / W = 1:1.5), and 3mL of phosphotungstic acid aqueous solution (pH = 2) were added. After sealing the autoclave, the atmosphere was replaced with hydrogen three times. Then, hydrogen was pressurized to 2MPa at ambient temperature. Stirring and heating were initiated, and the reaction system was heated to 250°C and allowed to react at this temperature for 12 hours. After completion of the reaction, the autoclave was quenched in an ice-water bath to ambient temperature. The liquid phase was extracted three times with chloroform using trioxane as an internal standard. The extract was mixed with tritiated chloroform and analyzed by nuclear magnetic resonance spectroscopy (NMR). The results showed that the PET conversion rate of this example was 91.3%, the p-toluic acid yield was 44.7%, and the p-xylene yield was 16.1%.

[0058] Example 9

[0059] In an autoclave reactor equipped with an electromagnetic stirrer, thermocouple, and programmable temperature controller, 0.05g of PET powder, 0.05g of PtW / MCM-48 catalyst (Pt loading of 5wt%, Pt / W = 1:1.5), and 3mL of phosphotungstic acid aqueous solution (pH = 3) were added. After sealing the autoclave, the atmosphere was replaced with hydrogen three times, and then pressurized with hydrogen to 2MPa at ambient temperature. Stirring and heating were activated, and the reaction system was heated to 260°C and allowed to react at this temperature for 12 hours. After completion of the reaction, the autoclave was quenched in an ice-water bath to ambient temperature. The liquid phase was extracted three times with chloroform using trioxane as an internal standard. The extract was mixed with tritiated chloroform and analyzed by nuclear magnetic resonance spectroscopy (NMR). The results showed that the PET conversion rate of this example was 96.0%, the p-toluic acid yield was 44.1%, and the p-xylene yield was 8.5%.

[0060] Example 10

[0061] In an autoclave reactor equipped with an electromagnetic stirrer, thermocouple, and programmable temperature controller, 0.05g of PET powder, 0.05g of PtW / MCM-48 catalyst (Pt loading of 5wt%, Pt / W = 1:1.5), and 3mL of phosphotungstic acid aqueous solution (pH = 2) were added. After sealing the autoclave, the atmosphere was replaced with hydrogen three times, and then pressurized with hydrogen to 2MPa at ambient temperature. Stirring and heating were activated, and the reaction system was heated to 260°C and allowed to react at this temperature for 10 hours. After completion of the reaction, the autoclave was quenched in an ice-water bath to ambient temperature. The liquid phase was extracted three times with chloroform using trioxane as an internal standard. The extract was mixed with tritiated chloroform and analyzed by nuclear magnetic resonance spectroscopy (NMR). The results showed that the PET conversion rate in this example was 100.0%, the p-toluic acid yield was 50.8%, and the p-xylene yield was 28.6%.

[0062] The embodiments described above provide a detailed description of the technical solutions of the present invention. It should be understood that the above are only specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, supplements or similar substitutions made within the scope of the principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for producing p-toluic acid and p-xylene by directional depolymerization of PET, characterized in that: A platinum-tungsten bimetallic catalyst for PET depolymerization is used, and the platinum-tungsten bimetallic catalyst for PET depolymerization is prepared by the following preparation method: A mixture comprising chloroplatinic acid, sodium tungstate, mesoporous molecular sieve MCM-48, and deionized water is prepared, the mixture is naturally dried until mud cracks form, and then the temperature is increased to further dry the mixture. The dried solid is reduced at 400-500 °C for 4-5 hours in a reducing gas atmosphere to obtain a platinum-tungsten bimetallic catalyst for PET depolymerization. The carrier of the platinum-tungsten bimetallic catalyst for PET depolymerization is mesoporous molecular sieve MCM-48, the molar ratio of platinum to tungsten elements is 1:0.5-2, and the platinum loading is 4 wt%-6 wt%.

2. The method for producing p-toluic acid and p-xylene by directional depolymerization of PET according to claim 1, wherein: The reducing gas atmosphere is a hydrogen-argon mixed atmosphere, and the heating rate during the reduction process is 1-3 °C / min.

3. The method for producing p-toluic acid and p-xylene by directional depolymerization of PET according to claim 1, characterized in that: The following steps are involved: S1. Constructing a reaction system using crushed PET, a platinum-tungsten bimetallic catalyst for PET depolymerization, and an acid solution, wherein the acid solution is a phosphotungstic acid aqueous solution; S2. Place the reaction system constructed in step S1 under a hydrogen atmosphere at a temperature of 240-280° C. for a depolymerization reduction reaction for 8-16 hours. After the reaction is completed, quench and cool to obtain p-toluic acid and p-xylene.

4. The method for producing p-toluic acid and p-xylene by directional depolymerization of PET according to claim 3, characterized in that: Shredded PET is pre-cleaned, pre-crushed waste PET in the form of flakes, granules or powder.

5. The method for producing p-toluic acid and p-xylene by directional depolymerization of PET according to claim 3, characterized in that: The molar ratio of platinum to tungsten elements in the platinum-tungsten bimetallic catalyst for PET depolymerization is 1:1.5, the platinum loading is 5 wt%, and the mass ratio of crushed PET to the platinum-tungsten bimetallic catalyst for PET depolymerization is 1:0.5-2.

6. The method for producing p-toluic acid and p-xylene by directional depolymerization of PET according to claim 3, characterized in that: The pH value of the phosphotungstic acid aqueous solution is 1-5.

7. The method for producing p-toluic acid and p-xylene by directional depolymerization of PET according to claim 3, characterized in that: The ratio of crushed PET to phosphotungstic acid aqueous solution is 1 g: 40-80 mL.

8. The method for producing p-toluic acid and p-xylene by directional depolymerization of PET according to claim 3, characterized in that: The depolymerization reduction reaction is controlled to be carried out at a pressure of 1-5 MPa.

9. A method for treating waste PET products, characterized in that: Waste PET is treated by using the method for producing p-toluic acid and p-xylene by directional depolymerization of PET as described in any one of claims 1 to 8.

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